mstform README

mstform is a form library written for
mobx-state-tree and
React. It manages form state for you and lets you
define validation rules. It understands about repeating sub-forms as well.

It doesn't put any requirements on your widgets. It works with any React
controlled component.

Features

- It knows about raw input (the value you type) and the converted input (the
value you want). You may type a string but want a number, for instance.
mstform converts this automatically.
- It can drive any React controlled component. It normalizes input components
so it can generate the right props for it -- whether it be a input type
string, type checked or a custom component that works in terms of objects -
mstform has you covered.
- Integrates deeply with a mobx-state-tree (MST) model. You give it a model
instance and it renders its contents. When you are ready to submit the form,
you have a mobx-state-tree model again. You can modify the mobx-state-tree
instance in code as well and the form is automatically updated.
- Thanks to MST it's easy to convert form contents to JSON and back again.
- It knows about types. If you use vscode for instance, your editor tells you
if you do something wrong. This works even in plain JavaScript if you enable
ts-check.

Philosophy

Form libraries are tricky.

Web forms are an integral part of most web applications. This means that you
need a lot of flexibility: you want to be able to mix form content with
non-form content, use whichever React components you like for input (from plain
HTML to fancy UI component libraries), and style it the way you want.

Web forms are also everywhere. That's why we'd like to automate as much as
possible and write as little form-specific code as possible.

But those two desires are in conflict with each other. It's tempting to start
auto-generating forms. It makes writing forms really easy. Unfortunately many
forms need special behavior, and it's difficult to capture this in an
auto-generating form library.

This way a form library that automates too much can be in the way when you want
to customize it to fit your application's requirements exactly. On the other
hand if your form library that doesn't do enough means you end up writing a lot
of custom code.

mstform balances simple usage with flexibility. It doesn't provide any React
widgets, or in fact any React components at all. It doesn't auto-generate forms
either. You write your own React components that render the form, and mstform
automates the management of values and errors. It aims to make the form code
that you do write look as straightforward as possible.

mstform is also built on a very powerful state management library:
mobx-state-tree (MST). Both the library and the application programmer can use
its features to make form construction more easy. As one example, MST makes it
trivial to serialize form contents to JSON and restore it again.

A Simple Example

``js
// @ts-check
import { observer } from "mobx-react";
import { types } from "mobx-state-tree";
import { Field, Form, FormState, converters } from "mstform";
import * as React from "react";
import { Component } from "react";

// we have a MST model with a string field foo
const M = types.model("M", {
foo: types.string,
});

// we expose this field in our form
const form = new Form(M, {
foo: new Field(converters.string, {
validators: [(value) => (value !== "correct" ? "Wrong" : false)],
}),
});

// we create an instance of the model
const o = M.create({ foo: "FOO" });

@observer
class Input extends Component {
render() {
const { type, field } = this.props;
return ;
}
}

@observer
class InlineError extends Component {
render() {
const { children, field } = this.props;
return (


render() {
// we get the foo field from the form
const field = this.formState.field("foo");
return (



);
}
}
`

What's going on in the example?

First we define a MST model M, which defines a string property foo. Next we
define a form for that model. We pass the model definition as the first
argument to the Form constructor.

A form needs to define fields for entries you want to expose in the form. Each
Field needs a converter. The converter specifies how to turn a raw value as
input by a user into a value you want to store in our MST model instance. Here
we use converters.string, a simple string, which is also stored as a string.
That's not a very fancy converter. A more complex converter would be
converters.number, which converts the input string into a number, and does
some validation to make sure that only numbers can be entered into the form.

The field definition also takes a number of options. The option we specify here
is a validation function. You can provide additional validator functions in a
list. If the validation function returns a string, that is the text of the
validation error and the validation has failed -- the entered value is not
stored in the underlying model instance. If the validation is successful, the
function should return false, null or undefined; not returning any value
is a successful validation.

We define a simple InlineError component that can display error text. It
takes a field and displays its error (which may be empty). Your UI component
library probably has a nicer component that helps to display errors -- Ant
Design for instance has Form.Item.

We also define a simple Input component that is our input; it takes a field
prop too and uses this to set the required input props (value and
onChange).

We then define a MyForm component that actually displays the form. To display
a form we need to initialize its _form state_. We create the form state with
form.state and store it on this in the constructor.

The form state maintains form-related state, such as errors to display. It also
can take state specific configuration (for instance how to save the form to a
backend), but in this case we don't supply any.

The form state needs a MST instance; this is the MST instance that you modify
with the form. When the user changes the form and it passes validation, the MST
instance is directly updated.

Here the MST instance is o from the global scope. but typically the object to
edit in the form comes in as a prop, and we can access it here.

Then in render we retrieve the field accessor for the foo field. This has
everything we need: error to show the current error, and inputProps for the
input component, so we pass the field to InlineError and Input.

I've enabled ts-check on top. If you're using vscode you can see
it reflect the correct types -- it knows raw is a string, for instance. This
can help to catch errors.

RepeatingForm

Many forms have a sub-form that repeats itself. The MST model
could look like this:

`js
const Animal = types.model("Animal", {
name: types.string,
size: types.string,
});

const Zoo = types.model("Zoo", {
animals: types.array(Animal),
});
`

Here we want a form that lets you add and remove animals:

`js
import { RepeatingForm } from "mstform";

const form = new Form(Zoo, {
animals: new RepeatingForm({
name: new Field(converters.string),
size: new Field(converters.string),
}),
});
`

We can now use it in our render method:

`js
// this represents all the sub forms
const animalForms = state.repeatingForm("animals");

const entries = o.animals.map((animal, index) => {
// get the sub-form we want
const animalForm = animalForms.index(index);
// and get the fields as usual
const name = animalForm.field("name");
const size = animalForm.field("size");
return (


`js
const Animal = types.model("Animal", {
name: types.string,
size: types.string,
});

const House = types.model("House", {
description: types.string,
pet: Animal,
});
`

We want a form that includes information about the pet:

`js
import { SubForm } from "mstform";

const form = new Form(Zoo, {
description: new Field(converters.string),
pet: new SubForm({
name: new Field(converters.string),
size: new Field(converters.string),
}),
});
`

We can now mix fields from the main form with those from the
sub-form in our render method:

`js
const description = state.field("description");
const name = state.subForm("pet").field("name");
`

Accessors

mstform defines a bunch of accessors:

- FieldAccessor, which you define with Field and get with field(). This
represents a field in the form that you can actually fill in and interact
with.

- SubFormAccessor which you define with SubForm and get with subForm().
This represents a sub-object in the underlying model instance.

- RepeatingFormAccessor which you define with RepeatingForm and get with
repeatingForm(). This represents an array of objects in the underlying
model instance.

- RepeatingFormIndexedAccessor which you define along with
RepeatingFormAccessor using RepeatingForm. You access it via the
index() method on a RepeatingFormAccessor. This represents a sub-object
in the underlying array instance.

- GroupAccessor. You define this in a second argument on forms. You can
access it via the group() method on any form accessor. This is a special
kind of accessor that only implements the isValid and isWarningFree methods.
It's a way to aggregate validation results from other accessors.

- Finally there is the FormState itself, which is the accessor at the root of
all things. You get it with form.state().

Accessors can contain other accessors. In particular, FormState,
SubFormAccessor and RepeatingFormIndexedAccessor allow you to access all
varieties of sub-accessor on it (except for FormState itself).
RepeatingFormAccessor allows only a single kind of sub-accessor, namely
RepeatingFormIndexedAccessor, which you access via index(). You cannot
access any sub-accessors on FieldAccessor. GroupAccessor doesn't allow
you access sub-accessors either.

All these accessors, except for GroupAccessor which is truly the odd one out,
have some properties in common:

- value: the underlying MST value that this accessor represents.

- path: The JSON path to the underlying MST value (see mobx-state-tree).

- fieldref: a generalization of the path to a pattern. foo/3/bar becomes
foo[].bar.

- context: The context object such as passed into form.state().

- isValid: Is true if the accessor (and all its sub-accessors) is valid.

- error: An error message (or undefined). Note that errors on non-field
accessors can only be set by external means such as with the getError
hook.

- warning: A warning message (or undefined). Warning messages can only
be set using the getWarning hook.

Supported converters

A converter specifies how to convert a raw value as it is entered in the form
to the converted value as it's stored in the MST instance, and back again. A
converter also specifies the empty form of the raw value (such as an empty
string), which is used by add forms. It also specifies which controlled props
to generate by default for React. Conversion may fail, in which case the
converter generates a validation error.

$3

The input raw value is a string. The converted value may be a string or some
other object:

- converters.string: value is a string.

- converters.literalString: value is a string.
literalType is a string literal type of which values are valid.

- converters.number: value is a number.

- converters.integer: value is an integer.

- converters.decimal({maxWholeDigits:x, decimalPlaces:y, allowNegative:z}).
You use this with the decimal mobx-state-tree type that is also exported
by this library, like in this model:

`js
const M = types.model({
d: decimal,
});
`

So the value that the converter delivers is a Decimal instance (from
decimal.js-light). It contains a decimal number with a maximum
maxWholeDigits (default 10) before the period and a maximum of
decimalPlaces (default 2) after the period. decimalPlaces also controls
the number of decimals that is initially rendered when opening the form.
With allowNegative (boolean, default true) you can specify if negative
values are allowed. With normalizedDecimalPlaces you can set the amount
of decimal places the converted number has. It should not be lower than
decimalPlaces, but can be higher. If it is, the given number is
automatically padded with additional decimal places set to 0.

Conversion error types are:

- default: Cannot be parsed, not a decimal number

- tooManyDecimalPlaces: we entered too many digits after the decimal
separator.

- tooManyWholeDigits: we entered too many digits before the decimal
separator.

- cannotBeNegative: you entered a negative number where this wasn't
allowed.

- converters.decimalString(maxWholeDigits: x, decimalPlaces: y, allowNegative: z}): like converters.decimal but has a string as its
value with a normalized representation of the decimal.

Number and decimal converters also respond to a handful of options through the
use of converterOptions. decimalSeparator specifies the character used to
separate the integral and fractional part of a number or decimal.
thousandSeparator specifies the character used to group thousands together.
renderThousands determines whether or not the thousand separators should be
rendered.
maxZeroesPadding can be set to limit the number of trailing zeros. This to
display a flexible minimum amount of decimals. Show more if we have more. Added
this new property for a customer that is wishing to display prices in at least 2
and at most 10 decimals. But, if, for instance, a price does not have more than
3 decimals, we do not want to show all trailing zeros
zeroIsEmpty can be set to treat 0 and similar inputs (e.g. 0.00 or 0.0000) as
empty values.

$3

converters.boolean: raw value is a boolean, value is also a boolean. The
default raw value in add forms is false.

$3

converters.stringArray: raw value is an array of strings. value is an
observable array of strings. Note that this is for using arrays that are
treated a value -- a list of which checkboxes are selected, for instance. When
you want the user to be able to add items to the array, using RepeatingForm
instead.

$3

converters.textStringArray: raw value is a string with newlines. Value is an
array of strings split by newline. You can use this with a textarea to edit an
array of strings by newline.

$3

converters.model(Model): does not do any conversion (model instance goes in,
model instance comes out), but allows you to specify that a MST model comes in
as a raw value and is the value. Typescript will be happy. This can be used to
support an input component such as a drop-down selection that generate a
reference to an object. This fits MST's types.reference.

$3

converters.modelReferenceArray(Model): does not do any conversion (model
reference instance array goes in, model reference instance array comes out), but
allows you to specify that a MST reference model array comes in as a raw value
and is the value. Typescript will be happy. This can be used to support a multi
input component such as a drop-down selection that generate a reference array to
an object. This fits MST's types.array(types.reference(Model)).

$3

converters.maybe(converter): This works on string converters as well as model
converters.

When you wrap it around any converter that takes a raw string value, the empty
value (such as the empty string) is accepted and converted into undefined.
This allows you to model empty values.

It can also be wrapped around a model converter, in which case it now accepts
empty. This is handy when you have a types.maybe(types.reference()) in MST.

converters.maybeNull(converter) is like converters.maybe but is designed to
work with types.maybeNull, so the empty value is null.

$3

converters.dynamic(converter, getOptions). This works on any converter that
expects a parameter object for its configuration. Currently this is only
converters.decimal.

This is a way to make the parameters for a converter dynamic and get
decided at run-time, based on the context you pass into state()
as well as the field accessor -- these get passed into the getOptions
function as arguments. So:

`js
const form = new Form(Foo, {
value: new Field(
converters.dynamic(converters.decimal, (context, accessor) => {
return { allowNegative: context.weWantNegatives };
})
),
});
`

allows negative values for this decimal dynamically if
context.weWantNegatives is set to true.

converters.maybe and converters.maybeNull wrap around converters.dynamic,
so it's converters.maybe(converters.dynamic(converters.decimal, getOptions)).

$3

converters.object: this accept any object as raw value and returns it,
including null. Prefer converters.model if you can. Warning: the default
raw value is null and using this with basic data types (string, boolean,
number and such) won't make the type checker happy as they don't accept "null".
Use more specific converters instead.

$3

Converters can be passed various options. Number and decimal converters respond
to decimalSeparator, thousandSeparator and renderThousands. These can be
set in a converterOptions argument on the state:

`js
const formState = form.state(o, {
converterOptions: {
decimalSeparator: ".",
thousandSeparator: ",",
renderThousands: false,
},
});
`

$3

A converter may fail to convert a raw value. In this case, the UI displays a
conversion error. You can control this conversion error with the
conversionError property for a field.

`js
const form = new Form(M, {
nr: new Field(converters.number, {
conversionError: "This conversion failed",
}),
});
`

You can also make conversionError a function. It takes a context
as its first argument. Context is an arbitrary object you can pass into the state method from your application:

`js
const form = new Form(M, {
nr: new Field(converters.number, {
conversionError: (context) =>
context.language === "en"
? "This conversion failed"
: "De conversie faalde",
}),
});
`

Some converters can return multiple types of conversion error. If
you care about showing these differences in the UI, you can set up
an object for conversionError:

`js
conversionError: {
default: "Not a number",
tooManyDecimalPlaces: "Too many decimal places",
tooManyWholeDigits: "Too many whole digits",
cannotBeNegative: "Cannot be negative"
}
`

This object must always contain a default key, which contains the fallback
conversion error if no error type matched.

It's also possible to use a function to dynamically generate the messages,
like this:

`js
conversionError: {
default: context => "Not a number",
tooManyDecimalPlaces: context => "Too many decimal places",
tooManyWholeDigits: context => "Too many whole digits",
cannotBeNegative: context => "Cannot be negative"
}
`

$3

You can define a new converter. For instance this is a converter which
takes a text in the UI and considers "t" as true and the rest as
false:

`ts
const boolean = new Converter({
emptyRaw: "f",
emptyImpossible: true,
convert(raw) {
return raw === "t";
},
render(value) {
return value ? "t" : "f";
},
});
`

Converter is a generic type, with . R is the type of the raw value
(as you have to render in a React component), and V is the type of the
converted value (as you have in the MST model).

A converter needs to define a convert and a render method. convert takes
a raw value and converts it to the MST value. render takes the MST value and
converts it to the raw value. You can trigger a conversion error by throwing
ConversionError inside convert; it takes the conversion error type as its
first argument.

emptyRaw is the raw value that should be shown if the field is empty in the
UI. We also set emptyImpossible -- it's impossible for the result of this
conversion to be empty (it's either true or false). In other cases,
an empty value can exist: for instance a converter to a string could produce
the empty string, or a maybe converter can produce undefined. In this
case you need to set the emptyValue property to what the value is when
it's not filled in. It's not allowed to set emptyValue when you
also define emptyImpossible to be true.

You can optionally set defaultControlled, the controlled props to be used by
default for this converter. You can also optionally set neverRequired; this
is handy for fields where the required status makes no sense -- a checkbox is
an example.

Something else you can define is isEmpty. This is a hook that overwrites the
default empty check, that works by taking the raw and comparing it to the
emptyRaw. This should be a function that takes the raw as argument, and
returns a boolean.

convert and render take an optional second argument, options. With
options, you can pass converterOptions and a context. context is an
arbitrary value you can pass in as a form.state() option from your
application:

`js
const formState = form.state(o, { context: { something: "FOO" } });
`

This is useful when you want to make a converter that depends on
an application-specific context.

Controlled props

A controlled component is a React
component that displays a value and defines an onChange handler that is
called when the value is changed by the user. The component itself does not
manage its value; this is done externally. mstform is a library that helps
you control these components for you.

Controlled components receive subtly different props:

- input type string has a value prop and an onChange with an event. It
gets the updated value from event.target.value.

- input type checkbox has a checked prop and an onChange that receives
event.target.checked with the updated value.

- There are also higher level widgets where value and onChange are
symmetrical. A date picker widget for instance could have a JS Date as
value and onChange directly returns a new Date instance.

mstform offers a controlled hook. It takes a function that given the field
accessor returns the right props for control. This can be used to ensure that
accessor.inputProps contains the right information for your particular
controlled component.

There are three controlled implementations built in:

- controlled.value - value and onChange processes
event.target.value.

- controlled.checked - checked and onChange processes
event.target.checked.

- controlled.object - value represents some object and onChange gets a
new object as an argument. Symmetrical value and onChange.

By default the converter determines which is used. If you use the string
converter or a derivative, controlled.value is used, and if you use the
boolean converter by default the controlled.checked is used. For
anything else the default is controlled.object.

You can always override controlled in the field configuration. For
example:

`js
import { observer } from "mobx-react";
import { types } from "mobx-state-tree";
import { Field, Form, FormState, converters, controlled } from "mstform";
import * as React from "react";
import { Component } from "react";

// we have a MST model with a string field foo
const M = types.model("M", {
foo: types.string,
});

// we expose this field in our form
const form = new Form(M, {
foo: new Field(converters.string, {
controlled: controlled.string,
}),
});
`

For backward compatibility with earlier versions of mstform, mstform also
supports fromEvent and getRaw in the field options. fromEvent is a flag
that indicates we want to pull the raw value to validate and convert from the
event.target.value. getRaw is a function that given the arguments to
onChange turns them into the updated raw value.

Add Mode

So far we've described how to use mstform with edit forms -- we display what's
in a MST instance and allow the user to edit it. There's another use case where
you want to create a new MST instance however: an add form.

Consider this MST model:

`js
const M = types.model("Foo", {
nr: types.number,
});
`

How do we create an add form for it? The add form needs an MST instance so that
it can store the user-entered values. But this model _requires_ you to create
an instance with a value for nr.

Let's do that and use an arbitrary number for nr. We could have picked
any number but 0 is probably the most clear, so we use that:

`js
const node = M.create({ nr: 0 });
`

Let's look at the form definition:

`js
const form = new Form(M, {
nr: new Field(converters.number),
});
`

If we create a normal edit form for this node, we would see the raw value "0"
in the input widget the form. That's not what we want to do in an add form; we
want to display an empty input widget (raw value "", the empty string). We
can accomplish this by setting the form in add mode when we create it:

`js
const state = form.state(node, { addMode: true });
`

This way the form is shown correctly, with the empty values. How does it know
what empty values to display in an add form? The converter actually specifies
this -- converters.number for instance knows that the empty value is the
empty string.

In case you already have a default value for a field and you do not want it to
be in add-mode, you can exclude them by including its fieldref in the
addModeDefaults option:

`js
const state = form.state(node, { addMode: true, addModeDefaults: ["nr"] });
`

Now the nr field is shown with the value 0 in it immediately.

If the field you reference from addModeDefaults is configured to be derived,
this is used to calculate the derived value automatically.

$3

Consider a repeating sub-form. Adding a new entry to a sub-form is much like
having an add form. Each time you add a new entry, the new sub-form should be
in add mode, even in edit forms. mstform automatically takes care of this if
you use the .push and .insert methods on the repeating form accessor, or if
you manipulate the underlying model directly. Existing records are shown in
edit mode, unless the whole form is in add mode.

If you use .push and .insert on the repeating form accessor, you can pass
in an additional argument with the addModeDefaults array. Here is an example:

`js
repeating.push({ a: 3, b: 5 }, ["b"]);
`

Here a is not mentioned and is considered to be in add-mode; instead of its
value, the empty raw is shown instead. But b is referenced and therefore 5
is visible immediately in the form.

Backend interaction (save and processing)

When we create the form state, we can pass it options on how to interact with
the backend. There are two features:

- You can save the form by specifying a save function. This save function can
return error messages in case the backend could not save successfully.

- You can configure the form to be validated and updated dynamically by the
backend using a process function. This can show messages and update the
form while the user is typing, under the control of a backend. This is
useful because validation needs to be defined on the backend anyway in
order to prevent any incorrect data to be submitted. This way you can
integrate this same validation code with the frontend. You can also supply
a processAll function which runs all validations supported by the
backend.

$3

You can write a function that defines how to save the underlying MST instance
(node), for instance by sending JSON to a backend:

`js
async function save(node) {
// we have defined a real 'save' function on the model that knows how
// to save the form to the backend. Should return a ProcessResult if
// there is a problem, or null or undefined if there is no problem.
return node.save();
}
`

Here is how we configure it:

`js
this.formState = form.state(o, {
backend: { save },
});
`

The save function should return undefined or null if the save succeeded and
there are no server validation errors. It can also return a special process
result - we discuss this below.

When you implement a form submit button, you should call state.save():

`js
@observer
export class MyForm extends Component {
constructor(props) {
super(props);
// we create a form state for this model
this.formState = form.state(o, {
backend: { save },
});
}

handleSave = async () => {
const success = await this.formState.save();
if (success) {
// success
} else {
// failure
}
};

render() {
// we get the foo field from the form
const field = this.formState.field("foo");
return (


state.save() does the following:

- Makes sure the form is completely valid before it's submitted to the
server. If not, the save is canceled and validation errors are displayed.

- Uses your supplied save function do to the actual saving.

- Processes the process result returned by the server.

- Returns true if saving succeeded, and false if not due to validation
errors or server processing problems.

If you don't specify your own save you get an exception when you call
state.save().

$3

Sometimes the backend knows more than the frontend, and you want to implement
some form behavior on the backend. Besides the low-level hooks (such as
getError) where you can integrate external behavior by hand, you can also
configure this behavior at a higher level.

If you implement the form processing protocol on your backend, your backend can
control validation messages (error and warning) messages, as well as control
default values for fields and clear them if they become invalid.

Here is how we configure it (in addition to save):

`js
async function process(node, path, liveOnly) {
// we have defined a real 'process' function on the model that knows how
// to invoke process on the backend. returns ProcessResult
return node.process();
}

this.formState = form.state(o, {
backend: { save, process },
});
`

The node argument is the underlying node that the form represents. path is
a JSON pointer (aka MST node path) to the field that was just changed by the
user modifying the form. liveOnly is a boolean value and is set to true
before you invoked save. This can be used to distinguish between "live
validations" (which always run) from those that only start running after you
attempt a save for the first time. The function should return a ProcessResult
structure.

The system keeps track of which field paths have been changed by the user. It
also makes sure that user input is debounced, so that a change in the form only
gets sent to the backend after the user stops changing the field for a field,
or when a set time has passed.

Once a field path has been changed, the node is sent to an asynchronous
"process" function, along with the field path that changed. You should
implement this function to define backend processing. It should send back a
structure with error messages, warning messages, and field updates.

The backend processing system ensures that the process function only runs one
at the time, and in sequence (in the order in which they were triggered). This
way, the consistency of updates is ensured.

The system also keeps track of fields that have changed but have not
yet been processed by the backend. In this case any updates from the backends
are ignored - new user input takes precedence.

$3

The process function takes the path of the field that has been changed.
save supports reprocessing of the whole form on the backend during saving. It
can in some circumstances be useful to explicitly trigger such a reprocess by
itself: run the backend process for the entire form at once. For this we have
processAll.

Here is how we configure it (in addition to save and process):

`js
async function processAll(node, liveOnly) {
// we have defined a real 'processAll' function on the model that knows how
// to invoke processAll on the backend. returns ProcessResult
return node.processAll();
}

this.formState = form.state(o, {
backend: { save, process, processAll },
});
`

You can invoke processAll on the form state to trigger it:

`js
formState.processAll();
`

$3

process, processAll and saveAll all return a ProcessResult.

Here is an example ProcessResult:

`js
const result = {
updates: [{ path: "/a", value: "Alpha" }],
accessUpdates: [{path: '/c', disabled: true }]
errorValidations: [
{ id: "one", messages: [{ path: "/b", message: "This is wrong" }] }
],
warningValidations: []
};
`

updates is a list of fields to update. Each field is indicated by path. The
rest of the structure is up to the developer and defines the update
information. By default, you need to supply a value with the new value to put
in the form there, but you can implement by your custom update procedure based
on other information if you pass an applyUpdate function (to be described
later).

accessUpdates is a list of fields to update as well. Each field is indicated
by path. The rest of the structure updates whether a field is disabled,
readOnly, hidden and required. Not passing a particular boolean results in
no change for that field. You can also use paths for accessors that aren't
fields, such as a repeating form, in which case for disabled, readOnly and
hidden the information is inherited by all fields in it. Setting required
only has meaning on the field accessor itself and is not inherited.

Both errorValidations and warningValidations are lists of validation
structures. These validation structures each have an id -- if a new
validation structure comes in, the previous validation structure with that id
(if it exists) is removed. A validation structure contains a list of validation
messages, each have a path. The path may be a path to a non-field such as a
sub-form, or repeating form as well, in case the validation error applies to
this.

The idea is that the backend associates validation functions with one or more
patterns of field paths (such as using the fieldref mechanism). Each validation
function has a unique id and can generate messages for arbitrary field paths in
the form. When a process request comes in, the backend only has to re-run
validation functions that match the path that just changed. Other fields that
are not affected do not have their messages affected - the frontend holds on to
these validation messages.

Warnings are like errors, but appear on field accessors with the warning
property. They do not make the form invalid - saving is still possible.

$3

Here is how we hook our process function into the backend.

`js
const M = types.model("M", {
foo: types.string,
});

const o = M.create({ foo: "FOO" });

async function myProcess(node, path) {
// call the backend, turn into ProcessResult and return it
}

form.state(o, { backend: { process: myProcess } });
`

This causes myProcess to be called whenever a field changes in the form
(debounced). You can control the debounce delay by passing delay (default
500):

`js
form.state(o, { backend: { process: myProcess, delay: 300 } });
`

As mentioned before you can also configure the applyUpdate function with
a custom one:

`js
import { applyPatch } from "mobx-state-tree";

function myApplyUpdate(node, update) {
// same behavior as the default
applyPatch(node, [
{ op: "replace", path: update.path, value: update.value },
]);
}

form.state(o, { backend: { process: myProcess, applyUpdate: myApplyUpdate } });
`

By default, process kicks off for every path in sequence. However, this can be too
strenuous on your system. If you want to process all paths at once, you can define
bulkProcess on your form state.

`js
form.state(o, { backend: { process: myProcess, bulkProcess: true } });
`

Now, your process function expects a list of paths, rather than one path. You can use
this to call one URL with all paths that need processing.

`js
const M = types.model("M", {
foo: types.string,
});

const o = M.create({ foo: "FOO" });

async function myProcess(node, path, liveOnly, paths?) {
// call the backend, turn into ProcessResult and return it
}

form.state(o, { backend: { process: myProcess, bulkProcess: true } });
`

paths is an optional argument, so it doesn't have to be defined.

$3

Before we described how to dynamically update validation information during
user interaction. We can also send back validation information when the user
saves the form. If the save failed to pass backend validation, we can also also
send back a ProcessResult instead of undefined or null (which both mean
success). This ProcessResult may be partial - if you don't include a
property, it is assumed to be the empty list.

$3

If you retrieve your errors in another way than using the process function, you
can use the getError hook:

`js
this.formState = form.state(o, {
getError: (accessor) =>
accessor.path === "/name" ? "Is wrong" : undefined,
});
`

The error property of the field will contain the "Is wrong" error message if
the field does not return undefined with the getError function. If a field
contains both an internally generated error message and one that is generated
via getError, the internally generated message trumps the one returned by the
getError hook.

Other accessors in mstform - SubForm, RepeatingForm and Form - also use
this error hook, allowing you to set errors on the complete form - or any
accessor within it. Indexed entries within repeating forms can also be set with
an error. If, for example, we want to raise an error when a RepeatingForm is
empty, we can raise an error on the repeating form accessor like this

`js
this.formState = form.state(o, {
getError: (accessor) =>
accessor instanceof RepeatingFormAccessor && accessor.length === 0
? "The repeating form must contain at least one form"
: undefined,
});
`

$3

mstform has a hook which allows you to include warning messages in its
accessors. Warnings are similar to errors, but don't make the form invalid. The
idea is that you can show warnings for certain fields in your form as a
notification to the user.

`js
const state = form.state(o, {
getWarning: (accessor) =>
accessor.raw < 0 ? "This value is negative" : undefined,
});
`

To implement warnings, pass a getWarning function. It is up to you to decide
how and when you wish to show these warnings in the UI. To check if the form
contains any warnings, you can use

`js
state.isWarningFree; // true or false
`

$3

You can pass an option into save() to ignore the required validation. This
can be useful if you have fields which are required in the form yet want allow
intermediate saves where this required setting is ignored.

Here's how to ignore the required validation:

`js
this.formState.save({ ignoreRequired: true });
`

This lets save proceed even if fields marked as required are not filled
in. It's up to you to construct the underlying MST model to allow empty values
(typically with types.maybe()) and to let the form accept them too (typically
with converters.maybe()).

$3

You can also ignore the getError validation during save:

`js
this.formState.save({ ignoreGetError: true });
`

This lets save proceed even though there are still external validation
errors. save still is blocked when you have an internal validation error -- a
raw value that cannot be successfully converted.

If you only define a save function for your backend and no process function
ignoreGetError is enabled automatically. This is to allow you to still save
forms even though they have externally defined errors.

$3

You can cause save to not affect the save status when you save::

`js
this.formState.save({ ignoreSaveStatus: true });
`

This causes the save status to remain before. Normally the save status is
updated to after when you save, which can affect the display of validation
messages and the state of the liveOnly flag that is passed to backend
process and processAll.

$3

By default, mstform displays inline validation errors as soon as you
make a mistake. This may not be desirable. You can turn it off by
passing another option:

`js
this.formState = form.state(o, {
validation: {
beforeSave: "no",
},
});
`

Now inline validation only occurs after you save the first time, not before.

It's also possible to turn off inline validation altogether:

`js
this.formState = form.state(o, {
validation: {
beforeSave: "no",
afterSave: "no",
},
});
`

In this case the user only sees updated validation errors once they press the
button that triggers state.save() and no errors are generated when the user
is filling in the form.

If you use ignoreSaveStatus the save status is not affect and remains
before.

You can reset the save status back to before with state.resetSaveStatus().

You can remove all internal and external errors and warnings by calling
state.clearAllValidations().

required fields

You can control which fields are required by setting the required flag in the
field definition:

`js
const form = new Form(M, {
name: new Field(converters.string, { required: true }),
});
`

This causes required property of the field accessor to be true, which you
can use during form rendering. It also causes it to be a validation error if
the field isn't filled in.

When the user enters an empty value (for instance the empty string), mstform
empties the underlying value if possible, changing the underlying object. This
is possible for the form defined above, as it uses a string converter (which can
be empty). A number converter cannot be empty however:

`js
const form = new Form(M, {
nr: new Field(converters.number),
});
`

In this case, the user _has_ to fill in a raw value that can be converted to a
number, otherwise the user gets the required error message and the underlying
value is not updated. Note that the required configuration in this case is
optional as it's implied by converters.number.

You can control the required error message by setting requiredError:

`js
const form = new Form(M, {
name: new Field(converters.string, {
required: true,
requiredError: "This is required!",
}),
});
`

You can also set requiredError to a function, in which cases it receives a
context argument (which you can pass in as an option to state()).

`js
const form = new Form(M, {
name: new Field(converters.string, {
required: true,
requiredError: (context) =>
context.language === "en"
? "This is required!"
: "Dit is verplicht!",
}),
});
`

requiredError (as a message or a function) can also be set on the state,
where it will be applied to every field on the form unless you override the
required error message on the field:

`js
this.formState = form.state(o, {
requiredError: "This is required!",
});
`

Empty and required fields

All fields have the isEmpty and isEmptyAndRequired properties.
isEmpty checks whether the raw value of the field equals the emptyRaw
value of the converter and if so it is considered empty. If the converter option
emptyImpossible is true isEmpty will always return false.

isEmptyAndRequired additionally checks whether the required property is true
for this field.

Dynamic disabled, hidden, required and readOnly fields

mstform has hooks that let you calculate hidden, disabled, required and
readOnly state based on the accessor. Here is a small example that
makes the foo form or field disabled. This uses the JSON Path functionality of
mstform to determine whether a field is disabled, but any operation can be
implemented here. You could for instance retrieve information about which
fields are disabled dynamically from the backend before you display the form.
The fieldref functionality described below is very useful for this.

`js
const state = form.state(o, {
isDisabled: (accessor) => accessor.path === "/foo",
});
`

To implement hidden behavior, pass in an isHidden function.

To implement readOnly behavior, pass in an isReadOnly function.

To implement required behavior, pass in an isRequired function. This does not
only affect the required property on the accessor, but also makes the field
require the form or field just as if you used the required flag in the field
definition. The required flag in the definition always makes something
required, no matter what isRequired says.

isDisabled returning true makes the disabled prop true in
accessor.inputProps. If isReadOnly is true, the readOnly flag is added to
accessor.inputProps; otherwise it's absent, but it's up to you to ensure your
React input widgets support a readOnly prop (HTML input does). There is no
such behavior for hidden or required; use accessor.hidden and
accessor.required in your rendering code to determine whether a form or field
wants to be hidden, or a field wants to be required. There is also an
inputAllowed flag on accessors, which checks if a form or field isn't disabled,
hidden or read-only.

When these properties are set on forms, they will automatically be passed down
to the children of said form. This works for regular forms, repeating forms and
subforms, and every kind of property except required.

Fieldref

Accessors that have a path property also define a fieldref property. The
fieldref is a generalized form of the path that is convenient for matching.

The path /foo results in the fieldref foo. The path /foo/bar results in
the fieldref foo.bar. The path /foo/1/bar results in the fieldref
foo[].bar, and so does /foo/2/bar or any other index. The path /foo/1 by
itself (for .repeating.index()) results in the fieldref foo[].

To create an isDisabled hook that makes the bar field disabled
in a repeating form, you can write:

`js
const state = form.state(o, {
isDisabled: (accessor) => accessor.fieldref === "foo[].bar",
});
`

Extra validation

Sometimes the information needed to validate the form cannot be known in
advance at form definition time. Instead, the form can be displayed multiple
times in the application, each time with different validation requirements.
mstform has a hook that lets you define additional validation behavior on the
form level.

`js
const state = form.state(o, {
extraValidation: (accessor, value) => {
if (accessor.path === "/foo") {
return value === "Wrong" ? "Wrong!" : false;
}
},
});
`

Note that you have to use the second value argument to get the value to use
to validate, as accessor.value still has the old value.

Validation groups

It can be useful to consider the validation status of a whole set of related
fields, without considering the validation status of the whole form. In a UI
you can then indicate that part of the form is invalid, which is especially
useful if the form is not visible in its entirety, for instance if is spread
out across tabs or a menu.

You can define validation groups with a second parameter you pass into Form,
SubForm or RepeatingForm:

`js
const M = types.model("M", {
a: types.string,
b: types.string,
c: types.string,
});

const form = new Form(
M,
{
a: new Field(converters.string),
b: new Field(converters.string),
c: new Field(converters.string),
},
{
one: new Group({ include: ["a", "b"] }),
two: new Group({ include: ["c"] }),
}
);
`

Here we define two groups, one and two. Group one is valid only if a
and b are valid. Group two is valid only if c is valid.

You can access a group on the state or form accessor and check its isValid
or isWarningFree property:

`js
const first = state.group("first");
if (first.isValid) {
// only executed if a and b are valid
}
if (first.isWarningFree) {
// only executed if none of the group members have warnings
}
`

When you define a group you can pass exclude instead of include:

`js
const form = new Form(
M,
{
a: new Field(converters.string),
b: new Field(converters.string),
c: new Field(converters.string),
},
{
one: new Group({ exclude: ["c"] }),
}
);
`

Group one now includes all accessors except c, and therefore a and b as
well.

Derived values

The value of some fields depends on the value of other fields; you can express
this relationship in a MST view. In some forms you want to automatically
calculate such a derived value but still allow the user to override it
explicitly. Then if the input to the calculation changes, the value is updated
again.

You express such derived values with mstform:

`js
const M = types
.model("M", {
calculated: types.number,
a: types.number,
b: types.number,
})
.views((self) => ({
sum() {
return self.a + self.b;
},
}));

const form = new Form(M, {
calculated: new Field(converters.number, {
derived: (node) => node.sum(),
}),
a: new Field(converters.number),
b: new Field(converters.number),
});
`

calculated starts out with the value based on the sum of a and b. The
user can modify calculated directly. When the user modifies a or b, the
derived value changes again to the result of the derived function.

When you access a repeating form, the node passed into the derived function is
the sub-node that the repeating form represents, so the derived value is
determined within that context.

Note that derived calculations occur if you actually access the field
to use it in a form; it doesn't work for fields that are never used.

Change hook

When you change one field it's sometimes useful to have some side effect, for
instance to change the value of another field. You can do so with the change
hook:

`js
const M = types
.model("M", {
a: types.number,
b: types.number,
})
.actions((self) => ({
setB(value: number) {
self.b = value;
},
}));

const form = new Form(M, {
a: new Field(converters.number, {
change: (node, value) => {
node.setB(value);
},
}),
b: new Field(converters.number),
});
`

We have defined an action that lets us modify b, which is represented
by the field b. We implement a change hook to call that action whenever
a is changed. This only happens if a passes validation -- changes
to a that result in an error message don't result in an execution
of the change hook.

Focus and blur hooks

You may want to react to field focus or blur events. You can do this with a
custom onFocus or onBlur event handler on the input element, but in some cases
you want to react generically to _all_ focus/blur events in a form. You can
pass a special hooks to the form state options for this:

`js
const state = form.state(o, {
focus: (ev, accessor) => {},
blur: (ev, accessor) => {},
});
`

The hook receives the event and the focused field accessor. You can use the
accessor to get the field name (accessor.name), value (accessor.value),
etc. When you define the hook, inputProps on the field accessor contains an
onFocus/onBlur handler, so if you use that with the field it is there
automatically.

In addition, you can set a field to re-render itself when you blur out of it,
using the postprocess option on fields. An example use case is rendering
extra zeroes in decimal fields, like so:

`js
const form = new Form(M, {
foo: new Field(converters.decimal({ decimalPlaces: 2, addZeroes: true }), {
postprocess: true,
}),
});
`

Update hook

When you want to react to changes to any field value in the form, you can
implement the update hook. This hook is triggered only when a change happens to
the _value_, not when the _raw_ is updated, so only when the underlying
instance that the form represents is updated. This means that if there are any
client-side validation messages, the update hook isn't yet triggered.

`js
const state = form.state(o, {
update: (accessor) => {},
});
`

Sources for references

When you use a reference field in a form (using the object or model
converter), you often display it with either a drop-down box or an autocomplete
widget. mstform offers support for populating these widgets, keeping track of
values that have appeared before and caching results.

In order to have a source your application needs to define what MST model
populates these sources and a container for them. Let's imagine we want to
allow the user to select a user object.

`js
const User = types.model("User", {
id: types.identifier,
username: types.string,
});

const UserContainer = types.model("UserContainer", {
entryMap: types.map(User),
});
`

We use types.map so we can keep track of User by their identifier so we can
quickly retrieve them again. You need to attach the UserContainer somewhere
to your state tree so that you can make references to users from other parts in
your state tree. Let's assume here it's in root.userContainer.

We have some way to load an array of users from the backend:

`js
async function loadUsers(q) {
const response = await window.fetch(...);
return response.json();
}
`

This queries the server based on some source-specific search parameters q
and returns an array of serialized User items, so:

`js
[
{
id: "a",
username: "Alpha",
},
{
id: "b",
username: "Beta",
},
];
`

It's up to the implementer to wrap loadUsers into a debounce facility so
that not every loadUsers call results in a server hit.

We can now define a source for users using a UserContainer instance and a
way to load users::

`js
import { Source } from "mstform";

const userSource = new Source({
entryMap: root.userContainer.entryMap,
load: loadUsers,
});
`

Note that you can also make entryMap a function that returns the
entryMap:

`js
import { Source } from "mstform";

const userSource = new Source({
entryMap: () => root.userContainer.entryMap,
load: loadUsers,
});
`

We define a model which contains a reference to a user:

`js
const M = types.model("M", {
user: types.maybe(types.reference(User)),
});
`

And a form for it:

`js
const form = new Form(M, {
user: new Field(converters.maybe(converters.model(User)), {
references: {
source: userSource,
},
}),
});
`

Let's now look at the user accessor. When the React component is mounted in
case of a select box, or in an event handler when we change the content of
autocomplete, we must load the references:

`js
const state = this.formState;
const user = state.field("user");

await user.references.load({{}});
`

references.load() contains an query argument -- these are additional
search parameters to pass through loadUser, such as what the user typed in
an autocomplete field.

Once references are loaded, we can access them in the UI (inside a React
render method):

`js
const state = this.formState;
const user = state.field("user");

const values = user.references.values({});
// display values somewhere
`

The source caches search requests, so that any future references.load() with
the same query resolve immediately without even hitting the backend. For this
reason, the search query accepted by load must be either JSON serializable or
alternatively you can provide a keyForQuery function to Source which turns
the search parameters into a unique cache key.

You can also make a field depend on another. Let's imagine that we have
a way to look for users that are friends of a user:

`js
async function loadFriends({user}) {
const response = await window.fetch(...);
return response.json();
}
`

Now we have a form with a user and a friend:

`js
const M = types.model("M", {
user: types.maybe(types.reference(User)),
friend: types.maybe(types.reference(User)),
});

const friendSource = new Source({
container: root.userContainer,
load: loadFriends,
});

const form = new Form(M, {
user: new Field(converters.maybe(converters.model(User)), {
references: {
source: userSource,
},
}),
friend: new Field(converters.maybe(converters.model(User)), {
references: {
source: friendSource,
dependentQuery: (accessor) => {
return { user: accessor.node.user };
},
},
}),
});
`

The dependentQuery bit here makes sure that when you load references for
friend the user is included automatically in this case the user
field.

Select widgets may need to refresh their list of options based on the values in
other fields. If you want to enable this you can call autoLoadReaction() on
references. If you don't call it (the default) then you are responsible for
this yourself. This is useful for autocomplete widgets which only reload when
the user interacts with them.

You can configure sources to use a defaultQuery if invoked without
arguments.

You can call load and values on sources directly too. If you don't
pass the query argument, it defaults to {} like for references.

You can call clear() on the source to remove _everything_ in it -- you should
only do this if you have nothing pointing to entries in its underlying
entryMap, or if all references are mobx-state-tree safeReference.

Note: mstform does not yet not implement any cache eviction facilities, either
from the container or from the search results.

Dirty state

For every field accessor we keep a copy of the initial value to determine
whether the fields value has been changed or not. This initial value does
include any value that has been set through addModeDefaults.

Every accessor has a property isDirty to check whether something has
changed, this can be any accessor like RepeatingFormAccessor,
FormAccessor, FieldAccessor, GroupAccessor.

When saving the form and the save was succesful the dirty state will be reset,
so the state after saving will be set as the initial value on every
FieldAccessor.

Manually resetting the dirty state can be done by calling resetDirtyState on
the FormState or resetDirtyState on any other accessor.

Restore state

Every accessor has a restore method to restore its value to the original state.

After succesfully saving a FormAccessor the original state is replaced.

Tips

- Don't name your form state this.state` on a React component as this has a
special meaning to React and can lead to odd bugs.