A [React hook](https://reactjs.org/docs/hooks-intro.html) for managing side-effects in your reducers.
npm install use-effect-reducerA React hook for managing side-effects in your reducers.
Inspired by the useReducerWithEmitEffect hook idea by Sophie Alpert.
If you know how to useReducer, you already know how to useEffectReducer.
💻 CodeSandbox example: Dog Fetcher with useEffectReducer
- Installation
- Quick Start
- Named Effects
- Effect Implementations
- Initial Effects
- Effect Entities
- Effect Cleanup
- Replacing Effects
- String Events
- API
- useEffectReducer hook
- exec(effect)
- exec.stop(entity)
- exec.replace(entity, effect)
- TypeScript
Install it:
``bash`
npm install use-effect-reducer
Import it:
`js`
import { useEffectReducer } from 'use-effect-reducer';
Create an effect reducer:
`js
const someEffectReducer = (state, event, exec) => {
// execute effects like this:
exec(() => {/ ... /});
// or parameterized (better):
exec({ type: 'fetchUser', user: event.user });
// and treat this like a normal reducer!
// ...
return state;
});
`
`js
// ...
const [state, dispatch] = useEffectReducer(someEffectReducer, initialState, {
// implementation of effects
});
// Just like useReducer:
dispatch({ type: 'FETCH', user: 'Sophie' });
`
No - internally, useEffectReducer (as the name implies) is abstracting this pattern:
`js
// pseudocode
const myReducer = ([state], event) => {
const effects = [];
const exec = (effect) => effects.push(effect);
const nextState = // calculate next state
return [nextState, effects];
}
// in your component
const [allState, dispatch] = useReducer(myReducer);
useEffect(() => {
allState.effects.forEach(effect => {
// execute the effect
});
}, [allState.effects]);
`
Instead of being implicit about which effects are executed and _when_ they are executed, you make this explicit in the "effect reducer" with the helper exec function. Then, the useEffectReducer hook will take the pending effects and properly execute them within a useEffect() hook.
An "effect reducer" takes 3 arguments:
1. state - the current stateevent
2. - the event that was dispatched to the reducerexec
3. - a function that captures effects to be executed and returns an effect entity that allows you to control the effect
`js
import { useEffectReducer } from 'use-effect-reducer';
// I know, I know, yet another counter example
const countReducer = (state, event, exec) => {
switch (event.type) {
case 'INC':
exec(() => {
// "Execute" a side-effect here
console.log('Going up!');
});
return {
...state,
count: state.count + 1,
};
default:
return state;
}
};
const App = () => {
const [state, dispatch] = useEffectReducer(countReducer, { count: 0 });
return (
Named Effects
A better way to make reusable effect reducers is to have effects that are named and parameterized. This is done by running
an effect object (instead of a function) and specifying that named effect's implementation as the 3rd argument to useEffectReducer(reducer, initial, effectMap).`js
const fetchEffectReducer = (state, event, exec) => {
switch (event.type) {
case 'FETCH':
// Capture a named effect to be executed
exec({ type: 'fetchFromAPI', user: event.user }); return {
...state,
status: 'fetching',
};
case 'RESOLVE':
return {
status: 'fulfilled',
user: event.data,
};
default:
return state;
}
};
const initialState = { status: 'idle', user: undefined };
const fetchFromAPIEffect = (_, effect, dispatch) => {
fetch(
/api/users/${effect.user})
.then(res => res.json())
.then(data => {
dispatch({
type: 'RESOLVE',
data,
});
});
};const Fetcher = () => {
const [state, dispatch] = useEffectReducer(fetchEffectReducer, initialState, {
// Specify how effects are implemented
fetchFromAPI: fetchFromAPIEffect,
});
return (
dispatch({ type: 'FETCH', user: 42 });
}}
>
Fetch user
Effect Implementations
An effect implementation is a function that takes 3 arguments:
1. The
at the time the effect was executed with exec(effect)
2. The object that triggered the effect
3. The effect reducer's dispatch function to dispatch events back to it. This enables dispatching within effects in the effectMap if it is written outside of the scope of your component. If your effects require access to variables and functions in the scope of your component, write your effectMap there.The effect implementation should return a disposal function that cleans up the effect:
`js
// Effect defined inline
exec(() => {
const id = setTimeout(() => {
// do some delayed side-effect
}, 1000); // disposal function
return () => {
clearTimeout(id);
};
});
`js
// Parameterized effect implementation
// (in the effect reducer)
exec({ type: 'doDelayedEffect' });// ...
// (in the component)
const [state, dispatch] = useEffectReducer(someReducer, initialState, {
doDelayedEffect: () => {
const id = setTimeout(() => {
// do some delayed side-effect
}, 1000);
// disposal function
return () => {
clearTimeout(id);
};
},
});
`Initial Effects
The 2nd argument to
can either be a static initialState or a function that takes in an effect exec function and returns the initialState:`js
const fetchReducer = (state, event) => {
if (event.type === 'RESOLVE') {
return {
...state,
data: event.data,
};
} return state;
};
const getInitialState = exec => {
exec({ type: 'fetchData', someQuery: '*' });
return { data: null };
};
// (in the component)
const [state, dispatch] = useEffectReducer(fetchReducer, getInitialState, {
fetchData(_, { someQuery }) {
fetch(
/some/api?${someQuery})
.then(res => res.json())
.then(data => {
dispatch({
type: 'RESOLVE',
data,
});
});
},
});
`Effect Entities
The
function returns an effect entity, which is a special object that represents the running effect. These objects can be stored directly in the reducer's state:`js
const someReducer = (state, event, exec) => {
// ... return {
...state,
// state.someEffect is now an effect entity
someEffect: exec(() => {
/ ... /
}),
};
};
`The advantage of having a reference to the effect (via the returned effect
) is that you can explicitly stop those effects:`js
const someReducer = (state, event, exec) => {
// ... // Stop an effect entity
exec.stop(state.someEffect);
return {
...state,
// state.someEffect is no longer needed
someEffect: undefined,
};
};
`Effect Cleanup
Instead of implicitly relying on arbitrary values in a dependency array changing to stop an effect (as you would with
), effects can be explicitly stopped using exec.stop(entity), where entity is the effect entity returned from initially calling exec(effect):`js
const timerReducer = (state, event, exec) => {
if (event.type === 'START') {
return {
...state,
timer: exec(() => {
const id = setTimeout(() => {
// Do some delayed effect
}, 1000); // Disposal function - will be called when
// effect entity is stopped
return () => {
clearTimeout(id);
};
}),
};
} else if (event.type === 'STOP') {
// Stop the effect entity
exec.stop(state.timer);
return state;
}
return state;
};
`All running effect entities will automatically be stopped when the component unmounts.
Replacing Effects
If you want to replace an effect with another (likely similar) effect, instead of calling
and calling exec(effect) to manually replace an effect, you can call exec.replace(entity, effect) as a shorthand:`js
const doSomeDelay = () => {
const id = setTimeout(() => {
// do some delayed effect
}, delay); return () => {
clearTimeout(id);
};
};
const timerReducer = (state, event, exec) => {
if (event.type === 'START') {
return {
...state,
timer: exec(() => doSomeDelay()),
};
} else if (event.type === 'LAP') {
// Replace the currently running effect represented by
state.timer
// with a new effect
return {
...state,
timer: exec.replace(state.timer, () => doSomeDelay()),
};
} else if (event.type === 'STOP') {
// Stop the effect entity
exec.stop(state.timer); return state;
}
return state;
};
String Events
The events handled by the effect reducers are intended to be event objects with a
property; e.g., { type: 'FETCH', other: 'data' }. For events without payload, you can dispatch the event type alone, which will be converted to an event object inside the effect reducer:`js
// dispatched as { type: 'INC' }
// and is the same as
dispatch('INC');
API
$3
The
hook takes the same first 2 arguments as the built-in useReducer hook, and returns the current state returned from the effect reducer, as well as a dispatch function for sending events to the reducer.`js
const SomeComponent = () => {
const [state, dispatch] = useEffectReducer(someEffectReducer, initialState); // ...
};
`The 2nd argument to
can either be a static initialState or a function that takes in exec and returns an initialState (with executed initial effects). See Initial Effects for more information.`js
const SomeComponent = () => {
const [state, dispatch] = useEffectReducer(
someEffectReducer,
exec => {
exec({ type: 'someEffect' });
return someInitialState;
},
{
someEffect(state, effect) {
// ...
},
}
); // ...
};
`Additionally, the
hook takes a 3rd argument, which is the implementation details for named effects:`js
const SomeComponent = () => {
const [state, dispatch] = useEffectReducer(someEffectReducer, initialState, {
log: (state, effect, dispatch) => {
console.log(state);
},
}); // ...
};
`$3
Used in an effect reducer,
queues the effect for execution and returns an effect entity.The
effect can either be an effect object:`js
// ...
const entity = exec({
type: 'alert',
message: 'hello',
});
`Or it can be an inline effect implementation:
js
// ...
const entity = exec(() => {
alert('hello');
});
`$3
Used in an effect reducer,
stops the effect represented by the entity. Returns void.`js
// Queues the effect entity for disposal
exec.stop(someEntity);
`Used in an effect reducer,
does two things:1. Queues the
entity for disposal (same as calling exec.stop(entity))
2. Returns a new effect entity that represents the effect that replaces the previous entity.TypeScript
The effect reducer can be specified as an
EffectReducer, where the generic types are:- The
state type returned from the reducer
- The event object type that can be dispatched to the reducer
- The effect object type that can be executed`ts
import { useEffectReducer, EffectReducer } from 'use-effect-reducer';interface User {
name: string;
}
type FetchState =
| {
status: 'idle';
user: undefined;
}
| {
status: 'fetching';
user: User | undefined;
}
| {
status: 'fulfilled';
user: User;
};
type FetchEvent =
| {
type: 'FETCH';
user: string;
}
| {
type: 'RESOLVE';
data: User;
};
type FetchEffect = {
type: 'fetchFromAPI';
user: string;
};
const fetchEffectReducer: EffectReducer = (
state,
event,
exec
) => {
switch (event.type) {
case 'FETCH':
// State, event, and effect types will be inferred!
// Also you should probably switch on
//
state.status first ;-) // ...
default:
return state;
}
};
``