Font Face Observer 

Font Face Observer is a small @font-face loader and monitor (3.5KB minified and 1.3KB gzipped) compatible with any webfont service. It will monitor when a webfont is loaded and notify you. It does not limit you in any way in where, when, or how you load your webfonts. Unlike the Web Font Loader Font Face Observer uses scroll events to detect font loads efficiently and with minimum overhead.

How to use

Include your @font-face rules as usual. Fonts can be supplied by either a font service such as Google Fonts, Typekit, and Webtype or be self-hosted. You can set up monitoring for a single font family at a time:

``js
var font = new FontFaceObserver('My Family', {
weight: 400
});

font.load().then(function () {
console.log('Font is available');
}, function () {
console.log('Font is not available');
});
`

The FontFaceObserver constructor takes two arguments: the font-family name (required) and an object describing the variation (optional). The object can contain weight, style, and stretch properties. If a property is not present it will default to normal. To start loading the font, call the load method. It'll immediately return a new Promise that resolves when the font is loaded and rejected when the font fails to load.

If your font doesn't contain at least the latin "BESbwy" characters you must pass a custom test string to the load method.

`js
var font = new FontFaceObserver('My Family');

font.load('中国').then(function () {
console.log('Font is available');
}, function () {
console.log('Font is not available');
});
`

The default timeout for giving up on font loading is 3 seconds. You can increase or decrease this by passing a number of milliseconds as the second parameter to the load method.

`js
var font = new FontFaceObserver('My Family');

font.load(null, 5000).then(function () {
console.log('Font is available');
}, function () {
console.log('Font is not available after waiting 5 seconds');
});
`

Multiple fonts can be loaded by creating a FontFaceObserver instance for each.

`js
var fontA = new FontFaceObserver('Family A');
var fontB = new FontFaceObserver('Family B');

fontA.load().then(function () {
console.log('Family A is available');
});

fontB.load().then(function () {
console.log('Family B is available');
});
`

You may also load both at the same time, rather than loading each individually.

`js
var fontA = new FontFaceObserver('Family A');
var fontB = new FontFaceObserver('Family B');

Promise.all([fontA.load(), fontB.load()]).then(function () {
console.log('Family A & B have loaded');
});
`

If you are working with a large number of fonts, you may decide to create FontFaceObserver instances dynamically:

`js
// An example collection of font data with additional metadata,
// in this case “color.”
var exampleFontData = {
'Family A': { weight: 400, color: 'red' },
'Family B': { weight: 400, color: 'orange' },
'Family C': { weight: 900, color: 'yellow' },
// Etc.
};

var observers = [];

// Make one observer for each font,
// by iterating over the data we already have
Object.keys(exampleFontData).forEach(function(family) {
var data = exampleFontData[family];
var obs = new FontFaceObserver(family, data);
observers.push(obs.load());
});

Promise.all(observers)
.then(function(fonts) {
fonts.forEach(function(font) {
console.log(font.family + ' ' + font.weight + ' ' + 'loaded');

// Map the result of the Promise back to our existing data,
// to get the other properties we need.
console.log(exampleFontData[font.family].color);
});
})
.catch(function(err) {
console.warn('Some critical font are not available:', err);
});
`

The following example emulates FOUT with Font Face Observer for My Family.

`js
var font = new FontFaceObserver('My Family');

font.load().then(function () {
document.documentElement.className += " fonts-loaded";
});
`

`css
.fonts-loaded {
body {
font-family: My Family, sans-serif;
}
}
`

Installation

If you're using npm you can install Font Face Observer as a dependency:

`shell
$ npm install fontfaceobserver
`

You can then require fontfaceobserver as a CommonJS (Browserify) module:

`js
var FontFaceObserver = require('fontfaceobserver');

var font = new FontFaceObserver('My Family');

font.load().then(function () {
console.log('My Family has loaded');
});
`

If you're not using npm, grab fontfaceobserver.js or fontfaceobserver.standalone.js (see below) and include it in your project. It'll export a global FontFaceObserver that you can use to create new instances.

Font Face Observer uses Promises in its API, so for browsers that do not support promises you'll need to include a polyfill. If you use your own Promise polyfill you just need to include fontfaceobserver.standalone.js in your project. If you do not have an existing Promise polyfill you should use fontfaceobserver.js` which includes a small Promise polyfill. Using the Promise polyfill adds roughly 1.4KB (500 bytes gzipped) to the file size.

Browser support

FontFaceObserver has been tested and works on the following browsers:

* Chrome (desktop & Android)
* Firefox
* Opera
* Safari (desktop & iOS)
* IE8+
* Android WebKit

License

Font Face Observer is licensed under the BSD License. Copyright 2014-2017 Bram Stein. All rights reserved.