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# EventTarget

The YUI Custom Event System enables you to define and use events beyond those available in the DOM — events that are specific to and of interest in your own application. Custom Events are designed to work much like DOM events. They can bubble, pass event facades, have their propagation and default behaviors suppressed, etc.

The APIs for working with custom events are provided by the `EventTarget` class. All other infrastructure classes extend `EventTarget`, but if you just need the custom event APIs, you can `extend` or `augment` your classes with `EventTarget` directly.

DEPRECATION NOTE: The `subscribers` and `afters` properties which used to sit on `CustomEvent` object instances have been deprecated and removed for performance reasons as of the 3.7.0 release.

If you're referring to the `subscribers` or `afters` properties directly just to access the set of subscribers, consider switching to the public `getSubs()` method instead which hides you from the implementation details.

If you have a use case which requires you to access the above properties directly you can set `Y.CustomEvent.keepDeprecatedSubs` to true, to restore them, but you will incur a performance hit if you enable this flag.

## Getting Started

To include the source files for EventTarget and its dependencies, first load the YUI seed file if you haven't already loaded it.

    <script src="http://yui.yahooapis.com/3.10.3/build/yui/yui-min.js"></script>

Next, create a new YUI instance for your application and populate it with the modules you need by specifying them as arguments to the `YUI().use()` method. YUI will automatically load any dependencies required by the modules you specify.

    <script>
    // Create a new YUI instance and populate it with the required modules.
    YUI().use('event-custom', function (Y) {
        // EventTarget is available and ready for use. Add implementation
        // code here.
    });
    </script>

For more information on creating YUI instances and on the `use()` method, see the documentation for the YUI Global Object.

## Video Overview

This video from YUIConf 2009 gives a good overview of the YUI event system API. The content covers DOM and custom events. Note: the synthetic event system was updated since this video.

## The Basics

You can get started using custom events and the `EventTarget` API without creating your own class. The YUI instance (typically `Y`) is an `EventTarget`, as is pretty much every other class in YUI. We'll go over the basics using `Y`, then move into creating your own `EventTarget`s.

If you've looked over the DOM Event system docs, this should look very familiar. That's because `Node`s are also `EventTarget`s.

### Subscribing to Events

    // Custom events can have any name you want
    Y.on('anyOldNameYouWant', function () {
        alert("Looky there!");
    });

    // Group subscriptions by passing an object or array to on()
    Y.on({
        somethingImportant: updateCalendar,
        birthday          : eatCake,
        weekendEnd        : backToTheGrindstone
    });

    // Some events have prefixes
    Y.once("fuji:available", climb);

    // Custom events have distinct "after" moments
    Y.after("spa-category|pedicure", gelatoTime);

All `EventTarget`s host methods `on`, `once`, `after`, and `onceAfter`. Both `once` and `onceAfter` will automatically detach the subscription after the callback is executed the first time. All subscription methods return a subscription object called an EventHandle. The distinction between `on` and `after` is discussed in the "after" phase section below.

### Firing Events

    // All subscribers to the myapp:ready event will be executed
    Y.fire('myapp:ready');

    // Pass along relevant data to the callbacks as arguments
    Y.fire('birthday', {
        name: 'Walt Disney',
        birthdate: new Date(1901, 11, 5)
    });

Notify event subscribers by calling `fire( eventName )`, passing any extra data about the event as additional arguments. Though `fire` accepts any number of arguments, it is preferable to send all event data in an object passed as the second argument. Doing so avoids locking your code into a specific `fire` and callback signature.

    // Subscription callbacks receive fire() arguments
    Y.on('birthday', function (name, birthdate) {
        var age = new Date().getFullYear() - birthdate.getFullYear();
        alert('Happy ' + age + ', ' + name + '!');
    });

    // Possible, but not recommended
    Y.fire('birthday', 'A. A. Milne', new Date(1882, 0, 18));

    // Instead, try to always pass only one object with all data
    Y.on('birthday', function (e) {
        var age = new Date().getFullYear() - e.birthdate.getFullYear();
        alert('Happy ' + age + ', ' + e.name + '!');
    });

    Y.fire('birthday', {
        name: '"Uncle" Walt Whitman',
        birthdate: new Date(1819, 4, 31)
    });

In the world of DOM events, the `fire` step is something the browser is responsible for. A typical model involves the browser receiving keyboard input from the user and firing `keydown`, `keyup`, and `keypress` events. Custom events put your code in the position of dispatching events in response to criteria that are relavant to your objects or application.

### Callback arguments and event facades

    // Simple notification events don't send event objects, only fire() data
    Y.on('talkie', function (data) {
        alert('(' + data.time + ') Walkie ' + data.message);
        // data.preventDefault is not defined. data is just a plain object
    });

    Y.fire('talkie', {
        message: 'roger, over.',
        time: new Date()
    });

    // Events configured to emitFacade will send an event object, merged with
    // fire() data
    Y.publish('bill:due', {
        emitFacade: true,
        defaultFn : payUp
    });

    Y.on('bill:due', function (e) {
        // Event facades have standard properties and methods as well as properties
        // from payload data passed to fire()
        if (e.payee === 'Rich Uncle Sal') {
            e.preventDefault(); // the `payUp` method won't be called (Sal can wait)
        }
    });

    // Objects passed as the second argument to fire() for facade events will have
    // their properties merged onto the facade received by the callback.
    Y.fire('bill:due', {
        payee: 'Great Aunt Myra',
        amount: 20
    });

Custom event callbacks are usually, but not always passed an EventFacade as their first argument. Custom events can be configured to send event facades or only the data they were fired with. Always passing event data in an object as the second argument to `fire` allows you to write all your callbacks to expect event data as a single first argument, whether it's an `EventFacade` or just a plain object. The `emitFacade` and `defaultFn` configurations are detailed below, in Publishing Events.

### Detaching Event Subscriptions

    // Subscription methods return a subscription handle...
    var subscription = Y.on('myapp:ready', initComponents);

    // ...with a detach method
    subscription.detach();

    // Or detach by signature
    Y.detach('myapp:ready', initComponents);

    // Or by subscription category
    Y.on('spa-category|pedicure', gelatoTime);

    // Detach subscriptions to all events in the spa-category subscription group
    Y.detach('spa-category|*');

The preferred method of detaching subscriptions is to use the EventHandle that is returned from the subscription methods. Alternately you can use the `detach` or `detachAll` methods which work as described in the Event user guide.

### Extending EventTarget

Add the `EventTarget` APIs onto any class using `Y.augment()`.

    function MyClass() {
        /* insert constructor logic here */
    }

    MyClass.prototype = {
        add: function (item) {
            // You can assume the APIs are available from your class instances
            this.fire("addItem", { item: item });
        },
        ...
    };

    // Make MyClass an EventTarget
    Y.augment(MyClass, Y.EventTarget);

    var instance = new MyClass();
    instance.on('addItem', function (e) {
        alert("Yay, I'm adding " + e.item);
    });

    instance.add('a new item'); // ==> "Yay, I'm adding a new item"

`Y.augment` works like a lazy `extend` or a mixin. It adds the APIs to the host class, but on the first call to any of the methods, it calls the `EventTarget` constructor on the instance to make sure the necessary internal objects are ready for use. If your class extends another, augmenting it won't interfere with that inheritance hierarchy.

`EventTarget`s can be set up with a number of default configurations for the events they `fire`. Pass the defaults as the fourth argument to `Y.augment`.

    // Make all events fired from MyClass instances send an event facade
    Y.augment(MyClass, Y.EventTarget, true, null, {
        emitFacade: true
    });

## Publishing Events

Some custom event configurations can be defaulted from class configuration, but others need to be specified on a per-event basis. Use the `publish` method to do this.

    // publish takes an event name and a configuration object
    Y.publish('somethingSpecial', {
        emitFacade: true,
        broadcast: 2,
        defaultFn: clapClapHallelujah,
        fireOnce: true
    });

### Event Facades

The most common configuration for custom events is `emitFacade`. This is because with the event facades comes a lot of additional functionality, such as preventable default behaviors and bubbling.

    function Recipe() {
        // publishing events is typically done at instantiation
        this.publish('add', {
            emitFacade: true,
            defaultFn: this._defAddFn
        });
    }

Event facades mirror the event objects you're familiar with from the DOM. They have properties like `e.type` and `e.target` and the same methods, allowing you to call `e.preventDefault()` to disable default behavior you've configured for the event or `e.stopPropagation()` to stop the event from bubbling.

    var gruel = new Recipe();

    gruel.on('add', function (e) {
        if (e.item === "brussel sprouts") {
            // call e.preventDefault() just as you would for DOM events
            e.preventDefault(); // brussel sprouts? eww!
        }
    });

`emitFacade` is typically passed as a default configuration to `Y.augment`. All other YUI infrastructure classes extend `EventTarget` and set `emitFacade` to `true` for you.

    Y.extend(MyClass, Y.Base, {
        add: function (item) {
            // This will fire with an event facade because Y.Base sets emitFacade to true
            this.fire('addItem', { item: item });
        },
        ...
    });

### `fireOnce` Events

Important, typically system-level or lifecycle related events can be configured as `fireOnce`. These events mimic things like `window.onload` or the `domready` event.

    Widget.prototype.render = function (where) {
        ...

        // Widget rendering only happens once
        this.publish('render', {
            defaultFn: this._defRenderFn,
            fireOnce: true,
            ...
        });

        this.fire('render', ...);
    };

After `fireOnce` events have been `fire()`d, any subsequent (late) subscriptions are immediately executed. This can introduce race conditions, however, since subscribers might expect to be called at a later time, after the code that follows the subscription has also executed. In this case, you can configure `fireOnce` events with the `async` flag and post-`fire` subscriptions will be executed in a `setTimeout`, allowing all subsequent code to run before the late subscriber is notified.

    // BEFORE
    Y.publish('myapp:ready', {
        fireOnce: true
    });

    // ... elsewhere in the code
    // If myapp:ready has been fired, setStuffUp executes right now, but might
    // expect MyApp.Stuff to be created already.  So, boom.
    Y.on('myapp:ready', setStuffUp);

    MyApp.Stuff = {};

    // AFTER
    Y.publish('myapp:ready', {
        fireOnce: true,
        async   : true
    });

    // ... elsewhere in the code
    // Even if myapp:ready has been fired, setStuffUp will execute later. So, no boom
    Y.on('myapp:ready', setStuffUp);

    MyApp.Stuff = {};

### Bubbling Events

Events that are configured with `emitFacade` support bubbling to other `EventTarget`s, allowing you to subscribe to them from other objects, much like DOM event bubbling. Add other `EventTarget`s to an instance's bubble path with `addTarget`.

    function LeafNode() { ... }

    LeafNode.prototype.rename = function (newName) {
        var oldName = this.name;
        this.name   = newName;

        this.fire("update", {
            prevVal: oldName,
            newVal : newName
        });
    };

    function TreeNode() { ... }

    TreeNode.prototype.add = function (node) {
        this._items.push(node);

        // The new child node's events will bubble to this TreeNode
        node.addTarget(this);
    };

    Y.augment(LeafNode, Y.EventTarget, true, null, { emitFacade: true });
    Y.augment(TreeNode, Y.EventTarget, true, null, { emitFacade: true });

    var rootNode = new TreeNode("ROOT"),
        branchA  = new TreeNode("branchA"),
        leaf1    = new LeafNode("leaf1");

    rootNode.add(branchA);                 //            ROOT
    rootNode.add( new LeafNode("leaf2") ); //           /    \
                                           //      branchA   leaf2
    branchA.add(leaf1);                    //     /       \
    branchA.add( new LeafNode("leaf3") );  //  leaf1     leaf3

    // Subscribe to 'update' events from any leaf or tree node under root
    rootNode.on('update', function (e) {
        alert(e.prevVal + " has been renamed " + e.newVal);
    });

    leaf1.rename("Flower!"); // ==> "leaf1 has been renamed Flower!"

### Event Prefixes

Individual events or all events fired by an `EventTarget` can be configured to include a prefix to help filter subscriptions to common event names by their origin. Prefixed event names look like `'prefix:eventName'`.

Taking the code snippet above, configuring a default `prefix` while augmenting the classes will allow for subscription to only `LeafNode` updates.

    // All events fired by LeafNode instances will be prefixed with "leaf:"
    Y.augment(LeafNode, Y.EventTarget, true, null, {
        emitFacade: true,
        prefix: 'leaf'
    });
    // ...and for TreeNodes, "tree:"
    Y.augment(TreeNode, Y.EventTarget, true, null, {
        emitFacade: true,
        prefix: 'tree'
    });

    ...

    // Listen specifically for changes from LeafNodes
    rootNode.on('leaf:update', function (e) {
        alert(e.prevVal + " has been renamed " + e.newVal);
    });

    leaf1.rename("Flower!"); // ==> "leaf1 has been renamed Flower!"
    branchA.rename("Chewbacca!"); // (nothing)

Subscribing with prefixes is similar to using DOM event delegation, though it is done using `on()` rather than `delegate()`.

Optionally, you can omit the prefix when subscribing on the object that fires the event.

    // prefix is optional when subscribing on the firing object...
    leaf1.on('leaf:update', worksJustLike);
    leaf1.on('update', function (e) {
        e.type; // 'leaf:update' -- the event type will remain prefixed
        ...
    });

    // ...but prefixes are required from other objects
    rootNode.on('update', function (e) {
        // will not capture leaf:update events
    });

Subscribe to all events of a specific type, regardless of prefix, using the wildcard prefix `*:eventName`.

    // Execute the callback if either the group object or one of its items fires an
    // `update` event
    rootNode.on('*:update', function (e) {
        switch (e.type) {
            case "leaf:update": ...
            case "tree:update": ...
        }
    });

### Adding Default Behavior

Custom events can be bound to behaviors just like DOM events (e.g. clicking on a link causes navigation to a new page). This is especially useful when doing CRUD operations that you want to expose to other objects in your system to prevent, alter, or enhance.

Add a default behavior to an event by configuring the event's `defaultFn`. By convention, default functions are named `_def(the name of the event)Fn`.

    function TreeNode(name) {
        this.name   = name;
        this._items = [];

        // Event publishing is typically done during instantiation
        this.publish('add', {
            defaultFn: this._defAddFn
        });
    }

    // Adding a child node is an interesting mutation operation. Move the mutation
    // logic from the method to a mutation event's default behavior
    TreeNode.prototype.add = function (node) {
        this.fire('add', { newNode: node });
    };

    // Default functions receive the event facade like other subscribers
    TreeNode.prototype._defAddFn = function (e) {
        this._items.push(e.newNode);

        e.newNode.addTarget(this);
    };

    ...

    branchA.add(leaf1); // without 'add' subscriptions, the behavior is the same

Unless configured with `preventable: false`, default behaviors can be disabled with `e.preventDefault()` just like the DOM. Unlike their DOM counterparts, though, event subscribers can change facade properties to alter the default behavior by way of effectively changing its input.

    TreeNode.prototype.add = function (node) {
        this.fire('add', {
            newNode: node,
            bubbleEvents: true
        });
    };

    // Default functions receive the event facade like other subscribers
    TreeNode.prototype._defAddFn = function (e) {
        this._items.push(e.newNode);

        if (e.bubbleEvents) {
            e.newNode.addTarget(this);
        }
    };

    ...

    // You can prevent default behavior from anywhere along the bubble path
    rootNode.on('tree:add', function (e) {
        if (e.newNode.name === "Leafy") {
            e.preventDefault();
        } else if (e.newNode.name === "James Bond") {
            e.bubbleEvents = false; // Shhhh
        }
    });

    rootNode.add( new LeafNode("Leafy") ); // Node NOT added
    rootNode.add( new LeafNode("James Bond") ); // Node added without event bubbling

### Broadcasting Events to Y or Between YUI instances

Event broadcasting is very similar to bubbling, but with some important distinctions:

1. Broadcasting is specific to the YUI instance and the `Y.Global` shared `EventTarget`
2. Events don't need to be configured with `emitFacade` to broadcast
3. Broadcasting happens after the default behavior, which also means...
4. Event behavior can't be prevented from broadcast subscribers
5. Broadcast can be defaulted for all events for an `EventTarget`

Broadcasting is essentially a "fast track" bubbling configuration allowing you to specify that events can be subscribed to from the YUI instance (with `broadcast: 1`) or from `Y.Global` (with `broadcast: 2`).

    // All events from instances of MyClass can be subscribed from Y.on()
    Y.augment(MyClass, Y.EventTarget, true, null, {
        emitFacade: true,
        prefix: 'awesome',
        broadcast: 1
    });

    // Respond to a 'thing' event from any instance of MyClass in the YUI sandbox
    Y.on('awesome:song', partyOn);

    var instance = new MyClass()

    instance.fire("song", { which: "Bohemian Rhapsody", whom: "Wayne" });

`Y.Global` is an `EventTarget` that is shared between all YUI instances, allowing cross-sandbox communication. To avoid feedback loops, it's best to add an instance identity to outgoing events and only respond to incoming events from other identities.

    YUI().use('node', 'event-custom', function (Y) {
        var id = "Alpha Beta Base"; // probably Y.guid() would be safer

        Y.Global.on('message', function (e) {
            if (e.origin !== id) {
                alert("message received from " + e.origin + ": " + e.message);

                murdock.fire("message", {
                    message: "We'll get you down.  And down safe.",
                    origin: id
                });
            }
        });

        function Character() {
            this.publish('message', { broadcast: 2 });
            ...
        }

        Y.augment(Character, Y.EventTarget, true, null, {
            emitFacade: true
        });

        var murdock = new Character();

        Y.one('#status').on('click', function () {
            murdock.fire("message", {
                message: "You're coming in too fast!",
                origin: id
            });
        });
    });

    YUI().use('node', 'event-custom', function (OtherY) {
        var id = "Lunar Shuttle";

        OtherY.Global.on('message', function (e) {
            if (e.origin !== id) {
                alert("message received from " + e.origin + ": " + e.message);
            }
        });

        function Character() {
            this.publish('message', { broadcast: 2 });
        }

        OtherY.augment(Character, OtherY.EventTarget, true, null, {
            emitFacade: true
        });

        var striker = new Character()

        OtherY.one('#report').on('click', function () {
            striker.fire("message", {
                message: "She's beginning to crack up",
                origin: id
            });
        });
    });

### Available Event Configurations and Defaults

Events can be configured with the following properties. Properties marked as "Class Configurable" can be passed to the `EventTarget` constructor configuration to default for all events.

Configuration | Description | Default | Class Configurable?
--- | --- | --- | ---
`prefix` | `e.type` will always include the configured prefix. Details above. | (empty) | YES
`context` | The default `this` object to execute callbacks with. Rarely set. | The instance | YES
`emitFacade` | If `true`, sends event facades to callbacks, allows bubbling and default functions, etc. This is commonly set to true for a class. Details above. | `false` | YES
`fireOnce` | If `true`, events will only fire once. Subscriptions made after firing will be immediately executed. Details above. | `false` | YES
`broadcast` | Details above. Fire the event from: `0` - Only the EventTarget instance `1` - The EventTarget and the YUI instance (`Y`) `2` - The EventTarget, `Y`, and `Y.Global` | 0 | YES
`bubbles` | For events configured to `emitFacade` allow bubbling events to other `EventTarget`s. | `true` | YES
`defaultFn` | Behavior associated with the event. Usually this is preventable (see `preventable` below). Details above. | (none) |
`preventable` | If set to `false`, `e.preventDefault()` will not disable execution of the event's `defaultFn`. | `true` |
`preventedFn` | Behavior associated with the event when `e.preventDefault()` is called from a subscriber. Use this function to reset partially applied transactional state. Incompatible with `preventable: false`. | (none) |
`stoppedFn` | Behavior associated with the event when `e.stopPropagation()` is called from a subscriber. Seldom used. | (none) |
`async` | Only applicable to events also configured with `fireOnce: true`. If `true`, new subscriptions to this event after it has already been fired will be queued to execute in a `setTimeout` instead of immediately (synchronously). | false |

### The "after" phase

Unlike DOM events, custom events also expose an "after" phase that corresponds to the time immediately after an event's default behavior executes. Subscribe to an event's "after" phase with the `after(...)` method. The signature is the same as `on(...)`.

    rootNode.after('tree:add', calc.updateTotals, calc);

The primary benefit of using `after()` subscriptions over `on()` subscriptions is that if any `on()` subscribers call `e.preventDefault()`, neither the event's configured `defaultFn` nor the `after()` subscribers will be executed. If an `after()` subscription is executed, you know that the `defaultFn` did as well.

Use `after()` to subscribe to events with a default behavior when you want to react to the event with a side effect.

Use `on()` to subscribe to events if you need to prevent or alter the default behavior or if they don't have default behavior.

## Event Lifecycle

The order of operations when firing an event is as follows:

### Simple Events (no facade)

1. `on()` subscribers are executed
2. `after()` subscribers are executed
3. `Y.on()` broadcast subscribers are executed.
4. `Y.after()` broadcast subscribers are executed.
5. `Y.Global.on()` broadcast subscribers are executed.
6. `Y.Global.after()` broadcast subscribers are executed.

If an `on()` or `after()` subscriber returns `false`, no more subscribers will be notified.

### Complex Events (with facade)

1. `on()` subscribers are executed
2. `on()` subscribers for each bubble target and their respective targets are executed until all targets' bubble paths are walked or a subscriber stops the propagation of the event.
3. If the event was prevented, any configured `preventedFn` will execute.
4. If not prevented, any configured `defaultFn` will execute.
5. If bubbling was stopped, any configured `stoppedFn` will execute.
6. `Y.on()` broadcast subscribers are executed.
7. `Y.after()` broadcast subscribers are executed.
8. `Y.Global.on()` broadcast subscribers are executed.
9. `Y.Global.after()` broadcast subscribers are executed.
10. `after()` subscribers are executed.
11. `after()` subscribers for each bubble target and their respective targets are executed.

The flow can be interrupted by `on()` subscribers doing any of these things:

`e.preventDefault()`
  1. The `defaultFn` will not be executed
  2. The `preventedFn` will execute
  3. No `after()` subscriptions will be executed
`e.stopPropagation()`
  1. The remainder of subscribers at this `EventTarget` WILL execute
  2. No bubble targets of this `EventTarget` will be notified
  3. The `stoppedFn` will execute
  4. The `defaultFn` and `after()` subscribers will execute
`e.stopImmediatePropagation()`
  Same as `e.stopPropagation()` except no more subscribers at this `EventTarget` will execute.
`e.halt()`
  Same as `e.preventDefault()` plus `e.stopPropagation()`.
`e.halt(true)`
  Same as `e.preventDefault()` plus `e.stopImmediatePropagation()`.
`return false`
  Same as `e.halt(true)`. Not recommended. Use the API methods.