<link type="text/css" rel="stylesheet" href="{{yuiBuildUrl}}/cssbutton/cssbutton-min.css">
<div class="intro">
    <p>
        The YUI Custom Event System enables you to define and use events beyond
        those available in the DOM &mdash; 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.
    </p>

    <p>
        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.
    </p>

    <p class="deprecated"><strong>DEPRECATION NOTE:</strong> 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.
    </p>

    <p>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.</p>

    <p>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.
    </p>

    <!--p>
        Bundled with `EventTarget` are <a
        href="http://en.wikipedia.org/wiki/Aspect_oriented_programming">Aspect
        Oriented Programming</a> methods that allow you to subscribe to the
        execution of object methods, and 
        their own.
    </p-->
</div>

<!-- insert Events Evolved video here -->

{{>getting-started}}

<h2>Video Overview</h2>

<iframe width="640" height="360" src="http://www.youtube.com/embed/s_7VjN3qxe8" frameborder="0" allowfullscreen></iframe>

<p>
    This video from YUIConf 2009 gives a good overview of the YUI event system
    API.  The content covers DOM and custom events.  Note: the <a
    href="../event/index.html#synthetic-events">synthetic event</a> system was
    updated since this video.
</p>

<h2>The Basics</h2>

<p>
    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.
</p>

<p>
    If you've looked over the <a href="../event/index.html#the-basics">DOM
    Event system docs</a>, this should look very familiar. That's because
    `Node`s are also `EventTarget`s.
</p>

<h3>Subscribing to Events</h3>

```
// 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);
```

<p>
    All `EventTarget`s host methods
    <a href="{{apiDocs}}/classes/EventTarget.html#method_on">`on`</a>, 
    <a href="{{apiDocs}}/classes/EventTarget.html#method_once">`once`</a>, 
    <a href="{{apiDocs}}/classes/EventTarget.html#method_after">`after`</a>, and
    <a href="{{apiDocs}}/classes/EventTarget.html#method_onceAfter">`onceAfter`</a>. 
    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
    <a href="{{apiDocs}}/classes/EventHandle.html">EventHandle</a>. The
    distinction between `on` and `after` is discussed in the
    <a href="#after">"after" phase</a> section below.
</p>

<h3 id="fire">Firing Events</h3>

```
// 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)
});
```

<p id="event-data-object">
    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.
</p>

```
// 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)
});
```

<p>
    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.
</p>

<h3>Callback arguments and event facades</h3>

```
// 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
});
```

<p>
    Custom event callbacks are <em>usually, but not always</em> passed an
    <a href="{{apiDocs}}/classes/EventFacade.html">EventFacade</a> as their
    first argument.  Custom events can be configured to send event facades or
    only the data they were fired with.  <a href="#event-data-object">Always
    passing event data in an object</a> 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
    <a href="#publishing-events">Publishing Events</a>.
</p>

<h3>Detaching Event Subscriptions</h3>

```
// 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|*');
```

<p>
    The preferred method of detaching subscriptions is to use the
    <a href="{{apiDocs}}/classes/EventHandle.html">EventHandle</a> that is
    returned from the subscription methods.  Alternately you can use the
    <a href="{{apiDocs}}/classes/EventTarget.html#method_detach">`detach` or
    `detachAll` methods</a> which work as described in the
    <a href="../event/index.html#detach-methods">Event user guide</a>.
</p>

<h3 id="extend-event-target">Extending EventTarget</h3>

<p>Add the `EventTarget` APIs onto any class using `Y.augment()`.</p>

```
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"
```

<p>
    `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.
</p>

<p>
    `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`.
</p>

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

<h2>Publishing Events</h2>

<p>
    Some custom event <a href="#configs">configurations can be defaulted</a>
    from class configuration, but others need to be specified on a per-event
    basis.  Use the `publish` method to do this.
</p>

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

<h3 id="facade">Event Facades</h3>

<p>
    The most common configuration for custom events is `emitFacade`.  This is
    because with the event facades comes a lot of additional functionality,
    such as <a href="#defaultFn">preventable default behaviors</a> and <a
    href="#bubbling">bubbling</a>.
</p>

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

<p>
    Event facades mirror the event objects
    <a href="../event/index.html#facade-properties">you're familiar with from
    the DOM</a>.  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.
</p>

```
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!
    }
});
```

<p>
    `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.
</p>

```
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 });
    },
    ...
});
```

<h3 id="once">`fireOnce` Events</h3>

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

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

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

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

<p>
    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.
</p>

```
// 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 = {};
```

<h3 id="bubbling">Bubbling Events</h3>

<p>
    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`.
</p>

```

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!"
```


<h3 id="prefix">Event Prefixes</h3>

<p>
    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'`.
</p>

<p>
    Taking the <a href="#bubbling">code snippet above</a>, configuring a default
    `prefix` while augmenting the classes will allow for subscription to
    only `LeafNode` updates.
</p>
```
// 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)
```

<p>
    Subscribing with prefixes is similar to
    <a href="../event/delegation.html">using DOM event delegation</a>, though it
    is done using `on()` rather than `delegate()`.
</p>

<p>
    Optionally, you can omit the prefix when subscribing on the object that
    fires the event.
</p>

```
// 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
});
```

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

```
// 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": ...
    }
});
```

<h3 id="defaultFn">Adding Default Behavior</h3>

<p>
    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
    <a href="http://en.wikipedia.org/wiki/Create,_read,_update_and_delete">CRUD</a>
    operations that you want to expose to other objects in your system to
    prevent, alter, or enhance.
</p>

<p>
    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`.
</p>

```
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
```

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

```
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
```

<h3 id="broadcast">Broadcasting Events to Y or Between YUI instances</h3>

<p>
    Event broadcasting is very similar to bubbling, but with some important
    distinctions:
</p>

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

<p>
    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`).
</p>

```
// 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" });
```

<p>
    `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.
</p>

```
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
        });
    });
});
```
<button id="status" class="yui3-button">Come in, Lunar Shuttle</button>
<button id="report" class="yui3-button">Can you read me, Alpha Beta?</button>

<script>
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
        });
    });
});
</script>

<!--h3>Monitoring Events</h3>

<p>TODO</p-->


<h3 id="configs">Available Event Configurations and Defaults</h3>

<p>
    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.
</p>

<table>
<thead>
    <tr>
        <th>Configuration</th>
        <th>Description</th>
        <th>Default</th>
        <th>Class Configurable?</th>
    </tr>
</thead>
<tbody>
    <tr>
        <td>`prefix`</td>
        <td>
            `e.type` will always include the configured prefix.
            <a href="#prefix">Details above</a>.
        </td>
        <td>(empty)</td>
        <td>YES</td>
    </tr>
    <tr>
        <td>`context`</td>
        <td>
            The default `this` object to execute callbacks with. Rarely set.
        </td>
        <td>The instance</td>
        <td>YES</td>
    </tr>
    <tr>
        <td>`emitFacade`</td>
        <td>
            If `true`, sends event facades to callbacks, allows bubbling and
            default functions, etc.  This is commonly set to true for a class.
            <a href="#facade">Details above</a>.
        </td>
        <td>`false`</td>
        <td>YES</td>
    </tr>
    <tr>
        <td>`fireOnce`</td>
        <td>
            If `true`, events will only fire once. Subscriptions made after
            firing will be immediately executed.
            <a href="#once">Details above</a>.
        </td>
        <td>`false`</td>
        <td>YES</td>
    </tr>
    <!--tr>
        <td>`monitored`</td>
        <td>
            Allows you to subscribe to the event lifecycle moments (publish,
            fire, and subscribe) as separate events.
            <a href="#monitor">Details above</a>.
        </td>
        <td>`false`</td>
        <td>YES</td>
    </tr-->
    <tr>
        <td>`broadcast`</td>
        <td>
            <a href="#broadcast">Details above</a>. Fire the event from:
            <ul>
                <li>`0` - Only the EventTarget instance</li>
                <li>`1` - The EventTarget and the YUI instance (`Y`)</li>
                <li>`2` - The EventTarget, `Y`, and `Y.Global`</li>
            </ul>
        </td>
        <td>0</td>
        <td>YES</td>
    </tr>
    <tr>
        <td>`bubbles`</td>
        <td>
            For events configured to `emitFacade` allow bubbling events to
            other `EventTarget`s.
        </td>
        <td>`true`</td>
        <td>YES</td>
    </tr>
    <tr>
        <td>`defaultFn`</td>
        <td>
            Behavior associated with the event. Usually this is preventable
            (see `preventable` below).  <a href="#defaultFn">Details above</a>.
        </td>
        <td>(none)</td>
        <td>&nbsp;</td>
    </tr>
    <tr>
        <td>`preventable`</td>
        <td>
            If set to `false`, `e.preventDefault()` will not disable execution
            of the event's `defaultFn`.
        </td>
        <td>`true`</td>
        <td>&nbsp;</td>
    </tr>
    <tr>
        <td>`preventedFn`</td>
        <td>
            <p>
                Behavior associated with the event when `e.preventDefault()` is
                called from a subscriber.  Use this function to reset partially
                applied transactional state.
            </p>
            <p>Incompatible with `preventable: false`.</p>
        </td>
        <td>(none)</td>
        <td>&nbsp;</td>
    </tr>
    <tr>
        <td>`stoppedFn`</td>
        <td>
            Behavior associated with the event when `e.stopPropagation()` is
            called from a subscriber.  Seldom used.
        </td>
        <td>(none)</td>
        <td>&nbsp;</td>
    </tr>
    <tr>
        <td>`async`</td>
        <td>
            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).
        </td>
        <td>false</td>
        <td>&nbsp;</td>
    </tr>
</tbody>
</table>

<h3 id="after">The "after" phase</h3>

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

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

<p>
    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` <em>nor the `after()`
    subscribers</em> will be executed.  If an `after()` subscription is
    executed, you know that the `defaultFn` did as well.
</p>

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

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

<h2>Event Lifecycle</h2>

<p>The order of operations when firing an event is as follows:</p>

<h3 id="simple-event-lifecycle">Simple Events (no facade)</h3>

<ol>
    <li>`on()` subscribers are executed</li>
    <li>`after()` subscribers are executed</li>
    <li>`Y.on()` broadcast subscribers are executed.</li>
    <li>`Y.after()` broadcast subscribers are executed.</li>
    <li>`Y.Global.on()` broadcast subscribers are executed.</li>
    <li>`Y.Global.after()` broadcast subscribers are executed.</li>
</ol>

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

<h3 id="complex-event-lifecycle">Complex Events (with facade)</h3>

<ol>
    <li>`on()` subscribers are executed</li>
    <li>
        `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.
    </li>
    <li>
        If the event was prevented, any configured `preventedFn` will execute.
    </li>
    <li>If not prevented, any configured `defaultFn` will execute.</li>
    <li>If bubbling was stopped, any configured `stoppedFn` will execute.</li>
    <li>`Y.on()` broadcast subscribers are executed.</li>
    <li>`Y.after()` broadcast subscribers are executed.</li>
    <li>`Y.Global.on()` broadcast subscribers are executed.</li>
    <li>`Y.Global.after()` broadcast subscribers are executed.</li>
    <li>`after()` subscribers are executed.</li>
    <li>
        `after()` subscribers for each bubble target and their respective
        targets are executed.
    </li>
</ol>

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

<dl>
    <dt>`e.preventDefault()`</dt>
        <dd>
            <ol>
                <li>The `defaultFn` will not be executed</li>
                <li>The `preventedFn` will execute</li>
                <li>No `after()` subscriptions will be executed</li>
            </ol>
        </dd>
    <dt>`e.stopPropagation()`</dt>
        <dd>
            <ol>
                <li>The remainder of subscribers at this `EventTarget` <strong>WILL</strong> execute</li>
                <li>No bubble targets of this `EventTarget` will be notified</li>
                <li>The `stoppedFn` will execute</li>
                <li>The `defaultFn` and `after()` subscribers will execute</li>
            </ol>
        </dd>
    <dt>`e.stopImmediatePropagation()`</dt>
        <dd>
            Same as `e.stopPropagation()` except no more subscribers at this
            `EventTarget` will execute.
        </dd>
    <dt>`e.halt()`</dt>
        <dd>
            Same as `e.preventDefault()` plus `e.stopPropagation()`.
        </dd>
    <dt>`e.halt(true)`</dt>
        <dd>
            Same as `e.preventDefault()` plus `e.stopImmediatePropagation()`.
        </dd>
    <dt>`return false`</dt>
        <dd>Same as `e.halt(true)`.  Not recommended.  Use the API methods.</dd>
</dl>

<!--h2 class="no-toc">Subscribing to Object Methods with `Y.Do.*`</h2>

<h3 class="no-toc">Before and After</h3>

<h3 class="no-toc">Altering the Wrapped Method Behavior</h3>

<h3 class="no-toc">`EventTarget` API methods</h3>

<p>
    TODO
</p-->
