5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith<link type="text/css" rel="stylesheet" href="{{yuiBuildUrl}}/cssbutton/cssbutton-min.css">

55ce3362a93ce4e133a3b100a080360709f700a4Luke Smith<div class="intro">

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith <p>

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith The YUI Custom Event System enables you to define and use events beyond

ad2670c2a72f6612b923c6f67823a561cd5ee9beLuke Smith those available in the DOM &mdash; events that are specific to and of

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith interest in your own application. Custom Events are designed to work

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith much like DOM events. They can bubble, pass event facades, have their

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith propagation and default behaviors suppressed, etc.

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith </p>

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith <p>

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith The APIs for working with custom events are provided by the

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith `EventTarget` class. All other infrastructure classes extend

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith `EventTarget`, but if you just need the custom event APIs, you can

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith `extend` or `augment` your classes with `EventTarget` directly.

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith </p>

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith <!--p>

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith Bundled with `EventTarget` are <a

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith href="http://en.wikipedia.org/wiki/Aspect_oriented_programming">Aspect

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith Oriented Programming</a> methods that allow you to subscribe to the

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith execution of object methods, and

5abe4e6ccb00e9414eb123e8d3d7e1a8d2a7c317Luke Smith 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>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>

Notify the YUI instance (`broadcast: 1`) or the YUI instance

<em>and</em> `Y.Global` with the event.

<a href="#broadcast">Details above</a>.

</td>

<td>0 (instance only)</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-->