<div class="intro">
<p>Synthetic events are usually named abstractions that bind to existing
DOM events to monitor user actions for specific patterns. However, at
heart they are no more than a set of callbacks executed in response to
various triggering methods in the DOM event system.</p>
<p>You can do all sorts of things with synthetic events, including:</p>
<ul>
<li>
redefine native DOM events that behave inconsistently across
browsers (e.g. <a href="focus.html">`focus` and `blur`</a>)
</li>
<li>
provide abstract events that attach to different DOM events based on
the environment (e.g. <a href="touch.html#move">`gesturemovestart`
and family</a>)
</li>
<li>
create events with different subscription signatures (e.g.
<a href="hover.html">`hover`</a>)
</li>
<li>
create configurable events that only execute subscribers when
criteria passed during subscription are met (e.g.
<a href="touch.html#flick">`flick`</a> or
<a href="key.html">`key`</a>)
</li>
<li>
create events that encapsulate common UX patterns (e.g.
<a href="outside.html">`clickoutside`</a>)
</li>
<li>
create fun little easter eggs (e.g. <a
href="http://yuilibrary.com/gallery/show/event-konami">`konami`</a>)
</li>
<li>and more...</li>
</ul>
</div>
<h2>The hooks</h2>
<p>Synthetic events hook into the subscription binding and unbinding methods. Specifically:</p>
<ol>
<li>`node.on("eventName", ...)`, `Y.on("eventName", ...)`, <a href="index.html#one-time-subscriptions">and family</a></li>
<li>`node.delegate("eventName", ...)` or `Y.delegate("eventName", ...)`</li>
<li>`node.detach(...)` or `subscription.detach()`</li>
</ol>
<p>With the exception of a separate `detachDelegate()` method, the names used when defining synthetic events are the same as these basic methods.</p>
```
Y.Event.define("tripleclick", {
on: function (node, subscription, notifier) {
// called in response to individual subscriptions
},
delegate: function (node, subscription, notifier, filter) {
// called in response to delegate subscriptions
},
detach: function (node, subscription, notifier) {
// called when individual subscriptions are detached in any way
},
detachDelegate: function (node, subscription, notifier) {
// called when delegate subscriptions are detached in any way
}
});
```
<h2>Subscriptions and Notifiers</h2>
<p>In addition to the subscribing Node, each method receives a
<em>subscription</em> and a <em>notifier</em>. Use the <em>subscription</em>
to store event handles or other data that may be needed by another method. Use
<em>`notifier.fire(e)`</em> to dispatch the event to the callbacks that were
bound to it.</p>
```
Y.Event.define("tripleclick", {
on: function (node, subscription, notifier) {
var count = 0;
subscription._handle = node.on("click", function (e) {
if (++count === 3) {
// Call notifier.fire(e) to execute subscribers.
// Pass the triggering event facade to fire()
notifier.fire(e);
} else {
...
}
});
},
detach: function (node, subscription, notifier) {
},
delegate: function (node, subscription, notifier, filter) { ... },
detachDelegate: function (node, subscription, notifier) { ... }
});
```
<p>Subscribers to the synthetic event should receive a `DOMEventFacade`. The
easiest way to provide one is to pass the triggering DOM event's facade to
`notifier.fire(e)`. The facade's `e.type` will be updated to the name of the
synth. You will also have the opportunity to add extra data to the event
before dispatching to the subscription callbacks.</p>
```
Y.Event.define('multiclick', {
on: function (node, sub, notifier) {
var count = 0,
timer;
sub._handle = node.on('click', function (e) {
count++;
if (timer) {
timer.cancel();
}
timer = Y.later(200, null, function () {
e.clicks = count;
count = 0;
// subscribers will get e with e.type == 'multiclick'
// and extra property e.clicks
notifier.fire(e);
});
});
},
...
});
```
<h2>Delegation support</h2>
<p>The `delegate` function implementation takes an extra argument, the `filter` that was passed <code>node.delegate(type, callback, <em>HERE</em>)</code>. It's your responsibility to make sense of this filter for your event.</p>
<p>Typically, it is just passed along to a `node.delegate(...)` call against another event, deferring the filtration to the core `delegate()` method.</p>
```
Y.Event.define("tripleclick", {
on: function (node, subscription, notifier) { ... },
detach: function (node, subscription, notifier) { ... },
delegate: function (node, subscription, notifier, filter) {
var activeNode = null,
count = 0,
timer;
subscription._handle = node.delegate("click", function (e) {
if (timer) {
timer.cancel();
}
if (this !== activeNode) {
activeNode = this;
count = 0;
}
if (++count === 3) {
// Call notifier.fire(e) just as with `on`
notifier.fire(e);
} else {
timer = Y.later(300, null, function () {
count = 0;
});
}
}, filter); // filter is passed on to the underlying `delegate()` call
},
detachDelegate: function (node, subscription, notifier) {
}
});
```
<h2>Extra Arguments</h2>
<p>Supply a `processArgs` method in the event definition to support a custom
subscription signature. The method receives two arguments:</p>
<ol>
<li>an array of the subscription arguments for analysis</li>
<li>
a boolean `true` if the subscription is being made through
`delegate(...)`
</li>
</ol>
<p>If this method is supplied, it</p>
<ul>
<li>
<strong>MUST</strong> remove the extra arguments from the arg array
that is passed in, and
</li>
<li>
<strong>SHOULD</strong> return the extra data relevant to the
subscription.
</li>
</ul>
<p>The same `processArgs` method is used by both `on` and `delegate`, but there
are various signatures to account for. The easiest way to accept extra
arguments is to require them from index 3 in the argument list. It's also best
to limit the number of extra arguments to one and require an object literal to
allow for future changes.</p>
```
// for an event that takes one extra param
processArgs: function (args, isDelegate) {
var extra = args[3];
// remove the extra arguments from the array
args.splice(3,1);
return extra;
}
// for an event that takes three extra args
processArgs: function (args, isDelegate) {
return args.splice(3,3);
}
```
<p>Requiring extra params start at index 3 of the `args` array results in the
following subscription signatures:</p>
```
var extraConfig = { ... };
// Third argument for node.on() and node.delegate
node.on('extraArgEvent', callback, extraConfig, thisOverride, arg...);
node.delegate('extraArgEvent', callback, extraConfig, filter, thisOverride, arg...);
// Fourth argument for Y.on() and Y.delegate
Y.on('extraArgEvent', callback, targetSelector, extraConfig, thisOverride, arg...);
Y.delegate('extraArgEvent', callback, parentSelector, extraConfig, filter, thisOverride, arg...);
```
<p>For some custom signatures, the placement of the extra argument for
implementers using `Y.on()` or `Y.delegate()` may look awkward. Sometimes you
can support extras at other indexes if you can reliably tell that the argument
is not part of
<a href="index.html#binding-this-and-additional-callback-arguments">the extended
signature for `on(...)` or `delegate(...)`</a>. See the <a
href="{{apiDocs}}/files/event_js_hover.js.html">source for the "hover"
event</a> for an example of supporting multiple signatures.</p>
<p>The return value of `processArgs` is assigned to `subscription._extras` for the `on` and `delegate` definition methods.</p>
```
Y.Event.define('multiclick', {
processArgs: function (args, isDelegate) {
// The args list will look like this coming in:
// [ type, callback, node, (extras...), [filter,] thisObj, arg0...argN ]
return args.splice(3,1)[1] || {};
},
// Custom subscription signatures don't change the params of on/delegate
on: function (node, sub, notifier) {
var clicks = 0,
// data returned from processArgs is available at sub._extras
min = sub._extras.minClicks || 3,
max = sub._extras.maxClicks || 10,
timer;
sub._handle = node.on('click', function (e) {
if (timer) {
timer.cancel();
}
if (++clicks === max) {
e.clicks = clicks;
notifier.fire(e);
} else {
timer = Y.later(200, null, function () {
if (clicks > min) {
e.clicks = count;
notifier.fire(e);
}
count = 0;
});
}
});
},
...
});
```
<p>Usage of this synthetic event then expects a third argument as a
configuration object with `minClicks` and `maxClicks` properties.</p>
```
node.on('multiclick', obj.method, {
minClicks: 5,
maxClicks: 8
}, obj);
// extra args are supplied before the delegate filter
container.delegate('multiclick', doSomething, {
minClicks: 3,
maxClicks: 55
}, '.clickable');
```
<h2>A Tip to Make Your Synth Definition Smaller</h2>
<p>If the only difference between your `on` and `delegate` definitions is which method is used to bind to the supporting events, then you can propably get away with defining `delegate` and aliasing it to `on` (and so with `detach` and `detachDelegate`). See the
<a href="{{apiDocs}}/files/event_js_hover.js.html">source for the "hover"
event</a> for an example of this approach.</p>