<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) {
        subscription._handle.detach();
    },

    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) {
        subscription._handle.detach();
    }
});
```

<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>
