@title Concepts: Event Delegation
@group concepts

Explains Javelin event delegation with @{class:JX.Stratcom}.

= Overview =

Javelin provides event delegation as a core feature of the library, orchestrated
with @{class:JX.Stratcom}. Event delegation means that the library listens to
every event in the document and then delegates them to handlers you install,
as opposed to you installing handlers on specific nodes for specific events you
are interested in.

Event delegation can greatly simplify event handling for many types of user
interactions, and can also be used to do more traditional event listening for
specific events on specific nodes. The goal is to provide a strictly more
powerful event model, which gives you a very general delegation toolkit for
interactions where delegation makes sense but refines into a very specific
toolkit when you need less generality.

Beyond DOM events, Stratcom provides a general event delegation framework which
Javelin classes integrate with.

= Event Delegation Basics =

Javelin routes events based on the **event type** and **sigil set**.

The **event type** is a string with a general description of the event, and
includes the DOM event types like 'click' and 'keydown'. It may also be a
class-specific event (JX.Duck might emit 'quack').

The **sigil set** is a list of sigils (see @{article:Concepts: Sigils and
Metadata}) for the event. If the event is a DOM event, Javelin builds the
sigil set by walking up the DOM tree from the event target and collecting all
the sigils on nodes (it also collects some other data into the sigil set,
see "Magic Sigils" below). If the event is a class event, Javelin walks up
the class hierarchy collecting class names. If the event is a raw event invoked
with @{method:JX.Stratcom.invoke}, the caller must provide the sigil set.

When you install an event handler, you specify the event type and the (possibly
empty) sigil set you want to listen for.

When an event is invoked, Javelin finds all the listeners for that event type
and compares their sigil sets with the event's sigil set. If all of the sigils
in a callback's sigil set appear in the event's sigil set, the callback is
invoked. Generally, this mechanism allows you to ignore events you are not
interested in.

= Listening to General DOM Events =

You can listen to general DOM events with @{method:JX.Stratcom.listen}. This
method allows you to select which types of events you want to receive, and
which elements must be involved in the event:

  lang=js
  JX.Stratcom.listen(
    'click',          // Node
    null,             // Sigil set
    function(e) {     // Callback
      // ...
    });

This listens to all clicks on all elements in the document. More likely, you
want to listen only to some clicks. You accomplish this by annotating your
document with Javelin sigils (see @{article:Concepts: Sigils and Metadata})
and specifying a set of sigils which must be present between the target node
and the document root. For instance, your document might look like this:

  lang=html
  Important!
  Some Other Link

If you install a handler like the one above, you'll get callbacks for every
click, no matter which link it is or even if it's on the document itself. If
you just want clicks on the "Important!" link, you can install a more specific
handler:

  lang=js
  JX.Stratcom.listen(
    'click',
    'important',    // Listen only to this sigil set
    function(e) {
      // ...
    });

Now you will receive a callback only when the event target or some ancestor of
it has the "important" sigil, i.e., only when the user clicks on the
"Important!" link. You can also specify multiple sigils; ancestors must have
all of the sigils for you to get a callback:

  lang=js
  JX.Stratcom.listen(
    'click',
    ['menu', 'item'], // Listen only for clicks on menu items.
    function(e) {
      // ...
    });

= Listening to Specific DOM Events =

You can listen to specific DOM events with @{method:JX.DOM.listen}. This method
works like @{method:JX.Stratcom.listen} but takes a DOM node as the first
parameter and listens only for events within that node:

  lang=js
  JX.DOM.listen(
    node,             // Node
    'click',          // Event type(s)
    null,             // Sigil set
    function(e) {     // Callback
      // ...
    });

This is similar to setting `node.onclick` or `node.addEventListener`, but
uses the Javelin delegation core. You can also provide a sigil set, which works
just like @{method:JX.Stratcom.listen} general events. This is useful if your
node is a container, like a `<div />`, with a lot of stuff in it.

= DOM Event Flow =

Events dispatched within the DOM propagate using a bubbling method, as detailed
by http://www.w3.org/TR/DOM-Level-3-Events/#event-flow
Listeners assigned using @{method:JX.Stratcom.listen} or @{method:JX.DOM.listen}
are called in order of the deepest node in the DOM of the nodes which match the
sigil set listened to.

In this example the second listener, subscribed to 'inner', will be called
before the listener subscribed to 'outer'

  lang=html

      Click Me

= Listening to Class Events =

Beyond DOM events, you can also listen to class events. Every class installed
by Javelin has static and instance methods called `listen` (see
@{method:JX.Base.listen}). The static method allows you to listen for all events
emitted by every instance of a class and its descendants:

  lang=js
  JX.Animal.listen(
    'meow',
    function(e) {
      // Listen for ANY 'meow' from any JX.Animal instance or instance which
      // extends JX.Animal.
    });

The instance method allows you to listen for all events emitted by that
specific instance:

  lang=js
  var cat = new JX.Cat();
  cat.listen(
    'meow',
    function(e) {
      // Listen for 'meow' from only this cat.
    });

= Magic Sigils =

Javelin implements general delegation by building and comparing sigil sets. Some
of these sigils are not DOM sigils, but derived from other things:

  - `id:*` ID sigils are generated when an examined node has an "id" property.
  - `obj:*` Object sigils are generated when an event affects a class
    instance.
  - `class:*` Class sigils are generated while walking an affected instance's
    class chain.
  - `tag:*` Tag sigils are generated by examining the tag names of DOM nodes.

For instance, you can listen to all clicks on `<a />` tags in a document like
this:

  lang=js
  JX.Stratcom.listen(
    'click',
    'tag:a',
    function(e) {
      // ...
    });
