d3-dispatch

Register named callbacks and call them with arguments. Dispatching is a convenient mechanism for separating concerns with loosely-coupled code. A variety of D3 components, such as d3-xhr, use d3-dispatch to emit events. Think of this like Node’s EventEmitter, except every listener has a well-defined name so it’s easy to remove or replace them.

Changes from D3 3.x:

• It is now an error to attempt to register a callback type that: conflicts with a built-in property on all objects, such as __proto__ or hasOwnProperty; conflicts with a built-in method on dispatch (e.g., on); conflicts with another type on the same dispatch (e.g., dispatch("foo", "foo")); is the empty string.

• The exposed dispatch.type field is now strictly a method for invoking callbacks. Use dispatch.on(type, …) to get or set callbacks, rather than dispatch[type].on(…).

• The instanceof operator now works as expected with dispatch objects.

# dispatch(types…)

Creates a new dispatch object for the specified types. Each type is a string representing the name of a callback type, such as "zoom" or "change"; for each type, a method is exposed on the returned dispatch object for invoking the callbacks of that type.

For example, if you create a dispatch for "start" and "end" callbacks:

var dispatcher = dispatch("start", "end");

You can then register callbacks for the different types using dispatch.on:

dispatcher.on("start", callback1);
dispatcher.on("start.foo", callback2);
dispatcher.on("end", callback3);

Lastly, you can invoke any "start" callbacks using dispatch.type:

dispatcher.start("pass arguments to callbacks here");

# dispatch.on(type[, callback])

Adds, removes or gets an callback of the specified type.

The type is a string, such as "start" or "end". To register multiple callbacks for the same type, the type may be followed by an optional namespace, such as "start.foo" and "start.bar". You can remove all registered callbacks for a given namespace by saying dispatch.on(".foo", null).

If a callback is specified, it is registered for the specified type. If a callback was already registered for the same type, the existing callback is removed before the new callback is added. If callback is not specified, returns the current callback for the specified type, if any. The specified callback is invoked with the context and arguments specified by the caller; see dispatch.type.

# dispatch.type(arguments…)

The type method (such as dispatch.start for the "start" type) invokes each registered callback for the specified type, passing the callback the specified arguments. The this context will be used as the context of the registered callbacks.

For example, if you wanted to dispatch your "custom" callbacks after receiving a native "click" event, while preserving the current this context and arguments, you could say:

selection.on("click", function() {
  dispatch.custom.apply(this, arguments);
});

You can pass whatever arguments you want to callbacks; most commonly, you might create an object that represents an event, or pass the current datum (d) and index (i). See function.call and function.apply for further information.