Infusion Event System

Infusion events are one of the most basic and widely used facilities in the framework. Almost every Infusion component exposes one or more events and/or listeners to events fired by itself or other components. A component opts in to the event system by mentioning fluid.component or some other grade derived from it such as fluid.viewComponent in its list of parent Component Grades.

An Infusion event (or more precisely, a particular firing of an Infusion event) represents

1. a particular instant in time (corresponding to when it is fired)
2. a particular array of JavaScript objects, forming its arguments or signature (sometimes called its payload).

An Infusion event can be fired at any time, and any collection of JavaScript objects can be supplied as its payload - that is, any JavaScript function call can serve as the initiation point where an event is fired.

Rather than firing and listening to events in raw JavaScript code, we recommend using the framework to encode firing and listening to events declaratively. We'll show how this syntax works first, and then later show procedural equivalents and more details.

Declaring an event on a component

A top-level options block named events is supported on every component derived from the core framework grade fluid.component. The keys in this block represent the event name, and the values represent the type of the event. Currently only two event types are supported, represented by the values null and preventable (this second value is almost never used). This table explains the meaning of the two values:

For every such entry in the events section of a component's options, the framework will construct a corresponding event firer with the same name in the events section of the constructed component. The most common use of an event firer is to call its member named fire with some set of arguments. Here is a simple, self-contained example:

fluid.defaults("examples.eventedComponent", {
    gradeNames: ["fluid.component"],
    events: {
        myEvent: null
    }
});

var myComponent = examples.eventedComponent();

myComponent.myEvent.fire(97, false);
// firer of event can supply whatever arguments they like,
// but these should conform to some agreed signature

As a real-world example, here is a block of configuration taken from Infusion's Reorderer component:

{
    events: {
        onShowKeyboardDropWarning: null,
        onSelect: null,
        onBeginMove: "preventable",
        onMove: null,
        afterMove: null,
        onHover: null, // item, state
        onRefresh: null
    }
}

This indicates that the Reorderer supports 6 events of the listed types, of which one, onBeginMove represents a preventable event - a listener may countermand the beginMove effect by returning true when the event is received. The implementor has helpfully annotated the signature which is fired by the onHover event - in general since JavaScript doesn't have a type system this has to be done informally. If you have a Reorderer instance stored in a variable named thatReorderer, for example, the onSelect event could be fired with a call like thatReorderer.onSelect.fire(item). The fire method of the event is a plain function which can be detached and treated as a general callback - for example, this sequence is valid:

var myCallback = myComponent.myEvent.fire;
myCallback(42, true);

In general you shouldn't fire any of a component's events unless invited to by its documentation - you may disrupt its state. However, registering listeners to a component's events is always safe.

Registering a listener to an event

Both as part of defaults, and also as supplied instantiation options, an Infusion component can accept a structure named listeners. In the simplest form, the keys of the listeners structure are taken from the set of events present in the component's Grade, and the values are either single listener specifications or arrays of listener specifications. A listener specification can take a number of forms - either being written as a simple String or Function, or as a full JSON object. The standard way of declaring a listener using Infusion's IoC system is to supply the name of a global function using the member funcName or to supply a reference to a function handle (usually an Invoker) somewhere in the component tree using the member func. If your listener would like to receive different arguments than the ones that the event was fired with, you can supply references to these using the member args. You can consult the page Event injection and boiling for the use of these more complex listener specifications. If you are happy with the existing arguments you can write a simple definition as a String or Function holding the value that would have been written in func/funcName. Here is a simple example of a listener definition, expanding our example from earlier:

examples.myListener = function (number, condition) {
    console.log("Event listener received number " + number + " and condition " + condition);
};

fluid.defaults("examples.eventedComponent", {
    gradeNames: ["fluid.component"],
    events: {
        myEvent: null
    },
    listeners: {
        myEvent: "examples.myListener"
    }
});

var myComponent = examples.eventedComponent();
myComponent.myEvent.fire(97, false);
// console logs "Event listener received number 97 and condition false"

You should use the String forms for listener specifications rather than raw Function objects. This enables your component's options to consist of standard JSON which is more easily stored and manipulated.

Namespaced listeners

There are two more complex options for the keys held by listeners - firstly, the listener name may be qualified with a namespace following a period character . - this follows the jQuery convention for namespaced events. For example, the key myEvent.myNamespace could be used above - this still attaches the listener to exactly the same event, the one named myEvent, but in this case the framework will make sure that only one listener will ever be attached to myEvent which mentions the same namespace myNamespace. Event namespaces are useful in order to specify functional roles for listeners, and to insist that only one listener can ever fill this role at the same time. It is a good idea to namespace your listeners whenever you can.

Here is an example again from Infusion's Reorderer component:

{
    listeners: {
        "onShowKeyboardDropWarning.setPosition": "fluid.moduleLayout.defaultOnShowKeyboardDropWarning"
    }
}

represents that the function with the global name fluid.moduleLayout.defaultOnShowKeyboardDropWarning should be attached as a listener to the event onShowKeyboardDropWarning under the namespace setPosition. setPosition is a name which encodes the purpose of the listener for readers of the component - it is the one to be notified whenever the position of an item changes. Any integrator of this component can override exactly this listener by supplying the same namespace in their own listener specification.

If one namespaced listener displaces another, the displaced listener is pushed onto a hidden stack of inactive listeners for that namespace. If the listener which displaces it is removed (for example, through the component which registered it being destroyed), the displaced listener will be popped back off the stack and become active again.

Listeners to events held elsewhere

Secondly, rather than a simple string, the key in a listeners structure can hold any IoC Reference which resolves to an event anywhere in the component tree - that is, even one belonging to a different component. In this case the listener on the right hand side will be attached to that event rather than one of this component's own events. The framework will make sure to automatically deregister the listener when this component is destroyed. Many more complex cases are possible, including the wholesale injection of events from one component to another, and the creation of new events derived from existing ones. You can consult the page Event injection and boiling for more details.

Full listener record form

In many cases, the short form above where the listener declaration just consists of a single String doesn't provide enough space for all the directives that you want to supply for how the listener is notified. In this case, you can use the long form record, where the listener record takes the form of a JSON structure with the following members (the key for this record stays the same as above):

Using events and listeners procedurally

Traditional procedural APIs corresponding to all the above declarations exist. However, these APIs, with the exception of the fire method of an event, are not encouraged for typical users of the framework. For managing the lifecycle of event firers, as well as adding and removing listeners, you are recommended to use the declarative configuration above.

Using an event firer procedurally

Once an event firer is constructed, it can be called with the following methods (these form a stable API - but only the fire method is recommended for typical users):

Constructing an event firer procedurally

The Fluid event system is operated by instances of an event firer which are created by a call to fluid.event.makeEventFirer(). It is recommended that users don't construct event firers by hand but instead rely on the framework's facilities for automatically constructing these given event blocks in component options. The signature of fluid.event.makeEventFirer is not a stable API:

var myFirer = fluid.makeEventFirer(options);