Registering JavaScript object methods as callbacks(注册一个对像方法为回调函数[解决回调函数的上下文问题])

Registering JavaScript object methods as callbacks

November 27, 2007
Tags: ajax

The registration of callback functions is very common in JavaScript web programming, for example to attach user interface event handlers (such as onclick), or to provide a function to handle an XHR response. Registering an object method as a callback function is not entirely straightforward, but there are a number of approaches that we can use.

Let's say we've got a constructor, an object, and a function that registers a callback:

function MyObject(val){
    this.val = val;
}
MyObject.prototype.alertVal = function(){
    alert(this.val);
}

var obj = new MyObject(8);

function register(callback){
    // some time later...
    callback();
}

The constructor stores its single argument, and the alertVal() method alerts it. The simple register() function takes a function and calls it. In a real situation the behaviour here would be much more interesting but this will do for illustration.

Why we don't want to just pass obj.alertVal to register()

Object methods are first-class functions in JavaScript and we could pass obj.alertVal to register() -- but it isn't quite what we want. Let's see what happens:

register(obj.alertVal);
// inside register(), callback === obj.alertVal
callback()
// inside MyObject.prototype.alertVal
alert(this.val);
// because callback() was not called on an object,
// this === the JavaScript global object and not obj;
// this.val is the global variable val, not obj.val

When a function is called as a method on an object (obj.alertVal()), "this" is bound to the object that it is called on (obj). And when a function is called without an object (func()), "this" is bound to the JavaScript global object (window in web browsers.) When we passed obj.alertVal to register() we were passing a reference to the function bound to obj.alertVal, but no reference to the object obj.

So, we need to bind our method to the object.

Closure with an anonymous function

In JavaScript, whenever a function is defined within another one a closure is created [JavaScript Closures for Dummies ] [JavaScript Closures ]. A closure remembers the variable bindings that were in scope when the function was created. These bindings are then available whenever the function is called. We can bind our method to our object instance with the following:

register(function(){obj.alertVal()});

Whenever the anonymous function is called, "obj" will be bound to the value that it had when the function was created. Which is exactly what we want.

(If we execute the above code outside a function it will behave differently. No closure will be created, instead, the current value of the global variable "obj" will be used whenever the anonymous function is called, not the value at function definition.)

If we want to register a method on the object currently bound to "this", we need to take an extra step:

var obj = this;
register(function(){obj.alertVal()});

If we don't explicitly bind "this" to a named variable (obj) and instead use register(function(){this.alertVal()}) we will lose our object reference. "this" will be bound to the JavaScript global object whenever the anonymous function is called.

Build a generic closure maker

Instead of building a closure each time we want to register a method as a callback, we could write a utility function to do it for us. For example:

function bind(toObject, methodName){
    return function(){toObject[methodName]()}
}

With such a function we can then register our method with:

register(bind(obj, "alertVal"));

Dojo (dojo.hitch()) and Prototype (bind() ) both have such utility functions (that also allow you to provide arguments to pass to the method when called, something that our "bind" doesn't do.)

If we want to register a method of "this", we don't need to explicitly bind it (as we did above) before calling "bind" -- the function call does the binding for us. register(bind(this, "alertVal")) works as expected.

Alter the register function to take the object too

If we changed our register function to:

function register(anObject, methodName){
    // some time later...
    anObject[methodName]();
}

We could register our call with:

register(obj, "alertVal");

dojo.connect and YUI's YAHOO.util.Event.addListener() [YUI Event Utility ] [YAHOO.util.Event API docs ] both include this binding style in their API.

Bind the method to the object at construction

We could bind our method to the object (or instance variables as shown here) in the constructor function:

function MyObject(val){
    this.alertVal = function(){
        alert(val);
    }
}

We could then register obj.alertVal directly as "val" is already bound:

obj = new MyObject(8);
register(obj.alertVal);

Douglas Crockford writes about this programming style in Private Members in JavaScript.

Circular references and memory leaks

Whichever method you use you need to be careful about avoiding circular references when registering event handlers (such as onclick) on document objects. For example, if we register an object method as an event handler on an element, such that the method is bound to the object, and the object has a reference back to the element, then we have a circular reference, and a potential leak (in this case a solution would be to have the object store the element's id rather than store a reference to the element object itself.) Here are some articles that discuss ways to cause memory leaks and ways to avoid them:

• Memory leak patterns in JavaScript by Abhijeet Bhattacharya and Kiran Shivarama Sundar (IBM developerWorks)
• Understanding and Solving Internet Explorer Leak Patterns by Justin Rogers (Microsoft)
• JavaScript Closures by Richard Cornford

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