进行OO javascript回调时的范围

Question

Folks - I'm trying to learn how to write OO Javascript, I come from as3 OO background... issue I'm having is with passing a class' method as a callback to another class...

in the example below, i create an instance of AppController class, and in it, i create an instance of ConnectionMonitor class. I pass one of AppController's methods to be called back by ConnectionMonitor. The calling back works fine, but seems that what's inside the callback function looses the scope of the class its in (AppController )...

any thoughts?

//in the HTML
<script>
    $(document).ready( function(){
        new AppContoller().init();
    });
</script>


//in the js file

//AppController Class
var AppContoller = function(){
    this.body = $("body");

    this.init = function(){
        this.connection = new ConnectionMonitor();
        this.connection.detectInitialConnection( this.initialConnectionDetected );
    }

    //callback function I pass
    this.initialConnectionDetected = function(bool){
        if(bool){
            trace("app connected? "+bool); // logs - "app connected? true"
            this.showOnlineSwf();  //thows error - "Object [object DOMWindow] has no method 'showOnline' "
        }
        else{

        }
    }

    this.showOnlineSwf = function(){
        trace("i'm online");

    }

}

//ConnectionMonitor Class
var ConnectionMonitor = function()
{   
    this.detectInitialConnection = function(callbackFunction){
        setTimeout(callbackFunction, 1000, [true]);
    }
}




function trace(arg){
    console.log(arg.toString());
}

Answer 1

Modify init to bind the callback to the original context:

this.init = function() {
    this.connection.detectInitialConnection(
        this.initialConnectionDetected.bind(this));
}

Unfortunately, bind is not supported in all browsers , but you can capture the current value of this in a closure to achieve a similar effect:

this.init = function() {
    var that = this;
    this.connection.detectInitialConnection(function(detected) {
        that.initialConnectionDetected(detected);
    });
}

Yet another approach is to make detectInitialConnection capable of handling an optional context parameter:

this.detectInitialConnection = function(callbackFunction, _this){
    setTimeout(function() {
        callbackFunction.apply(_this || this, arguments);
    }, 1000, [true]);
}

You'd then call it like this:

this.init = function() {
    this.connection.detectInitialConnection(
        this.initialConnectionDetected, this);
}

The point in each of these examples is to retain a reference to the value of this from the context in which detectInitialConnection was called.

Answer 2

setTimeout will run the passed function bound to window . You can override this with ES5's bind :

this.connection.detectInitialConnection( this.initialConnectionDetected.bind(this) );

This doesn't work in older browsers, but the provided MDN link has a workaround.

Answer 3

this is tricky in javascript especially as it relates to constructors and passing functions around. If you do this it will work:

//AppController Class
var AppContoller = function(){
this.body = $("body");
var self = this;
this.init = function(){
    this.connection = new ConnectionMonitor();
    this.connection.detectInitialConnection( this.initialConnectionDetected );
}

//callback function I pass
this.initialConnectionDetected = function(bool){
    if(bool){
        trace("app connected? "+bool); // logs - "app connected? true"
        self.showOnlineSwf();
    }
    else{

    }
}

this.showOnlineSwf = function(){
    trace("i'm online");

}

}

Answer 4

this.initialConnectionDetected will pass the function, but without the information that it should be executed on the instance.

Calling this.foo() will pass this information, but only when called directly, not when passed around and called later (which is what you're doing).

You have to bind this information (ie the this value, which is the instance) using eg .bind :

this.connection.detectInitialConnection(this.initialConnectionDetected.bind(this));

Note that .bind is not available in older browsers, but there are shims available which mimic the behaviour of it.

Answer 5

The other answers are correct, but nobody seems to mention that Function.prototype.bind is not supported in all browsers . To make it work, you'll want to include Kris Kowal's es5-shim library .