js uses function binding technology to change the scope of event handlers

js uses function binding technology to change the scope of event handlers_javascript tips

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Release： 2016-05-16 17:57:51

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The first, and most common, is to register the event directly in the html tag by specifying the HTML attribute with the same name as the event handler. The code is as follows:

Copy code The code is as follows:

function eventHandler() {
alert("The current scope is the input element itself");
}

The second way is to assign a function to an event handler attribute. In this way, the element object is first obtained. The general code is as follows:

Copy the code The code is as follows:

input对象的Click事件执行环境截图
One disadvantage of registering a click event handler for the input element through the above three methods is that the scope of this handler (this) is always within the input object. In object-oriented programming, you need to explicitly specify this in a specific scope. In order to change the scope of this, you have to use a binding function technology of js.
The so-called "bound function" is to create a function that can call another function with specified parameters in a specific environment. It can be used well with event handlers to maintain the function while passing the function as a variable. The scope of the function (also the execution environment of this). The definition form of the binding function is as follows:

Copy the code The code is as follows:

 
function bind( fn,scope) { 
return fn.apply(scope||this,arguments); 
} 

This binding function accepts two parameters, the first one needs to be executed Function, the second one is a specific execution environment and returns a function that calls the given function in the given scope, passing all parameters together. Using binding function technology and DOM2-level event handlers, you can register an event handler for an element that executes in a specific scope. The specific processing method is as follows:
First modify the previously defined registration event method as follows:

Copy the code The code is as follows:

 
function addHandler(obj, type, handler, scope) { 
function fn(event) { 
var evt = event ? event : window.event; 
evt.target = event.target || event.srcElement; 
return handler.apply(scope || this,arguments); 
} 
obj.eventHash = obj.eventHash || {};//This is required The object that registers the event handler defines a hash object that saves the event, and saves the event handler and scope in the queue of the event type 
(obj.eventHash [type] = obj.eventHash [type] || [ ]).push({ "name": type, "handler": handler, "fn": fn, "scope": scope }); 
if (obj.addEventListener) { 
obj.addEventListener(type , fn, false); 
} else if (obj.attachEvent) { 
obj.attachEvent("on" type, fn); 
} else { 
obj["on" type] = fn; 
} 
} 

Using the modified registration event method can make the element's event handler execute in the specified environment.

Copy code The code is as follows:

When the above code is executed, the this scope of the handler eventHandler is under the window object.

this在window对象下的执行环境截图

The event registration through the binding function introduced earlier creates an event hash object for the element object to save the event handler. This hash object starts when the element removes the event handler. When it comes to emergency situations, the corresponding event handler can be accurately removed according to it. The code to remove the event handler is as follows:

Copy code The code is as follows:

 
function removeHandler ( obj, type, handler, scope) { 
obj.eventHash = obj.eventHash || {}; 
var evtList = obj.eventHash [type] || [], len = evtList.length; 
 if (len > 0) { 
for (; len--; ) { 
var curEvtObj = evtList[len]; 
if (curEvtObj.name == type && curEvtObj.handler === handler && curEvtObj.scope === scope) { 
if (obj.removeEventListener) { 
obj.removeEventListener(type, curEvtObj.fn, false); 
} else if (obj.detachEvent) { 
obj.detachEvent("on" type, curEvtObj.fn); 
} else { 
obj["on" type] = null; 
} 
evtList.splice(len, 1); 
break; 
} 
} 
} 
} 

This concludes the introduction of using function binding technology to register event handlers for a specific execution environment. Similarly, function binding can also be used to enable callback functions to be executed in a given execution environment.

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