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JavaScript的this,call(),apply(),bind() ---(转载)

2009-03-04 16:23 本站整理 浏览(4)

this是什么??

为了建立一个scope chain, 每个JavaScript的代码执行上下文都提供了this关键字。In its most common usage, [code]this serves as an identity function, providing our neighborhoods a way of referring to themselves. We can’t always rely on that behavior, however: Depending on how we get into a particular neighborhood,
this
might mean something else entirely. In fact, how we get into the neighborhood is itself exactly what
this
generally refers to. 需要注意特殊的四种情况:
Calling an Object’s Method
在典型的面向对象编程时,我们需要一种方式去指向和引用我们调用的对象.
this
serves the purpose admirably, providing our objects the ability to examine themselves, and point at their own properties.
<script type="text/javascript"> 

       var deep_thought = { 

        the_answer: 42, 

        ask_question: function () { 

         return this.the_answer; 

        } 

       }; 

    

       var the_meaning = deep_thought.ask_question(); 

    </script>

This example builds an object named
deep_thought
, sets its
the_answer
property to 42, and creates an
ask_question
method. When
deep_thought.ask_question()
is executed, JavaScript establishes an execution context for the function call, setting
this
to the object referenced by whatever came before the last ”.”, in this case:
deep_thought
. The method can then look in the mirror via
this
to examine its own properties, returning the value stored in
this.the_answer
: 42.
Constructor
Likewise, when defining a function to be used as a constructor with the
new
keyword,
this
can be used to refer to the object being created. Let’s rewrite the example above to reflect that scenario:
<script type="text/javascript"> 

       function BigComputer(answer) { 

        this.the_answer = answer; 

        this.ask_question = function () { 

         return this.the_answer; 

        } 

       } 

    

       var deep_thought = new BigComputer(42); 

       var the_meaning = deep_thought.ask_question(); 

    </script>

Instead of explicitly creating the
deep_thought
object, we’ll write a function to create
BigComputer
objects, and instantiate
deep_thought
as an instance variable via the
new
keyword. When
new BigComputer()
is executed, a completely new object is created transparently in the background.
BigComputer
is called, and its
this
keyword is set to reference that new object. The function can set properties and methods on
this
, which is transparently returned at the end of
BigComputer
’s execution.
Notice, though, that
deep_thought.the_question()
still works just as it did before. What’s going on there? Why does
this
mean something different inside
the_question
than it does inside
BigComputer
? Put simply, we entered
BigComputer
via
new
, so
this
meant “the new object.” On the other hand, we entered
the_question
via
deep_thought
, so while we’re executing that method,
this
means “whatever
deep_thought
refers to”.
this
is not read from the scope chain as other variables are, but instead is reset on a context by context basis.
Function Call
What if we just call a normal, everyday function without any of this fancy object stuff? What does
this
mean in that scenario?
<script type="text/javascript"> 

       function test_this() { 

        return this; 

       } 

       var i_wonder_what_this_is = test_this(); 

    </script>

In this case, we weren’t provided a context by
new
, nor were we given a context in the form of an object to piggyback off of. Here,
this
defaults to reference the most global thing it can: for web pages, this is the
window
object.
Event Handler
For a more complicated twist on the normal function call, let’s say that we’re using a function to handle an
onclick
event. What does
this
mean when the event triggers our function’s execution? Unfortunately, there’s not a simple answer to this question.
If we write the event handler inline,
this
refers to the global
window
object:
<script type="text/javascript"> 

       function click_handler() { 

        alert(this); // alerts the window object 

       } 

    </script> 

    ... 

    <button id='thebutton' onclick='click_handler()'>Click me!</button>

However, when we add an event handler via JavaScript,
this
refers to the DOM element that generated the event. (Note: The event handling shown here is short and readable, but otherwise poor. Please use a real addEvent function instead.):
<script type="text/javascript"> 

       function click_handler() { 

        alert(this); // alerts the button DOM node 

       } 

    

       function addhandler() { 

        document.getElementById('thebutton').onclick = click_handler; 

       } 

    

       window.onload = addhandler; 

    </script> 

    ... 

    <button id='thebutton'>Click me!</button>

Complications

Let’s run with that last example for a moment longer. What if instead of running
click_handler
, we wanted to ask
deep_thought
a question every time we clicked the button? The code for that seems pretty straightforward; we might try this:
<script type="text/javascript"> 

function BigComputer(answer) { 

   this.the_answer = answer; 

   this.ask_question = function () { 

    alert(this.the_answer); 

   } 

} 

function addhandler() { 

   var deep_thought = new BigComputer(42), 

    the_button = document.getElementById('thebutton'); 

   the_button.onclick = deep_thought.ask_question; 

} 

window.onload = addhandler; 

</script>

对上面的代码,我们期望点击按钮,
deep_thought.ask_question被执行,我们得到返回结果
“42.” 但为什么得到的结果反而是
undefined?哪里错了
?
The problem is simply this: We’ve passed off a reference to the
ask_question
method, which, when executed as an event handler, runs in a different context than when it’s executed as an object method. 简而言之,ask_question 中的this关键字是指向产生事件的DOM元素节点,而不是
BigComputer对象
. DOM元素节点并没有
the_answer属性,所以返回结果是
undefined而不是
“42.”
setTimeout
exhibits similar behavior, delaying the execution of a function while at the same time moving it out into a global context.
This issue crops up all over the place in our programs, and it’s a terribly difficult problem to debug without keeping careful track of what’s going on in all the corners of your program, especially if your object has properties that do exist on DOM elements or the
window
object.

Manipulating Context With
.apply()
and
.call()

We really do want to be able to ask
deep_thought
a question when we click the button, and more generally, we do want to be able to call object methods in their native context when responding to things like events and
setTimeout
calls. Two little-known JavaScript methods,
apply
and
call
, indirectly enable this functionality by allowing us to manually override the default value of
this
when we execute a function call. Let’s look at
call
first:
<script type="text/javascript"> 

var first_object = { 

   num: 42 

}; 

var second_object = { 

   num: 24 

}; 

function multiply(mult) { 

   return this.num * mult; 

} 

multiply.call(first_object, 5); // returns 42 * 5 

multiply.call(second_object, 5); // returns 24 * 5 

</script>

In this example, we first define two objects,
first_object
and
second_object
, each with a
num
property. Then we define a
multiply
function that accepts a single argument, and returns the product of that argument, and the
num
property of its
this
object. If we called that function by itself, the answer returned would almost certainly be
undefined
, since the global
window
object doesn’t have a
num
property unless we explicitly set one. We need some way of telling
multiply
what its
this
keyword ought refer to; the
call
method of the
multiply
function is exactly what we’re looking for.
call方法的第一个参数定义了this关键字在被调用方法的执行上下文中指向和对象,call方法的剩余参数则是被调用方法的参数。因此当
multiply.call(first_object, 5)被执行,
multiply函数被调用
,
5
为传入方法的第一个参数,
this
执行
first_object对象。
Likewise, when
multiply.call(second_object, 5)
is executed, the
multiply
function is called,
5
is passed in as the first argument, and the
this
keyword is set to refer to object
second_object
.
apply方法和
call方法基本一致
,但是允许你以数组的形式向被调用的函数传递参数, which can be quite useful when programatically generating function calls. Replicating the functionality we just talked about using
apply
is trivial:
<script type="text/javascript"> 

... 

multiply.apply(first_object, [5]); // returns 42 * 5 

multiply.apply(second_object, [5]); // returns 24 * 5 

</script>

apply
and
call
are very useful on their own, and well worth keeping around in your toolkit, but they only get us halfway to solving the problem of context shifts for event handlers. It’s easy to think that we could solve the problem by simply using
call
to shift the meaning of
this
when we set up the handler:
function addhandler() { 

var deep_thought = new BigComputer(42), 

   the_button = document.getElementById('thebutton'); 

the_button.onclick = deep_thought.ask_question.call(deep_thought); 

}

上面的代码仍然存在问题:
call是立即执行函数的,因此我们提供的
onclick
handler是函数的执行结果而不是函数本身.我们需要JavaScript的另一个特性来解决这个问题:bind方法。

The Beauty of
.bind()

I’m not a huge fan of the Prototype JavaScript framework, but I am very much impressed with the quality of its code as a whole. In particular, one simple addition it makes to the
Function
object has had a hugely positive impact on my ability to manage the context in which function calls execute:
bind
performs the same general task as
call
, altering the context in which a function executes. The difference is that
bind
returns a function reference that can be used later, rather than the result of an immediate execution that we get with
call
.
If we simplify the
bind
function a bit to get at the key concepts, we can insert it into the multiplication example we discussed earlier to really dig into how it works; it’s quite an elegant solution:
<script type="text/javascript"> 

var first_object = { 

   num: 42 

}; 

var second_object = { 

   num: 24 

}; 

function multiply(mult) { 

   return this.num * mult; 

} 

Function.prototype.bind = function(obj) { 

   var method = this, 

    temp = function() { 

     return method.apply(obj, arguments); 

    }; 

   return temp; 

} 

var first_multiply = multiply.bind(first_object); 

first_multiply(5); // returns 42 * 5 

var second_multiply = multiply.bind(second_object); 

second_multiply(5); // returns 24 * 5 

</script>

First, we define
first_object
,
second_object
, and the
multiply
function, just as before. With those taken care of, we move on to creating a
bind
method on the
Function
object’s
prototype
, which has the effect of making
bind
available for all functions in our program. When
multiply.bind(first_object)
is called, JavaScript creates an execution context for the
bind
method, setting
this
to the
multiply
function, and setting the first argument,
obj
, to reference
first_object
. So far, so good.
The real genius of this solution is the creation of
method
, set equal to
this
(the
multiply
function itself). When the anonymous function is created on the next line,
method
is accessible via its scope chain, as is
obj
(
this
couldn’t be used here, because when the newly created function is executed,
this
will be overwritten by a new, local context). This alias to
this
makes it possible to use
apply
to execute the
multiply
function, passing in
obj
to ensure that the context is set correctly. In computer-science-speak,
temp
is a closure that, when returned at the end of the
bind
call, can be used in any context whatsoever to execute
multiply
in the context of
first_object
.
This is exactly what we need for the event handler and
setTimeout
scenarios discussed above. The following code solves that problem completely, binding the
deep_thought.ask_question
method to the
deep_thought
context, so that it executes correctly whenever the event is triggered:
function addhandler() { 

var deep_thought = new BigComputer(42), 

   the_button = document.getElementById('thebutton'); 

the_button.onclick = deep_thought.ask_question.bind(deep_thought); 

}

Beautiful.