javascript - Javascript 对象和 this 关键字

Question

我知道这引用了对象所有者。但是我很难尝试让课堂工作，同时试图确定这是指什么。

猜猜最好只显示代码：

function Ajax_Class(params) {
// Public Properties
this.Response = null;

// Private Properties
var RequestObj = null;

// Prototype Methods
this.OnReset    = function() { };
this.OnOpenConn = function() { };
this.OnDataSent = function() { };
this.OnLoading  = function() { };
this.OnSuccess  = function() { alert("default onSuccess method."); };
this.OnFailure  = function() { alert("default onFailure method."); };

// Public Methods
this.Close = function() {   // Abort current Request
    if (RequestObj) {
        RequestObj.abort();
        RequestObj = null;
        return true;
    } else return false;
};

// Private Methods
var Instance = function() {     // To create instance of Http Request
    try { return new XMLHttpRequest(); }
    catch (error) {}
    try { return new ActiveXObject("Msxml2.XMLHTTP"); }
    catch (error) {}
    try { return new ActiveXObject("Microsoft.XMLHTTP"); }
    catch (error) {}

    // throw new Error("Could not create HTTP request object.");
    return false;
};

var ReadyStateHandler = function() {
    // Switch through possible results
    switch(RequestObj.readyState) {
        case 0:
            this.OnReset();
        break;

        case 1:
            this.OnOpenConn();
        break;

        case 2:
            this.OnDataSent();
        break;

        case 3:
            this.OnLoading();
        break;

        case 4:
            // Check Status
            if (RequestObj.status == 200)  {
                // Handle Response
                Response = HandleResponse();
                // Call On Success
                this.OnSuccess();
                // Hide Loading Div
                LoadingDiv(true);
            } else {
                this.OnFailure();
            }

        break;
    } // End Switch
};

var Open = function() {
    // In case it's XML, Override the Mime Type
    if ((params["ResponseType"] == "XML") && (RequestObj.overrideMimeType)) 
        RequestObj.overrideMimeType('text/xml');

    // 
    if ((params["User"]) && (params["Password"]))
        RequestObj.open(params["Method"], params["URL"],  params["Async"], params["User"], params["Password"]);
    else if (params["User"])
        RequestObj.open(params["Method"], params["URL"],  params["Async"], params["User"]);
    else
        RequestObj.open(params["Method"], params["URL"],  params["Async"]);

    // Set Request Header ?
    if (params["method"] == "POST") 
        //this.SetRequestHeader("Content-Type", "application/x-www-form-urlencoded; charset=UTF-8");
        RequestObj.setRequestHeader("Content-type", "application/x-www-form-urlencoded");

};

var Send = function() {
    if (params["Data"])     RequestObj.send(params["Data"]);
    else                    RequestObj.send(null);
};

var HandleResponse = function() {
    if (params["ResponseType"] == "JSON")
        return ParseJSON(RequestObj.responseText);
    else if (params["ResponseType"] == "XML")
        return RequestObj.responseXML;
    else 
        return RequestObj.responseText;
};

// Method ParseJSON
var ParseJSON = function(obj) {
    if (obj)
        return JSON.parse(obj);
}; // End ParseJSON

// Method LoadingDiv -> Either Shows or Hides the Loading Div
var LoadingDiv = function(hide) {
    // Hide the Modal Window
    if (hide) { document.getElementById("Async").style.display = "none"; return false; }

    // Show Modal Window
    document.getElementById("Async").style.display = "block";

    // Reset the Position of the Modal_Content to 0, x and y
    document.getElementById("Async_Container").style.left = "0px";
    document.getElementById("Async_Container").style.top = "0px";

    // Center the Modal Area, no matter what the content
    var Screen_Width, Screen_Height;
        // Get screen data
        if (typeof(window.innerWidth) == "number") { Screen_Width = window.innerWidth; Screen_Height = window.innerHeight; }            //Non-IE
        else if (document.documentElement && (document.documentElement.clientWidth || document.documentElement.clientHeight)) {         //IE 6+ in 'standards compliant mode'
            Screen_Width = document.documentElement.clientWidth;
            Screen_Height = document.documentElement.clientHeight;
        } else if (document.body && (document.body.clientWidth || document.body.clientHeight)) {                                        //IE 4 compatible
            Screen_Width = document.body.clientWidth;
            Screen_Height = document.body.clientHeight;
        }

        // Set Modal_Content Max Height to allow overflow
        document.getElementById("Async_Container").style.maxHeight = Screen_Height - 200 + "px";

        // Get Modal_Container data
        var El_Width = document.getElementById("Async_Container").offsetWidth;
        var El_Height = document.getElementById("Async_Container").offsetHeight;


    // Set the Position of the Modal_Content
    document.getElementById("Async_Container").style.left = ((Screen_Width/2) - (El_Width/2)) + "px";
    document.getElementById("Async_Container").style.top = ((Screen_Height/2) - (El_Height/2)) + "px";
};


// Constructor

// Check the Params
// Required Params
if (!params["URL"]) return false;
// Default Params
params["Method"]        = (!params["Method"]) ? "GET" : params["Method"];   // GET, POST, PUT, PROPFIND
params["Async"]         = (params["Async"] === false) ? false : true;
params["ResponseType"]  = (!params["ResponseType"]) ? "JSON" : params["ResponseType"];  // JSON, TXT, XML
params["Loading"]       = (params["Loading"] === false) ? false : true;
// Optional Params
// params["User"])
// params["Password"]

// Create Instance of Http Request Object
RequestObj = Instance();

// Handle Ajax according to Async
if (params["Async"]) {
    // Handle what should be done in case readyState changes
    if (params["Loading"]) LoadingDiv();
    // State Handler || Response is Handled within the code
    RequestObj.onreadystatechange = ReadyStateHandler;
    // Open & Send
    Open();
    Send();
} else {
    // Handle the Loading Div
    if (params["Loading"]) LoadingDiv();
    // Open & Send
    Open();
    Send();
    // Handle Response
    this.Response = HandleResponse();
    // No State Handler, Application has been waiting for modifications.
    //this.OnSuccess(Response);
    // Handle the Loading Div
    LoadingDiv(true);
}

// no problems?
return true;

} // End Ajax

然后，在页面内部：

window.onload = function() {

update_states = function() {
    this.OnSuccess = function() {
        alert("overriden onSuccess Method");
    };
};
    update_states.prototype = new Ajax_Class({URL:"ajax/states.php?id=5&seed="+Math.random()});
    run_update_states = new update_states;      

} // Window Onload

我想要做的是有一个默认的 OnSuccess （等等）方法，如果有需要，我可以用子类方法覆盖默认方法，但它仍然会在 HttpRequest 状态更改时自动调用。

如果有人能指出我正确的方向，我将不胜感激，如果我能理解为什么这在这种情况下不起作用以及如何使其引用正确的对象，我会感到惊讶。

谢谢你的帮助。

score 1 · Accepted Answer

this是根据事物的调用方式设置的。这些是它的不同设置方式：

func()- 一个正常的函数调用。 this设置为全局对象（window在浏览器中）。
obj.method()- 方法调用。 this设置为obj。
func.call(obj, arg1, arg2, ...)- 使用.call(). this设置为obj。
func.apply(obj, args)- 使用.apply(). this设置为obj。
func.bind(obj, arg1, arg2, ...)- 使用.bind(). this创建一个在调用时设置为的新函数obj（在内部，它.apply()在其实现中使用）。

需要注意的典型事情。

调用常规函数，即使是在对象的方法中也会导致this指针window位于该函数内。
回调函数通常会收到this与其可能嵌入的代码不同的值。您可能必须将原始this值保存到局部变量中（通常self按约定调用），以便您可以从回调函数中引用它。

score 1 · Accepted Answer

也许您正在尝试执行以下操作：

function Base() {}

Base.prototype.showName = function() {
  alert('Base method: ' + this.name);
}

function Foo(name, arg){

   this.name = name;

  // Extend this if arg is an object
  if (typeof arg == 'object') {
    for (var p in arg) {

      // This references the new instance if called with new keyword
      if (arg.hasOwnProperty(p)) {
        this[p] = arg[p];
      }
    }
  }
}

Foo.prototype = new Base();

// Re-assign constructor property (not very useful but some to do like this)
Foo.prototype.constructor = Foo; 

var inst0 = new Foo('foo 0');

// Override inherited method
var inst1 = new Foo(
             'foo 1',
             {showName: function(){alert('Instance method: ' + this.name);}}
            );

inst0.showName(); // Base method: foo 0
inst1.showName(); // Instance method: foo 1

但是还有其他（更简单？更好？）的方法可以做到这一点。让原型继承和 javascript 的动态特性为你工作，不要仅仅因为你对它们更舒服就试图让它模仿其他继承模式。

score 0 · Accepted Answer

0

你试过这个吗

run_update_states = new update_states();

注意括号。

于 2012-09-06T22:55:06.473 回答

javascript - Javascript 对象和 this 关键字

3 回答 3

Related

Reference