有人可以解释一下这个涉及回调的图像加载器函数吗

Question

代码：

function loadImages(sources, callback){
    var images = {};
    var loadedImages = 0;
    var numImages = 0;
    for (var src in sources) {
        numImages++;
    }
    for (var src in sources) {
        images[src] = new Image();
        images[src].onload = function(){
            if (++loadedImages >= numImages) {
                callback(images);
            }
        };
        images[src].src = sources[src];
    }
}
window.onload = function(images){
    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
    var sources = {
        darthVader: "darth-vader.jpg",
        yoda: "yoda.jpg"
    };
    loadImages(sources, function(images){
        context.drawImage(images.darthVader, 100, 30, 200, 137);
        context.drawImage(images.yoda, 350, 55, 93, 104);
    });
};

---

混乱

function loadImages(sources, callback){

3：两个参数传递给该函数，其中一个参数本身就是一个函数：回调

    var images = {};

4：最后，图像设置为 ... nul（？）

    var loadedImages = 0;
    var numImages = 0;
    // get num of sources
    for (var src in sources) {
        numImages++;
    }
    for (var src in sources) {
        images[src] = new Image();
        images[src].onload = function(){
            if (++loadedImages >= numImages) {
                callback(images);

5：此时我的大脑很困惑.. ..

            }
        };
        images[src].src = sources[src];
    }
}
 
window.onload = function(images){

据我了解，

1：参数“images”为空。

    var canvas = document.getElementById("myCanvas");
    var context = canvas.getContext("2d");
 
    var sources = {
        darthVader: "darth-vader.jpg",
        yoda: "yoda.jpg"
    };
 
    loadImages(sources, function(images){

2：现在它作为参数传递给这个内联函数——仍然没有指向任何地方...它现在应该调用上面定义的 loadImages 方法...

        context.drawImage(images.darthVader, 100, 30, 200, 137);

它从哪里获取 darthvader 的上下文？我只看到“来源”上面有 darthVader

        context.drawImage(images.yoda, 350, 55, 93, 104);
    });
};

来源： http://www. html5canvastutorials.com/tutorials/html5-canvas-image-loader/

编辑：问题::

从步骤 4: 到 5: (特别是在第二个 for 循环），正在创建一个新数组 (images[src]) 并将其传递给在步骤 2: 之前定义为内联的回调() 函数。它实际上从哪里获取源图像？

Answer 1

装载机的

function loadImages(sources, callback){
    //images is set to an object literal
    //this is the same as writing "var images = new Object();
    var images = {};

    //the counter for the number of images loaded
    var loadedImages = 0;

    //the total number of images
    var numImages = 0;

    //count the total number of images to load
    for (var src in sources) {
        numImages++;
    }

    //iterate through every image in sources
    //"src" will be the key used in the object passed to the function (i.e. "yoda")
    for (var src in sources) {
        //set image[*keyname*] to a new Image object
        //i.e. images.yoda = new Image(), images.darthVader = new Image();
        images[src] = new Image();

        //attach an onload event listener to the image
        images[src].onload = function(){
            //add one to the number of images loaded
            //if the number of images loaded is equal to the total number of images, call the callback
            if (++loadedImages >= numImages) {
                //pass the object containing the images to load as a parameter of the callback function
                callback(images);
            }
        };
        //set the source of the created image to the src provided in the sources object
        //i.e. images.yoda.src = sources.yoda
        images[src].src = sources[src];
    }
}

用法

window.onload = function(images){
    //get the canvas
    var canvas = document.getElementById("myCanvas");

    //get the drawing context of the canvas
    var context = canvas.getContext("2d");

    //initialize a new object with two sources
    //accessible as sources.darthVader and sources.yoda
    var sources = {
        darthVader: "darth-vader.jpg",
        yoda: "yoda.jpg"
    };

    //load the images in sources using the provided callback function
    loadImages(sources, function(images){
        //draw the images that were loaded on the canvas
        context.drawImage(images.darthVader, 100, 30, 200, 137);
        context.drawImage(images.yoda, 350, 55, 93, 104);
    });
};

Answer 2

对于你的第一个问题：

var images = {};

大括号在 javascript 中意味着它创建一个新对象，所以它不是 NUL 而是一个空对象。 Javascript 中的对象类似于数组，但它存储键值对而不是索引值。

---

要了解回调发生了什么：

回调是一个函数指针，由调用者传递给函数“loadImages”。

---

source 是函数的参数，也由调用者传递。

在下面的行中，从该数组中读取图像 URL：

    images[src].src = sources[src];

---

正在创建一个新数组（images[src]）

否，但在数组内创建了一个新项目（数组元素）。

---

此加载器函数的缺点是它仅在回调中告诉您图像已完成 - 但它不会告诉您哪一个！

Answer 3

我已向您的 JavaScript 内联添加了注释：

function loadImages(sources, callback){
    //  "{}" is object literal syntax.  images is a new empty object, not null.
    var images = {};
    var loadedImages = 0;
    var numImages = 0;

    for (var src in sources) {
        numImages++;
    }
    for (var src in sources) {
        //  In a for..in loop, the variable (src) is the key of the object (sources).
        //  The value is retrieved via object[key] (eg, sources[src]).

        //  In the first iteration of this loop, the images object is given a
        //  property of "darthVader", and "yoda" in the second iteration.
        //  Note: images[src] does not create a new array, but adds a property
        //  named the value of src to images using square bracket notation.
        images[src] = new Image();
        images[src].onload = function() {
            if (++loadedImages >= numImages) {
                callback(images);  // "callback" is passed images as a parameter
            }
        };
        images[src].src = sources[src];
    }
}

//  The "images" parameter referenced here is never used.  It's pretty pointless.
//  It would be an Event object, so calling it "images" is a misnomer.
window.onload = function(images){ 
    var canvas = document.getElementById("myCanvas"); 
    var context = canvas.getContext("2d"); 

    var sources = { 
        darthVader: "darth-vader.jpg", 
        yoda: "yoda.jpg" 
    }; 

    //  Don't confuse "images" here with the "images" parameter passed to 
    //  onload.  This is the callback's own private parameter.  It get's its
    //  value from the caller.
    loadImages(sources, function(images){ 
        context.drawImage(images.darthVader, 100, 30, 200, 137); 
        context.drawImage(images.yoda, 350, 55, 93, 104); 
    }); 
};

编辑：有关方括号表示法的注释。鉴于此设置：

var obj = {};
var propertyName = "foo";

以下几行是等效的：

obj.foo = 1;
obj["foo"] = 1;
obj[propertyName] = 1;

上述每一行都会向 obj 添加一个名为“foo”的属性。