javascript - 有人可以解释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);
    });
};

---

困惑

function loadImages(sources, callback){

3: 两个参数传递给这个函数，一个本身就是一个函数:回调

    var images = {};

4: 最后，images 被设置为 ... 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:此时我的大脑一片困惑....

            }
        };
        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);

它从哪里获得 darthvader 的上下文？我只看到“消息来源”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::

从第 4 步到第 5 步:(特别是在第二个 for 循环中)，正在创建一个新数组 (images[src]) 并将其传递给 callback() 函数，该函数在第 2 步之前被定义为内联函数: .它实际上从哪里获取源图像？

Answer 1

加载器

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);
    });
};