image.onload 到一个方法中

Question

所以我一直在尝试让图像预加载器工作，因为我知道我需要先加载图像才能在画布上绘制它们。我遵循了这个描述：How do image preloaders work?，但我不确定如何将img.onload 变成一个方法，或者以某种方式制定解决方法，因为该函数无法访问它应该访问的变量。

这是我的代码。不包括 Canvas 类，因为它无关紧要。 提前致谢。

    window.onload = function () {
        var MyApp = new App();
        MyApp.Initialize();
    }
    class App {
        canvas: Canvas;
        imageURLs: string[] = [];
        images = [];
        imagePreLoader: ImagePreLoader;
        constructor() {
            this.canvas = new Canvas(window.innerWidth, window.innerHeight, "mainCanvas");
            this.ImagePreLoader = new ImagePreLoader(this.imageURLs, this.images);
        }
        Initialize() {
            this.imageURLs.push("0.png");
            this.imageURLs.push("1.png");
            this.imagePreLoader.startLoadingAllImages(this.Loop.bind(this));
        }
        Loop() {
            this.canvas.clear();
            this.Draw();
            requestAnimationFrame(this.Loop.bind(this));
        }
        Draw() {
            for (var i = 0; i < this.images.length; i++) {
                this.canvas.drawImage(this.images[i], i * 100, 100);
            }
        }
    }
    class ImagePreLoader {
        imagesLoaded = 0;
        imageURLs: string[];
        images;
        constructor(urlArray: string[], imageArray) {
            this.imageURLs = urlArray;
            this.images = imageArray;
        }
        startLoadingAllImages(callback): void {
            for (var i = 0; i < this.imageURLs.length; i++) {
                var img = new Image();
                this.images.push(img);
                img.onload = function () { //This doesn't work as the function can't access the variables of the class.
                    this.imagesLoaded++;
                    if (this.imagesLoaded >= this.imageURLs.length) {
                        callback();
                    }
                }
                img.src = this.imageURLs[i];
            }
        }
    }

Answer 1

改变：

img.onload = function () { //This doesn't work as the function can't access the variables of the class.
    this.imagesLoaded++;
    if (this.imagesLoaded >= imageURLs.length) {
        callback();
    }
}

到：

img.onload = () => { 
    this.imagesLoaded++;
    if (this.imagesLoaded >= imageURLs.length) {
        callback();
    }
}

使用 lambda，即() => { ... }，使this 关键字在这种情况下按预期工作。在这种情况下，function 将从调用者那里获得它的作用域 (this)，给它一个错误的作用域。