javascript - 为什么javascript不在 Canvas 上绘制图像？

Question

我在将图像绘制到 Canvas 时遇到问题。没有错误，但仍然没有绘制图像。请检查代码块，看看您是否能看到我看不到的内容:

function loadPacman(posX, posY) {
    this.posX = posX;
    this.posY = posY;
    pacman = new Image();
    pacman.src="http://www.frogbug.se/pacman/img/naziman.png";
    pacman.addEventListener("load", function(){
            canvas.drawImage(pacman, this.posX, this.posY)
        }, false);
}

function loadGhost(posX, posY) {
    this.posX = posX;
    this.posY = posY;
    ghost = new Image();
    ghost.src="http://www.frogbug.se/pacman/img/ghost.png";
    ghost.addEventListener("load", function(){
            canvas.drawImage(ghost, this.posX, this.posY)
        }, false);
}

这是我在页面加载时加载的函数:

function onLoad() {
    var xy = document.getElementById('canvas');
    canvas = xy.getContext('2d');
    //calculate the x and y for canvas
    x = xy.width;
    y = xy.height;

    //divide the width and length of the canvas to get small cubes
    //which is 40x40
    xtile = Math.floor(x/40);
    ytile = Math.floor(y/40);

    //draw lines around the canvas
    canvas.strokeRect(0, 0, x, y);

    //the array for the map
    var map = getMapArray();

    //draw the map
    drawMap(map);
    //fillCanvas("0010", 0, 40, 40, 40);

    //load the ghost and pacman
    loadPacman(0, 0);
    loadGhost((xtile*40)-40, (ytile*40)-40);
}

提前致谢!您可以在此处查看完整的源代码等: http://www.picturds.com/pacman_serious/

Answer 1

从加载回调中的 this.posX 和 this.posY 中删除 this。 this 在该上下文中是图像元素，与分配给 this.posX 时的 this 不同(即 window)。

http://jsfiddle.net/5dkx4/