将图像绘制到画布中时为什么需要延迟？

Question

这是代码：

var spriteFolder = "../../assets/Painter/sprites/";
var sprites = {};
sprites.background = new Image();
sprites.background.src = spriteFolder + "spr_background.jpg";
sprites.cannon_barrel = new Image();
sprites.cannon_barrel.src = spriteFolder + "spr_cannon_barrel.png";
sprites.cannon_red = new Image();
sprites.cannon_red.src = spriteFolder + "spr_cannon_red.png";
sprites.cannon_green = new Image();
sprites.cannon_green.src = spriteFolder + "spr_cannon_green.png";
sprites.cannon_blue = new Image();
sprites.cannon_blue.src = spriteFolder + "spr_cannon_blue.png";


var Canvas2D = {
    canvas: undefined,
    canvasContext: undefined
};

Canvas2D.initialize = function(canvasName) {
    Canvas2D.canvas = document.getElementById(canvasName);
    Canvas2D.canvasContext = Canvas2D.canvas.getContext("2d");
};

Canvas2D.clear = function() {
    Canvas2D.canvas.clearRect(0, 0, Canvas2D.canvas.width, Canvas2D.canvas.height);
};

Canvas2D.drawImage = function(sprite, position, rotation, origin) {
    Canvas2D.canvasContext.save();
    Canvas2D.canvasContext.translate(position.x, position.y);
    Canvas2D.canvasContext.rotate(rotation);
    Canvas2D.canvasContext.drawImage(sprite,
        0, 0, sprite.width, sprite.height,
        -origin.x, -origin.y, sprite.width, sprite.height
    );
    Canvas2D.canvasContext.restore();
};

function init() {
    Canvas2D.initialize("myCanvas");
    setTimeout(function() {
        Canvas2D.canvasContext.drawImage(sprites.background, 0, 0);
    }, 1000);
}
document.addEventListener("DOMContentLoaded", init);

如果我不使用setTimeout ，则不会绘制图像。 为什么？

Answer 1

因为您的图像尚未加载。 由于您的1秒超时，他们有足够的时间加载。

请看MDN https://developer.mozilla.org/zh-CN/docs/Web/API/Canvas_API/Tutorial/Using_images中的示例

您需要使用类似的方法-请参见img.onload()

    function draw() {
      var ctx = document.getElementById('canvas').getContext('2d');
      var img = new Image();
      img.onload = function(){
        ctx.drawImage(img,0,0);
        ctx.beginPath();
        ctx.moveTo(30,96);
        ctx.lineTo(70,66);
        ctx.lineTo(103,76);
        ctx.lineTo(170,15);
        ctx.stroke();
      };
      img.src = 'https://mdn.mozillademos.org/files/5395/backdrop.png';
    }