javascript - 使用 Object.call 访问父作用域

Question

我想在 onload 函数中访问 img 属性，我该怎么做？我向 Picture 对象添加了 img 属性，并在 Picture 对象的范围内调用 onload 函数，但我仍然无法访问 this.img。

// picture
    function Picture(x, y, w, h, imgurl){
        this.x = x;
        this.y = y;
        this.w = w;
        this.h = h;
        this.imgurl = imgurl;
        this.draw = drawPic;
        this.overcheck = overRect;
    } // end picture

    function drawPic(){
            this.img = new Image(); // add img to this scope
            this.img.src = this.imgurl;
            this.img.onload = function(){
                //ctx.drawImage(this.image, this.that.x, this.that.y, this.that.w, this.that.h);
                ctx.drawImage(this.img, this.x, this.y, this.w, this.h); //error
            } // end onload
            this.img.onload.call(this);
    } // end drawPic

Answer 1

使用对this的引用

function drawPic() {
  var self = this;
  this.img = new Image();
  this.img.src = this.imgurl;
  this.img.onload = function() {
    ctx.drawImage(self.img, self.x, self.y, self.w, self.h);
  };
}