can't draw a image in canvas

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I design a object named "Lottory"

function Lottery(canvas, cover_img, width, height){
    this.canvas = canvas;
    this.context = this.canvas.getContext("2d");
    this.width = width;
    this.height = height;
    this.cover_img = cover_img;
}

and a method init()

Lottery.prototype = {
    init: function(){
        var _this = this;
        var image = new Image();
        image.src = this.cover_img;
        image.onload = function(){
            _this.context.drawImage(image, 0, 0, _this.width, _this.height);    
        }
    }
}

When I use this, it can't draw a image on my canvas, Why?

var canvas = document.getElementById("cv");
var context = canvas.getContext("2d");
canvas.height = window.innerHeight;
canvas.width = window.innerWidth;

var cover = new Lottery(canvas,"index_blur.jpg", canvas.height, canvas.width);
cover.init();

asked 1 min ago

Zk Liao

27

can't draw a image in canvas