绘制到HTML5 Canvas的图像在首次加载时无法正确显示

Question

I'm following a tutorial on importing and displaying images on an HTML5 canvas. Everything works fine, until I try to change the image itself. For example, I'll have a yellow circle as my image, and the script works fine. But if I open the image itself in Paint and change the circle to red, and refresh the page, the circle won't show up until I click or refresh again a second time manually. Here is the code snippet I'm using:

var topMap = new Image();
topMap.src = "topMap.png";

function drawMap() {
    context.clearRect(0, 0, WIDTH, HEIGHT);
    context.drawImage(topMap, 0, 0);
}

function init() {
    drawMap();
}

init();

Answer 1

The circle is probably not showing up because you are drawing it before the image is loaded. Change your last statement to:

// Lets not init until the image is actually done loading
topMap.onload = function() {
  init();
}

The reason it works after you hit refresh is because the image is now pre-loaded into the cache.

You always want to wait for any images to load before you attempt to draw them or nothing will show up on the canvas instead.