Image Preloading in Canvas

I'm drawing an Image on the canvas using the drawImage function. This is how Im setting the src property of the image:

var img = new Image(); // Create new Image object

img.src = 'test.php?filename=myfile.jpg'

and then

oCanvas.width = 600; oCanvas.height = 400; oContext.drawImage(img, 0, 0, 600, 400);

The problem is that if the src isn't set then I开发者_JAVA技巧 get the foll error:"uncaught exception: [Exception... "Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIDOMCanvasRenderingContext2D.drawImage]" . I know I get this error coz the image hasnt finished loading. I added an alert just before the call to drawImage function to let the image finish loading and it seems to work. But it's a pain in the neck. How do I check if the image has finished loading? BTW I have to set the src property by calling a php file.

Define this before you set the image's src attribute.

// Create new Image object
var img = new Image(); 

img.onload = function() {
    // add it to the canvas
    oCanvas.width = 600; oCanvas.height = 400;
    oContext.drawImage(img, 0, 0, 600, 400);
};

img.src = 'test.php?filename=myfile.jpg';

(it needs to be defined first because otherwise IE7 will not fire it).

Also, if you wanted to do something with that error, use JavaScript's try/catch.

try {
    oCanvas.width = 600; oCanvas.height = 400;
    oContext.drawImage(img, 0, 0, 600, 400);
} catch (exception) {
    alert('something went wrong!');
};