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Phubar Baz Musings of an HTML5 Programmer

I am working on a Javascript app (CloudGraph) that uses HTML5 canvas and JQuery. I'm using ExplorerCanvas to support canvas in IE. I recently came across an interesting problem.

What I was trying to do is restore the user's settings when the page is loaded by reading some information from a cookie that I set the last time the user accessed the application. One of these settings is the size of the canvas. I decided that the best place to do this would be when the document is ready using JQuery $(document).ready().

This worked fine in browsers that natively support the canvas element. But in IE it kept getting errors the first time I would hit the page. It seemed that the excanvas element wasn't initialized yet because I was getting null reference and unknown properties errors. If I refreshed the page the errors went away but the resized canvas wasn't drawing on the entire area of the canvas. It was like the clipping rectangle was still set to the default canvas size.

I found that the canvas element when using excanvas has a div child element which is where the actual drawing takes place. When I changed the width and height of the canvas element in document.ready it didn't change the width and height of the child div. Initially my solution was to also change the div element when changing the canvas element and that worked.

But then I realized that having to refresh the page every time I started the app in IE really sucked. That wouldn't be acceptable for users. Since it seemed like the canvas wasn't getting initialized before I was trying to use it I decided to try to initialize my app at a different time. I figured the next best place was in the onload event. Sure enough, moving my initialization to onload fixed all of the problems.

So, it looks like the canvas shouldn't be manipulated until the onload event when using ExplorerCanvas. There might be ways to do it when the document is ready; I found some posts on initializing excanvas manually. But for me waiting until onload worked just fine.

Posted on Monday, June 7, 2010 10:21 AM | Back to top

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