Code Highlighting

Monday, December 17, 2012

Html and Svg: handling events to and fro

I've been working on a website where I embed a couple of svg's in an html page. Actually, I embed the same svg in three places on one page. The svg in question has some script-driven animation, so I needed to use an object tag, rather than a simple img.
First challenge was this: I needed to trigger the svg animation for all svg's one after the other, which meant I had to call into the svg's script:

        function bounceBall(ball) {
            var svgView = getSvgView(ball);

        function getSvgView(ball) {
            var svgDoc;
            try {
                if (ball.getSVGDocument)
                    svgDoc = ball.getSVGDocument();
                else if (ball.contentDocument)
                    svgDoc = ball.contentDocument;

                if (svgDoc) {
                    return svgDoc.defaultView;
            } catch (e) { }
            return null;

When I initially call this function, the SVG dom might not be loaded yet. That's why I include the try/catch, and return null if an error occurs. Elsewhere in the code I set a setTimeout to retry 200 milliseconds later. The onload event is not quite reliable enough.

Next up was the click handler. If you simply add an onclick to the object tag, nothing happens when you click the svg. That makes perfect sense: the onclick is registered and handled within the svg, and never makes it to the html dom. I needed to have a click on the image open a little div in html though. From svg I could call the html javascript functions using top.someFunctionName(). I had the same svg image three times though, and it needed to do something different each time. Here's what I came up with:


<object type="image/svg+xml" data="/Content/Images/ball.svg" class="ball" style="left: 130px; top: 160px;" onclick="showPopup('homepopup2');"></object>

"But wait!" you say, "You just told me that doesn't work!". And it doesn't, but it would be pretty convenient if it did:

Html dom javascript:

        function setClickEvent(ball) {
            if (ball.onclick) {
                var svgwin = getSvgView(ball);
                if (svgwin) {
                    svgwin.eventHandler = ball.onclick;
                } else {
                    // if the svg view is not available,
                    // try again in 200 ms.
                    window.setTimeout(function() {
                    }, 200);

There you go. On load I simply funnel the onclick handler into the svg dom. The svg implementation is trivial:

    var eventHandler = null;

    function handleClick(){

<circle cx="15" cy="15" r="5" id="ball" onclick="handleClick();" />

If you need this sort of thing more often - or for more events - you could work out a neat wrapper with  registerEventHandler(eventName, eventHandler) and triggerEvent(eventName) methods to reuse (and a html-side script that automatically hooks it up). I don't currently foresee a need for it myself though.


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