BJAX: A Quick Hack for Using GWT with Bookmarklets

Dietrich Kappe, Thursday, August 30, 2007 @ 5:52 am

BJAX = Browser Extensions + Ajax

I've continued my explorations on how to use GWT in unconventional ways, starting with how to load a GWT application through a bookmarklet. That's been made much easier with GWT 1.4 through the introduction of a "cross site" feature:

Cross-site script inclusion is now supported. The compiler produces a "-xs" (meaning "cross-site") version of your module's startup script that can be included without being restricted by the same-origin policy. Be careful, though. Including scripts from other sites that you don't fully trust is a big security risk.

There has been some confusion about this "option." Some developers thought that "-xs" was a command-line flag you had to pass to the GWT compiler. In fact, the GWT compiler produceds a file with "-xs" in its name as a part of its normal workings -- no special flags necessary. This file, when included in a page via a script tag, loads the GWT application as a Javascript (a ".js" file) into the current browser window. That's opposed to the iframe/html way it's been up to now.

Obviously iframe/html way is not suitable for things like bookmarklets, as the cross-domain thing gets in the way. With the pure Javascript approach, our problem is solved, right? Not so fast. If you try including the "-xs" Javascript file into a page via a bookmarklet, you get the dreaded hanging page with a perpetual "Loading..." message in the status bar. Why is that?

A little tour through an "-xs" file reveals that GWT assumes that the Javascript file is loaded during the normal loading/rendering of the page. Specifically, it uses document.write to insert script tags and uses an onload event handler to kick off the application code.

var $wnd = window, $doc = document, external = $wnd.external, gwtOnLoad, bodyDone, base = '',metaProps = {}, values = [], providers = [], answers = [], onLoadErrorFunc, propertyErrorFunc;[...]var oldOnLoad = $wnd.onload;$wnd.onload = function(evt){    if (oldOnLoad) {      $wnd.onload = oldOnLoad;      $wnd.onload(evt);    }    bodyDone = true;    maybeStartModule();}[...]$doc.write('<\/script>');

Of course this doesn't work so well with a bookmarklet. For one, the onload event is already long gone by the time we run our code, and for another, document.write after a page has rendered will cause precisely the "hanging page" behavior we see when we try to load the GWT app via a bookmarklet. Really, this is less of a specific bookmarklet problem and more of a general "how do I get GWT to load after a page has already rendered" problem. So, what can we do to remedy this it?

First, we can simply use document.appendChild to insert the script tag, second, we can assume that the body has finished loading, and third, we will have to tell GWT where to find the application code. Why this last step? Because GWT uses a "marker script" to figure out the source URL of the GWT application:

function computeScriptBase(){    var thisScript, markerScript;    $doc.write('<\/script />');    markerScript = $doc.getElementById('__gwt_marker_MyApp');    if (markerScript) {      thisScript = markerScript.previousSibling;    }    function getDirectoryOfFile(path){      var eq = path.lastIndexOf('/');      return eq >= 0?path.substring(0, eq + 1):'';    }

    ;    if (thisScript && thisScript.src) {      base = getDirectoryOfFile(thisScript.src);    }    if (base == '') {      base = getDirectoryOfFile($doc.location.href);    }     else if (base.match(/^\w+:\/\//)) {    }     else {      var img = $doc.createElement('img');      img.src = base + 'clear.cache.gif';      base = getDirectoryOfFile(img.src);    }    if (markerScript) {      markerScript.parentNode.removeChild(markerScript);    }}

When you use document.appendChild, this code stops working. Now there's probably a way to make this code work with appendChild, but I haven't noodled about it enough. That's why this post is termed a "Quick Hack."  We do some simple surgery with the "-xs" file as follows:

function insertScript(src) {    var script = document.createElement("script");    script.type = "text/javascript";    script.src = src;    $doc.body.appendChild(script);}[...]function computeScriptBase(){    base = "http://labs.pathf.com/MyApp/";    bodyDone = true;}[...]insertScript(base + strongName);

With these changes, the GWT app can now be load via a bookmarklet.

Next week, I hope to demonstrate a few interesting hacks using GWT and bookmarklets. Maybe I'll even give the dowdy Craig's list a much needed overhaul. Stay tuned...

Technorati Tags: ajax, bjax, bookmarklet, gwt

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