lördag 13 Dec 2008, 12:34

Google Chrome has some limited support for Greasemonkey scripts (by using the –enable-greasemonkey command line argument), so I have been looking at making some of my last.fm Greasemonkey scripts work in Chrome. There are a few limitations. I will go through each one, with their workarounds:

Chrome ignores script metadata including @include and @exclude
In Chrome all Greasemonkey scripts are applied to all pages. The workaround is to include checking in the script, either based on the URL or a unique tag id / class name known to be present on the correct page.

No GM_setValue GM_getValue
This means that by default there is no way of saving / loading settings and if a script uses these methods it will fail. A workaround is to use cookies. This can be added to your script to allow it to work in Chrome:

function createCookie(name,value,days) {
if (days) {
var date = new Date();
date.setTime(date.getTime()+(days*24*60*60*1000));
var expires = "; expires="+date.toGMTString();
}
else var expires = "";
document.cookie = name+"="+value+expires+"; path=/";
}

function readCookie(name,defaultvalue) {
var nameEQ = name + "=";
var ca = document.cookie.split(';');
for(var i=0;i < ca.length;i++) {
var c = ca;
while (c.charAt(0)==' ') c = c.substring(1,c.length);
if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
}
return defaultvalue;
}

if(!window.GM_setValue)
{
window.GM_setValue = function(valuename, value) { createCookie(valuename,value,365); };
}
if(!window.GM_getValue)
{
window.GM_getValue = function(valuename, defaultvalue) { return readCookie(valuename,defaultvalue); };
}

No unsafeWindow
This does not actually appear to be needed in Chrome, but will cause an error if used. Adding this should prevent errors:

if(!window.unsafeWindow)
{
window.unsafeWindow = window;
}

No cross-domain HTTP requests
The GM_xmlhttpRequest function does not work in Chrome. It is possible to use the xmlhttpRequest object, but this only works for the same domain. A workaround is to use a proxy which will convert the XML feed to JSON format. An example using a Yahoo Pipe is shown below:

// Gets XML feeds, converting them to JSON
// using the getXML Yahoo Pipe - http://pipes.yahoo.com/sanand/getxml
function getFeed(feed,callback) {
var newScript = document.createElement('script');
newScript.type = 'text/javascript';
var tim = new Date();
newScript.src = 'http://pipes.yahoo.com/pipes/hrDp8A0v3RGr1Dm_yZ1_DQ/run?_render=json&_callback='; + callback + '&url='+escape(feed) + '&cache=' + tim.toGMTString();
document.getElementsByTagName("head").appendChild(newScript);
}