Question

I have a Greasemonkey script that works just fine in Firefox and Opera. I struggle with getting it to work in Chrome, however. The problem is injecting a function into the page that can be invoked by code from the page. Here’s what I’m doing so far:

First, I get a helper reference to the unsafeWindow for Firefox. This allows me to have the same code for FF and Opera (and Chrome, I thought).

var uw = (this.unsafeWindow) ? this.unsafeWindow : window;

Next, I inject a function into the page. It’s really just a very thin wrapper that does nothing but invoking the corresponding function in the context of my GM script:

uw.setConfigOption = function(newValue) {
    setTimeout(setConfigOption, 0, newValue);
}

Then, there’s the corresponding function right in my script:

setConfigOption = function(newValue) {
    // do something with it, e.g. store in localStorage
}

Last, I inject some HTML into the page with a link to invoke the function.

var p = document.createElement('p');
p.innerHTML = '<a href="javascript:setConfigOption(1)">set config option to 1</a>';
document.getElementById('injection-point').appendChild(p);

To summarize:
In Firefox, when the user clicks that injected link, it will execute the function call on the unsafeWindow, which then triggers a timeout that invokes the corresponding function in the context of my GM script, which then does the actual processing. (Correct me if I’m wrong here.)

In Chrome, I just get a “Uncaught ReferenceError: setConfigOption is not defined” error. And indeed, entering “window.setConfigOption” into the console yields an “undefined”. In Firebug and the Opera developer console, the function is there.

Maybe there’s another way to do this, but a few of my functions are invoked by a Flash object on the page, which I believe makes it necessary that I have functions in the page context.

I took a quick look at the alternatives to unsafeWindow on the Greasemonkey wiki, but they all look pretty ugly. Am I completely on the wrong track here or should I look more closely into these?

RESOLUTION: I followed Max S.’ advice and it works in both Firefox and Chrome now. Because the functions I needed to be available to the page had to call back into the regular ones, I moved my whole script to the page, i.e. it is completely wrapped into the function he called ‘main()’.

To make the extra uglyness of that hack a little bit more bearable, I could at least drop the usage of unsafeWindow and wrappedJSObject now.

I still haven’t managed to get the content scope runner from the Greasemonkey wiki working. It should do the same and it seems to execute just fine, but my functions are never accessible to <a> elements from the page, for example. I haven’t yet figured out why that is.

Answer 1

The only way to communicate with the code running on the page in Chrome is through the DOM, so you’ll have to use a hack like inserting a <script> tag with your code. Note that this may prove buggy if your script needs to run before everything else on the page.

EDIT: Here’s how the Nice Alert extension does this:

function main () {
  // ...
  window.alert = function() {/* ... */};
  // ...
}

var script = document.createElement('script');
script.appendChild(document.createTextNode('('+ main +')();'));
(document.body || document.head || document.documentElement).appendChild(script);