在调用函数之前，在Greasemonkey脚本中执行代码

Question

I'm working on a Greasemonkey script. What I need to do is execute a script before a function is called, or execute the script at the beginning of the function.

The problem is that the function is located in the document and not inside the Greasemonkey file. It will be like overwriting the function, but without overwriting it because it has to be executed after the script finished.

Here's my full Greasemonkey code, I don't know what I'm missing:

<pre>// ==UserScript==
// @name           appname
// @version        1.0.0
// @author         me
// @description    blah
// @include        http://www.runhere.net/*
// @exclude        http://www.notinhere.com/*
// @run-at         document-end
// ==/UserScript==

function addJQuery(callback) {
    var script = document.createElement("script");
    script.setAttribute("src", "http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js");
    script.addEventListener('load', function() {
        var script = document.createElement("script");
        script.textContent = "(" + callback.toString() + ")();";
        document.body.appendChild(script);
    }, false);
    document.body.appendChild(script);
}

function main() {
    var originalFunction = unsafeWindow.add_comment;
    unsafeWindow.add_comment = function(go_last_page) {
        alert("if it is shown, then works!");
        originalFunction.apply(unsafeWindow, new Array(true));
    }
}

//Load JQuery and execute function
addJQuery(main);​</pre>

The function I need to call is located in a page and is called add_comment . It has a single parameter that is a boolean type. I'm not familiar with javascript, but I need to do this simple extension.

I'll really appreciate your help.

Answer 1

Replace the function with a wrapper function that calls your function and then the original function.

var originalFunction = someObject.someFunction;

someObject.someFunction = function() {

    executeMyScript();
    return originalFunction.apply(someObject, arguments);

}

Answer 2

You can save the function to a variable, then overwrite the function.

Example:

var _func = functionIWant;
functionIWant = function(){
   // Do whatever
   _func(); // Call original function
}

Answer 3

That code is injecting main() into the target page via the addJQuery() method. This means that is improper to use unsafeWindow -- which will be undefined.

Also, you probably do not need to use .apply() in this case. Lastly, the code is using a variable, go_last_page , that does not seem to be defined anywhere.

So the code would be:

function main () {
    var originalFunction    = add_comment;
    add_comment             = function (/*go_last_page*/) {
        alert ("if it is shown, then works!");
        /* This next line is probably not needed.  "this" and "arguments" are
            special JS variables.
        originalFunction.apply (this, arguments);
        */
        originalFunction (true);  //-- Simplified call is probably sufficient.
    }
}