Question

I have to write an extension for saving user credentials to my database server (like in LastPass). How can I catch submit action from authentication form on active tab to grab entered login and password for saving using my extension and show dialog to a user, if he wants to save credentials or not?

Answer 1

Add a content script in your manifest and set it to run on some webpage:

"content_scripts": [{
    "matches": ["http://*.example.com/*"],
    "js": ["form_submits.js"],
    "run_at": "document_start"
}]

Also add permissions:

"permissions": [
    "tabs", "http://*.example.com/*"
],

In this content script listen to event “onsubmit” on forms, then contact with your extension using chrome.extension.sendRequest():

for (var i = 0; i < document.forms.length; i++) {
    document.forms[i].addEventListener("submit", function(){
        var form = this.form;
        var data = [form.elements["user"], form.elements["password"]];
        // contact with your background page and send the form data.
        chrome.extension.sendRequest({'name':'form_submit', 'data':data}, function(){ /* callback from request */ }, false);
    });
}

In your background script add listener to these calls:

chrome.runtime.onMessage.addListener(function(request, sender, callback) {
    switch (request.name) {
         case 'form_submit':
             var data = request.data;
             // do something with your form credentials.
             break;
     }
});

****Edited.****

You could also execute JavaScript directly on current active Tab and add form submit listeners:

First get the current window and active tab:

chrome.windows.getCurrent(function callback)
chrome.tabs.getSelected(integer windowId, function callback)

http://code.google.com/chrome/extensions/windows.html#method-getCurrent

http://code.google.com/chrome/extensions/tabs.html#method-getSelected

Then execute JavaScript in it:

chrome.tabs.executeScript(integer tabId, object details, function callback)

http://code.google.com/chrome/extensions/tabs.html#method-executeScript

A comprehensive Google Chrome extensions API is here:

http://code.google.com/chrome/extensions/getstarted.html