Chrome Extension - Alert on Tab Switch

Question

I am new to developing chrome extensions. I simply want to alert the user when tab switch / window switch takes place. I have made a content_script for all URLs.

I am using chrome API chrome.tabs.query to get the URL content.

My manifest.json file is as :

 {
//first three required lines
"manifest_version": 2,
"name" : "Epoch",
"version" : "1.0",

"description":"Chrome extension for alert on tab switch",
"author":"sharique",

"permissions": ["tabs"],


"content_scripts": [
{
    "matches": [
    "<all_urls>"
    ],
    "js": ["content.js"]
}
],

 "browser_action":{

    "default_popup":"popup.html"
}
}

content.js is as :

chrome.windows.onFocusChanged.addListener(function() {

    chrome.tabs.query({
      active: true,
      currentWindow: true
},

function(tabs) {

     var tabURL = tabs[0].url;
     var current_tab = [];
     current_tab = tabURL.split('/');
     alert("You are switched to : " + current_tab[2]);
});

});

popup.html is blank now.

I read it here that there is a bug into this listener. So is there any another way to detect tab changes in chrome ?

Going throw chrome extension document is pretty hectic, please suggest me some methods or provide some links in the comment so that I can proceed my maiden development to chrome extension.

Thank you.