Question

Is there a way to have pageAction defined for one website? I saw the samples and both of them use background.js to show chrome.pageAction.show(tabId) in that file.

Answer 1

To show a pageAction for a specific website, two methods exist.

Using a content script

Run the content script at the specific website, and pass a message to the background requesting the page action:

// contentscript.js
chrome.extension.sendMessage('showPageAction');
// background.js
chrome.extension.onMessage.addListener(function(message, sender) {
    if (message == 'showPageAction') {
        chrome.pageAction.show(sender.tab.id);
    }
});

Part of manifest.json:

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

Valid values for matches are exactly defined in the documentation, match patterns.
^{Note that the match pattern in this example matches http://example.com/ and not http://example.com/index.html. If you want to match anything on the website, use http://example.com/* (appended a asterisk).}

Using the chrome.tabs API

If you don’t want to use content scripts, you can use chrome.tabs events to show the page action:

// background.js
chrome.tabs.onUpdated.addListener(function(tabId, changeInfo, tab) {
    if (changeInfo.url == 'http://example.com/') {
        chrome.pageAction.show(tabId);
    }
});

I recommend the content script method, unless you’re already using the chrome.tabs API. Why? If you request the "tabs" permission in the manifest file, your users will see the following warning on installation:

Install <name of your extension>?
  It can access:
  Access your tabs and browsing activity