How to detect Facebook Like button pressed and trigger event?

Here is the example:

• http://www.facebook.com/UEvents?v=app_4949752878&ref=ts

These guys get people to click on Like button (in Step 1) first before letting the user download the video in Step 2.

Intere开发者_Python百科stingly, the Like button was detected and the page refreshed to show the blurred out image in Step 2 so user can click through to download.

And it seems to remember that I click on Like (when I reload the page) so the Like button does not show any more and it just only shows Step 2.

What is their trick of the trade?

When a Facebook tab is loaded, the fb_sig_is_fan parameter is passed in specifying if the current user viewing the tab is a fan. Clicking on the Like button will trigger a reload of the tab content, resulting in an updated fb_sig_is_fan being passed in so the app can decide to show a different image.

Actually information if user is page fan or not exists in signed request. Since you need firstly to decode signed request like it is here:

 $signed_request = $_REQUEST["signed_request"];
 list($encoded_sig, $payload) = explode('.', $signed_request, 2); 
 $data = json_decode(base64_decode(strtr($payload, '-_', '+/')), true);

And after that access page liked variable:

$pageLiked = $data['page']['liked'];

Now FB is on the way of deprecating FBML so don't consider <fb:visible-to-connection> as something that will work for a long base.

http://developers.facebook.com/docs/reference/javascript/FB.Event.subscribe/

FB.Event.subscribe('edge.create', function(response) {
// do something with response.session
});

One can detect when a user clicks on like button using edge.create. Also if the user has already liked the page, it can detected using FQL Like Table.

<fb:visible-to-connection>
put here code for users that like the page (no <fb:comments>) 
</fb:visible-to-connection>