When should I request a Facebook access token?

Facebook changed their Graph API recently to require an access token. Therefore I quickly made a Facebook application to be able to retrieve an access token, use a URL similar to the one below, generated my access token and implemented the token in my iPhone application. Lately I have been thinking about if I have done something wrong. Should I request the access token everytime a user of my application retrieves data from the Graph API? I am starting to doubt whether or not the access token is unique to each user or just to the Facebook application.

Can I hardcode an 开发者_JAVA技巧access token into my application or should I make a request on the URL below every time I need to retrieve data through the Graph API?

I am generating my access token with the following URL:

https://graph.facebook.com/oauth/access_token?client_id=MYID&client_secret=*MY_SECRET*&grant_type=client_credentials

Access token granted by user have expiration time. If token expired then you need to request it again. But if you ask user to grant you "offline_access" permissions then you'll have almost not limited access token. You can store it in database or file and use next time.

Here FB documentation about permissions: http://developers.facebook.com/docs/authentication/permissions/

Enables your app to perform authorized requests on behalf of the user at any time. By default, most access tokens expire after a short time period to ensure applications only make requests on behalf of the user when the are actively using the application. This permission makes the access token returned by our OAuth endpoint long-lived.