how to resume facebook session key after user change facebook password

i have iphone application that using facebook connect. users login to the iphone application using facebook connect. and then i receive their sessionKey back to my server, and i am using the sesssionkey to post actions to users wall.

the iphone application keeps the user logged in allways. which means that every times the user will open the application, he will not need to make login again.

the problem is that, when the user change his facebook password the user sessionKey also changes.

and when the user enter to my iphone applicaiton, the user is shown as loggedin, and then i recieve to my server the user old sessionkey, and when i try to post to the user wall facebook tells me that the session is inavlid.

it is ok because the user change 开发者_如何转开发his password and the session changes too.

my question is how can i check in the iphone if the session key has changed ? and if it changes to promote the user to make login again.

It sounds like you've answered your own question. If you have a seemingly infinite session which is suddenly invalid, just ask the user to log in again when you first discover the error.