ios – 如何使用完成处理程序打开FBSession,该处理程序不会被保留并在每次会话状态更改时调用？

传递给方法的完成块将始终在发生任何状态更改时使用,然后立即被清除,以便在发生另一个状态更改时忽略它.

typedef void(^FacebookLoginCompletionBlock)(id<FBGraphUser> user);

- (void)loginToFacebookFromWithCompletionBlock:(FacebookLoginCompletionBlock)completionBlock
{
    // If the user is not logged into Facebook attempt to open a session with "publish_actions" permissions.
    if([[FBSession activeSession] isOpen] == NO)
    {
        // Copy the completion block to a local variable so it can be nil'd out after it is used to prevent retain loops.
        __block FacebookLoginCompletionBlock copiedCompletionBlock = completionBlock;

        [FBSession openActiveSessionWithPublishPermissions:@[ @"publish_actions" ] defaultAudience:FBSessionDefaultAudienceFriends allowLoginUI:YES completionHandler:^(FBSession *session,NSError *error)
            {
                // Only attempt to run any of this code if there is a completion block to call back to. If completion block is nil than it has already been used and this is a state change that we do not care about.
                if(copiedCompletionBlock != nil)
                {
                    // Because this method is only concerned with the user logging into Facebook just worry about the open state occuring with no errors.
                    if(status == FBSessionStateOpen && error == nil)
                    {
                        // If the user successfully logged into Facebook download their basic profile information so the app can save the information to display to the user what account they are logged in under.
                        [FBRequestConnection startForMeWithCompletionHandler:^(FBRequestConnection *connection,NSError *error)
                            {
                                if(copiedCompletionBlock != nil)
                                    copiedCompletionBlock(user);

                                // nil out the copied completion block so it is not retained and called everytime the active FBSession's state changes.
                                copiedCompletionBlock = nil;
                            }];
                    }
                    // Because this method is only concerned with the user logging into Facebook if any other state is triggered call the completion block indicating that there was a failure.
                    else
                    {
                        if(copiedCompletionBlock != nil)
                            copiedCompletionBlock(nil);

                        // nil out the copied completion block so it is not retained and called everytime the active FBSession's state changes.
                        copiedCompletionBlock = nil;
                    }
                }

                // This block will exist the lifespan of the application because for some bizarre reason Facebook retains the completion handler for their open active session methods. Throw in some code that will display an error to the user if any session state changes occur that Facebook thinks the user should be aware of. Your code should be always checking if a active Facebook session exists before taking any action so not being aware of these changes should not be any issue. Worst case scenario you can listen for FBSessionDidSetActiveSessionNotification,FBSessionDidUnsetActiveSessionNotification,FBSessionDidBecomeOpenActiveSessionNotification or FBSessionDidBecomeClosedActiveSessionNotification notifications.
                if ([error fberrorShouldNotifyUser] == YES)
                {
                    NSString *alertTitle = @"Error logging into Facebook";
                    NSString *alertMessage = [error fberrorUserMessage];

                    if ([alertMessage length] == 0)
                        alertMessage = @"Please try again later.";

                    UIAlertView *alertView =  [[UIAlertView alloc] initWithTitle:alertTitle message:alertMessage delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil];

                    [alertView show];
                }
            }];
    }
    // If the user is already logged into Facebook immediately call the completion block with the user object that should have been saved when the user previously logged in.
    else
    {
        if(completionBlock != nil)
            completionBlock([self facebookUser]);
    }
}