Question

Sometimes when I try to inject JS script into UIWebView just after Facebook iOS login flow is completed, my iOS app gets frozen during fast-app-switch (from Safari to my app).

In the XCode console I get this error message:

failed to return after waiting 10 seconds. main run loop
mode:kCFRunLoopDefaultMode…

Sometimes the app doesn’t get frozen but the error message appears all the same.

I worked this around by running the JS injection code after an interval of 1 second. I implemented it as follows:

In the callback for FB session changes, I invoke my JS callback in 1 second using NSObject‘s performSelector:withObject:afterDelay: method. Here is my code sample:

/*
 * Callback for session changes.
*/
- (void)sessionStateChanged:(FBSession *)session state:(FBSessionState) state error:(NSError   *)error
{
  switch (state) {
    case FBSessionStateOpen:
      if (!error) {
        // We have a valid session
        [self performSelector:@selector(jsCallback:) withObject:session afterDelay:1];
      }
    break;

  // the rest of the method is omitted
}

- (void)jsCallback:(FBSession *)session {
     UIWebView *webView = ...
     NSString *jsScript = ...
     [webView stringByEvaluatingJavaScriptFromString:jsScript];
}

It works but I’m concerned that it is not a reliable solution. For example, in some situations 1 second might not be enough.

Any ideas how to achieve the same effect in a more reliable way?

Answer 1

Well, it turned out to be simple. I made use of UIApplicationState, like so:

/*
 * Callback for session changes.
*/
- (void)sessionStateChanged:(FBSession *)session state:(FBSessionState) state error:      (NSError   *)error
{
  switch (state) {
    case FBSessionStateOpen:
      if (!error) {
        // We have a valid session
        UIApplicationState appState = [[UIApplication sharedApplication] applicationState];

        NSLog(@"We have a valid session, app state: %u",appState);

        if (UIApplicationStateActive == appState){
            [self jsCallback:session];
        } else {
            self.fbSessionJustOpened = YES;
        }
      }
    break;

    // the rest of the method is omitted
}

In UIApplicationDelegate‘s applicationDidBecomeActive method:

- (void)applicationDidBecomeActive:(UIApplication *)application
{  
    if (self.isFbSessionJustOpened){
        self.fbSessionJustOpened = NO; // reset flag to NO        
        [self jsCallback:[FBSession activeSession]];
    }
}