Question

In my multi-tab app a thread cares about listening for messages coming from the server and it must be able to close a UIImagePickerController in case the camera is active while a specific message is received. The thread is run through NSThread detachNewThreadSelector.

I managed to invoke the right sequence of calls on the thread side through the ApplicationDelegate but the picker is not dismissed:

- (void) closeCameraController {
    [cameraTabController closeSubViewCamera];  // Invokes cancel: on the cameraSubView.
} 

The same sequence works properly when I start it through an event mapped on a button of the camera overlay (a ‘Cancel’ button):

- (IBAction) cancel {
    [[self parentViewController] dismissModalViewControllerAnimated:NO];        
}

Answer 1

From Apple’s Docs:

Note: For the most part, UIKit classes should be used only from an
application’s main thread. This is particularly true for classes
derived from UIResponder or that involve manipulating your
application’s user interface in any way.

You can use NSObject’s performSelectorOnMainThread:withObject:waitUntilDone:

- (void) closeCameraController {
    [cameraTabController performSelectorOnMainThread:@selector(closeSubViewCamera) 
           withObject:nil
        waitUntilDone:YES];
}