How do I check if an UIViewController is currently being displayed?

Question

How do I check if an UIViewController is currently being displayed?

My UIViewControllers are listening for NSNotifications - even if when they are not displayed (ie not shown). So I could have 10 UIViewController in the background observing NSNotifications from NSNotificationCenter. When an NSNotification is posted and received by the UIViewController, I'd like to find out if it is currently being shown. If it is not, I will just set a boolean so that it will processed when the View is presented. If it currently being display, I will do more things like update tables immediately, and so forth...

Answer 1

You need to check if your viewcontroller is on top of the stack of navigationcontroller's viewcontroller array. A sample code is,

if (self.navigationController.topViewController == self) {
    //the view is currently displayed
}

You can use this inside the viewWillAppear method to check whether the current view is visible.

Answer 2

Check to see if it's attached to the window. If it's not nil it's in hierarchy which is attached to the screen (of course it could be off the bounds of the screen, covered by some other view or have the hidden flag set)

if (myViewController.view.window) {
  // I'm attached to the window
} else {
  // not attached to the window
}

Answer 3

You can use flags in viewWillAppear and viewWillDisappear methods for this.

Answer 4

Why don't you remove the notification listener in viewWillDisappear and add it in viewWillAppear?

Edit: misread his question, sorry.

Suggested answer: set your own flag (BOOL) in viewDidDisappear and viewDidAppear.

Answer 5

Specify title to each ViewController and then get the title of current ViewController by the code given bellow.

NSString *currentController = self.navigationController.visibleViewController.title;

Then check it by your title like this

if([currentController isEqualToString:@"myViewControllerTitle"]){

    //write your code according to View controller. 

}

Answer 6

I think that checking of viewController.view.superview should works.

Answer 7

It's too late to replay on this question.

To check the instance of a UIViewController is currently on the top of the screen or to check if it is showing on screen, you can put a check like:

// Get the topmost view showing on the screen as below
    UIViewController * currentVC = ((UINavigationController*)app.window.rootViewController).visibleViewController;

// Now check whether the viewcontroller you want to show is the same as the currently showing view controller.
    if (currentVC.class == myVC.class) {  // Here myVC is any/new instance of the viewcontroller you would like to check or show (if not shown).
         // If both are same then it returns true and executes this block of code.
    }