Knowing when -viewWillAppear triggered but -viewWillDisappear hasn't

Question

Is there a way to know that a view controller is somewhere in the view controller lifecycle between -viewWillAppear and -viewWillDisappear?

I ask because I want to be damned sure that a method isn't fired when my view is either not on screen, or is about to disappear from the screen. When the view is about to disappear from the screen, certain objects which I cannot explicitly check at runtime may or may not be deallocated, so, obviously, interacting with them can lead to message sent to deallocated instance errors.

At present, I'm keeping track with a BOOL, like so:

- (void)viewWillAppear:(BOOL)animated {
    isOnScreen = YES;
    [super viewWillAppear:animated];
}

- (void)willWillDisappear:(BOOL)animated {
    isOnScreen = NO;
    [super viewWillAppear:animated];
}

And my method looks like this:

if (isOnScreen) [self doSomething];

Is there a simpler way to do this?

Answer 1

Use viewDidDisappear

- (void)viewDidDisappear:(BOOL)animated {
    // Do stuff that has to be done when this view is off screen.
}

Answer 2

your way seems to be the simplest approach, if not the most robust. (simply checking if that instance of the view exists seems like the correct approach (if it hasn't been dealloced yet)).

I also don't REALLY understand the purpose of this, unless you have another view controller running methods that pertain to the view controller being showed that you are using the boolean for. In that case, its more a design problem than an upkeep problem.