Is there a UIView resize event?

As Uli commented below, the proper way to do it is override layoutSubviews and layout the imageViews there.

If, for some reason, you can't subclass and override layoutSubviews, observing bounds should work, even when being kind of dirty. Even worse, there is a risk with observing - Apple does not guarantee KVO works on UIKit classes. Read the discussion with Apple engineer here: When does an associated object get released?

original answer:

You can use key-value observing:

[yourView addObserver:self forKeyPath:@"bounds" options:0 context:nil];

and implement:

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if (object == yourView && [keyPath isEqualToString:@"bounds"]) {
        // do your stuff, or better schedule to run later using performSelector:withObject:afterDuration:
    }
}

In a UIView subclass, property observers can be used:

override var bounds: CGRect {
    didSet {
        // ...
    }
}

Without subclassing, key-value observation with smart key-paths will do:

var boundsObservation: NSKeyValueObservation?

func beginObservingBounds() {
    boundsObservation = observe(\.bounds) { capturedSelf, _ in
        // ...
    }
}

Create subclass of UIView, and override layoutSubviews

Swift 4 keypath KVO -- This is how I detect autorotate and moving to iPad side panel. Should work work any view. Had to observe the UIView's layer.

private var observer: NSKeyValueObservation?

override func viewDidLoad() {
    super.viewDidLoad()
    observer = view.layer.observe(\.bounds) { object, _ in
        print(object.bounds)
    }
    // ...
}
override func viewWillDisappear(_ animated: Bool) {
    observer?.invalidate()
    //...
}

You can create a subclass of UIView and override the

setFrame:(CGRect)frame

method. This is the method called when the frame (i.e. the size) of the view is changed. Do something like this:

- (void) setFrame:(CGRect)frame
{
  // Call the parent class to move the view
  [super setFrame:frame];

  // Do your custom code here.
}

Pretty old but still a good question. In Apple's sample code, and in some of their private UIView subclasses, they override setBounds roughly like:

-(void)setBounds:(CGRect)newBounds {
    BOOL const isResize = !CGSizeEqualToSize(newBounds.size, self.bounds.size);
    if (isResize) [self prepareToResizeTo:newBounds.size]; // probably saves 
    [super setBounds:newBounds];
    if (isResize) [self recoverFromResizing];
}

Overriding setFrame: is NOT a good idea. frame is derived from center, bounds, and transform, so iOS will not necessarily call setFrame:.

If you're in a UIViewController instance, overriding viewDidLayoutSubviews does the trick.

override func viewDidLayoutSubviews() {
    // update subviews
}