Why UIImageView hides only after delay?

Question

After a UIView is tapped, I hide it and initialize new object with UIView and Quartz drawRect.

- (void)viewTapped:(UITapGestureRecognizer *)recognizer {   
    self.vignetteView.hidden=true;
    lupeItself = [[LoupeView alloc] initWithView:_pageView setZoomImageName:_zoomPageImageName setDelegate:self];
} 

Code above is hiding the UImageView only after some 2 seconds delay. But if last line (LoupeView alloc etc.) is removed, the it is hided instantly. Why? How to make the view hide instantly?

Answer 1

The .hidden = true change will not become visible until the execution path returns to the main runloop. The second line is probably blocking for a few seconds, preventing these changes from occuring (or drawRect is taking a long time further down the pipeline).

The simplest fix would be to defer the second line until the next runloop iteration:

self.vignetteView.hidden = YES;
// defer execution so the above changes are immediately visible
[[NSOperationQueue mainQueue] addOperationWithBlock:^{
    lupeItself = [[LoupeView alloc] initWithView:_pageView setZoomImageName:_zoomPageImageName setDelegate:self];
}];

Also, a minor point: you should use the constants YES and NO for BOOL properties and arguments, instead of true and false.