determine if uiview is displayed

is there a possibility to determine if an uiview obj is going to be displayed. imagine: you have 2 uiviews in an uiscrollview. now you are going to switch per gesture from the first view to the second. the first vi开发者_如何学运维ew now is NOT in the viewport. now you are going to go back to the first view. and now I want to be notified that this view is in viewport, or is redisplayed. the same has to be for the second view. I have not found any callback or something like this.

You make sure your UiViewController overrides viewWillAppear: (before it appears this method is called) or viewDidAppear: (after this method is called).

See: http://developer.apple.com/iphone/library/documentation/uikit/reference/UIViewController_Class/Reference/Reference.html#//apple_ref/occ/instm/UIViewController/viewWillAppear:

That depends what you mean by "switch". If one view is just scrolled out of the visible area of the scrollview, but still remains attached as a subview to it, then you may want to check if the bounds of your view overlap those of the scrollviews visible area.

You could do this by using UIScrollView Delegate's scrollViewDidScroll: method to implement a check for overlaps while the user is scrolling.

If however your view is actually removed from the viewstack, then you may want to subclass UIView and implement willMoveToSuperview: to check if the view has been added to the scrollview again.