When do I need to call setNeedsDisplay in iOS?

Question

When creating an iOS app, I\'m confused as to when exactly I need to call setNeedsDisplay? I know that it has something to do with updating/redrawing the UI; h

Answer 1

setNeedsDisplay: should be called when you want to refresh your view explicitly. It just sets an internal flag, and the iOS UI system will call drawRect: at an appropriate time later.

It sounds like it should be always called when you updating any property which may change the presentation. But it's not. Almost all the standard UI controls already handled that. I believe whenever you modify the properties of standard UI components (views), setNeedsDisplay: would be triggered internally, and the affected region will be redrawn. (In all the situations you listed)

However, if you create your own view, implement its own drawRect:, and want to update that when something has been changed, you must call setNeedsDisplay: explicitly.