setNeedsLayout and setNeedsDisplay

Question

What is the real difference between UIView methods setNeedsLayout and setNeedsDisplay?

As usual documentation is foggy about this.

Nightrider

Answer 1

Actually the documentation is pretty clear about this:

• setNeedsLayout will layout subviews

  Call this method on your application’s main thread when you want to adjust the layout of a view’s subviews.

• setNeedsDisplay will call for a redraw of your view (drawRect:, etc).

  You can use this method or the setNeedsDisplayInRect: to notify the system that your view’s contents need to be redrawn.

Ladylove

no, I was wondering more in terms of both are used to redraw the view, right? Both will wait until the next draw cycle to be redrawn, right? – Nightrider

Layout is not about drawing. Layout is about positioning/sizing subviews. But yes, both delay to the next runloop cycle. So calling one multiple times in a row will only result in one relayout/redraw. – Ladylove

Please provide an example of when setNeedsDisplay is needed. – Meshuga

Call setNeedsDisplay to make sure your view redraws itself. A common use case is to call this after changing a property of your class that's used during drawing. – Ladylove

@Meshuga Say you want to draw a line between two dots (two UIView, exactly), this is implemented in drawRect: of DrawLineView. After you change the position of one button or both of them, you need to call [drawLineView setNeedsDisplay]; to redraw the line between new position of the two dots. – Ordinal

@Meshuga blog.fujianjin6471.com/2015/06/11/… – Ordinal