iOS Programming: The Big Nerd Ranch Guide

Redrawing Views

When a UIView instance receives the message setNeedsDisplay, it will redraw its image. View subclasses send themselves setNeedsDisplay when their drawable content changes. For example, a UILabel will mark itself for re-display if it is sent the message setText:. (It has to redraw its image if the text it displays changes.)

The redrawing of a view’s image is not immediate; instead, it is added to a list of views that need updating. To understand when views actually get redrawn, we need to talk about the run loop – the infinite loop that comprises an iOS application. The run loop’s job is to listen for input (a touch, Core Location updates, data coming in through a network interface, etc.) and then find the appropriate handlers ...