iOS Programming: The Big Nerd Ranch Guide, Second Edition

For the More Curious: Redrawing Views

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

When a view has marked itself for re-display, it is not immediately redrawn; instead, it is added to a list of views that need updating. Your application is a giant infinite loop called the run loop. The run loop’s job is to check for input (a touch, Core Location updates, data coming in through a network interface, etc.) and then find the appropriate handlers for that event (like an ...