Programming iOS 6, 3rd Edition

Drawing, Animation, and Threading

When you change a visible view property, even without animation, that change does not visibly take place there and then. Rather, the system records that this is a change you would like to make, and marks the view as needing to be redrawn. You can change many visible view properties, but these changes are all just accumulated for later. Later, when all your code has run to completion and the system has, as it were, a free moment, then it redraws all views that need redrawing, applying their new visible property features. Let’s call this the redraw moment. (I’ll explain what the redraw moment really is later in this chapter.)

You can see that this is true simply by changing some visible aspect of a view and changing it back again, in the same code: on the screen, nothing happens. For example, suppose a view’s background color is green. Suppose your code changes it to red, and then later changes it back to green:

// view starts out green
view.backgroundColor = [UIColor redColor];
// ... time-consuming code goes here ...
view.backgroundColor = [UIColor greenColor];
// code ends, redraw moment arrives

The system accumulates all the desired changes until the redraw moment happens, and the redraw moment doesn’t happen until after your code has finished, so when the redraw moment does happen, the last accumulated change in the view’s color is to green — which is its color already. Thus, no matter how much time-consuming code lies between the change from green ...