Programming iOS 4

Bounds and Center

Suppose we wish to give a view a subview inset by 10 pixels, as in Figure 14-3. The utility function CGRectInset makes it easy to derive one rectangle as an inset from another, but what rectangle should we use as a basis? Not the superview’s frame; the frame represents a view’s position within its superview, and in that superview’s coordinates. What we’re after is a CGRect describing our superview’s rectangle in its own coordinates, because those are the coordinates in which the subview’s frame is to be expressed. That CGRect is the view’s bounds property.

Figure 14-3. A subview inset from its superview

So, the code to generate Figure 14-3 looks like this:

UIView* v1 = [[UIView alloc] initWithFrame:CGRectMake(113, 111, 132, 194)];
v1.backgroundColor = [UIColor colorWithRed:1 green:.4 blue:1 alpha:1];
UIView* v2 = [[UIView alloc] initWithFrame:CGRectInset(v1.bounds, 10, 10)];
v2.backgroundColor = [UIColor colorWithRed:.5 green:1 blue:0 alpha:1];
[self.window addSubview: v1];
[v1 addSubview: v2];
[v1 release]; [v2 release];

You’ll very often use a view’s bounds in this way. When you need coordinates for drawing inside a view, whether drawing manually or placing a subview, you’ll often refer to the view’s bounds.

Note

The screen also has bounds, and functions in that sense as the window’s superview, even though a UIScreen isn’t a view. Moreover, the window’s frame is always ...