We have seen that a subview moves when its superview’s bounds origin is changed. But what happens to a subview when its superview’s bounds size is changed? (And remember, this includes changing the superview’s frame size.)

Of its own accord, nothing happens. The subview’s bounds and center haven’t changed, and the superview’s bounds origin hasn’t moved, so the subview stays in the same position relative to the top left of its superview. In real life, however, that often won’t be what you want. You’ll want subviews to be resized and repositioned when their superview’s bounds size is changed. This is called layout.

The need for layout is obvious in a context such as Mac OS X, where the user can freely resize a window, potentially disturbing your interface. For example, you’d want an OK button near the lower right corner to stay in the lower right corner as the window grows, while a text field at the top of the window should stay at the top of the window, but perhaps should widen as the window widens.

There are no user-resizable windows on an iOS device, but still, a superview might be resized dynamically. For example, you might respond to the user rotating the device 90 degrees by swapping the width and height values of a view; now its subviews should shift to compensate. Or you might want to provide a reusable complex view, such as a table cell containing several subviews, without knowing its precise final dimensions in advance.

Layout is performed in two primary ways, which can ...

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