A major part of a view controller’s job is to know how to rotate the view. The user will experience this as rotation of the app itself: the top of the app shifts so that it is oriented against a different side of the device’s display. There are two complementary uses for rotation:
The app rotates to compensate for the orientation of the device, so that the app appears right way up with respect to how the user is holding the device. The challenge of compensatory rotation stems, quite simply, from the fact that the screen is not square. This means that if the app rotates 90 degrees, the interface no longer fits the screen, and must be changed to compensate.
The app rotates when a particular view appears in the interface, or when the app launches, to indicate that the user needs to rotate the device in order to view the app the right way up. This is typically because the interface has been specifically designed, in the face of the fact that the screen is not square, to appear in one particular mode (portrait or landscape).
In the case of the iPhone, no law says that your app has to perform compensatory rotation. Most of my iPhone apps do not do so; indeed, I have no compunction about doing just the opposite, forcing the user to rotate the device differently depending on what view is being displayed. The iPhone is small and easily reoriented with a twist of the user’s wrist, and it has a natural right way up, especially because it’s ...