Content Modes
Each view has a content mode that controls how the view recycles its content in response to changes in theview’s geometry and whether it recycles its content at all. When a view is first displayed, it renders its contentas usual and the results are captured in an underlying bitmap. After that, changes to the view’s geometry donot always cause the bitmap to be recreated. Instead, the value in the contentMode property determineswhether the bitmap should be scaled to fit the new bounds or simply pinned to one corner or edge of theview.
詳情可以查看View Programming Guide裏的Content Mode
各個選項都很簡單 Scale To Fill 和 Aspect Fit的區別就是方向上的適應整個View
Redraw：to redraw themselvesduring scaling and resizing operations
Setting your view’s content mode to this value forces the system tocall your view’s drawRect: method in response to geometry changes. In general, you should avoid using thisvalue whenever possible, and you should certainly not use it with the standard system views.
view Mode 有下面這幾個選項:
