UIImageView的基本用法

      UIViewContentModeScaleTo Fill,         UIViewContentModeScaleAs pectFit,            // contents scaled to fit with fixed aspect. remainder is transparent         UIViewContentModeScaleAs pectFill,          // contents scaled to fill with fixed aspect. some portion of content may be clipped.         UIViewContentModeRedraw,                            // redraw on bounds change (calls -setNeedsDisplay)         UIViewContentModeCenter,                            // contents remain same size. positioned adjusted.         UIViewContentModeTop,         UIViewContentModeBottom,         UIViewContentModeLeft,         UIViewContentModeRight,         UIViewContentModeTopLeft ,         UIViewContentModeTopRigh t,         UIViewContentModeBottomL eft,         UIViewContentModeBottomR ight,