UIImageView的基本用法

   UIViewContentModeScaleToFill,
    UIViewContentModeScaleAspectFit,      //
contents scaled to fit with fixed aspect. remainder is transparent
    UIViewContentModeScaleAspectFill,     //
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,
    UIViewContentModeTopRight,
    UIViewContentModeBottomLeft,
    UIViewContentModeBottomRight,