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,