tableview 中的imageview 有时候可能需要裁剪 或得想要的尺寸 

cell.bigPic.contentMode = UIViewContentModeScaleAspectFill;

cell.bigPic.clipsToBounds = YES;

cell.bigPic.userInteractionEnabled = YES;

可选择的枚举值

typedef NS_ENUM(NSInteger, UIViewContentMode) {

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,

};