ios截取图片，并保持图片原有比例，同时填充Imageview

ios截取图片，并保持图片原有比例，同时填充Imageview：

 _imageBig.autoresizingMask = UIViewAutoresizingFlexibleHeight| UIViewAutoresizingFlexibleWidth;
 _imageBig.clipsToBounds = YES;
 _imageBig.contentMode = UIViewContentModeScaleAspectFill;