iOS超出屏幕的尺寸的图片如何按比例缩放显示

_imageView.contentMode = UIViewContentModeScaleAspectFit;

_imageView.autoresizesSubviews = YES;

_imageView.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;