iOS图片按比例显示

只需加这么一段代码，如下：

imageView.contentMode = UIViewContentModeScaleAspectFit;
imageView.autoresizesSubviews = YES;
imageView.autoresizingMask = UIViewAutoresizingFlexibleLeftMargin | UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;