防止图片变形

UIImageView *imageView = [[UIImageView alloc] init];

// 防止图片变形

imageView.contentMode = UIViewContentModeScaleAspectFill;

imageView.clipsToBounds=YES;//  是否剪切掉超出 UIImageView 范围的图片

[imageView setContentScaleFactor:[[UIScreen mainScreen] scale]];

_imageView = imageView;

[self addSubview:imageView];

防止图片压缩的处理