instancetype完成工厂模式

@interface MONImage : UIImage

+ (instancetype)imageNamed:(NSString *)pName;

@end

@implementation MONImage

+ (instancetype)imageNamed:(NSString *)pName
{
    UIImage * source = [UIImage imageNamed:pName];
    CGImageRef cgImage = source.CGImage;
    if (cgImage)
        return [[self alloc] initWithCGImage:cgImage];
    // try it another way
    return nil;
}

@end