双重for循环九宫格

- (void)viewDidLoad {
    [super viewDidLoad];
    
    CGFloat margin = 30;
    CGFloat size = (CGRectGetWidth(self.view.frame) - margin * 3) / 2.0;
    for (NSInteger row = 0; row < 2; row ++) {
        for (NSInteger col = 0; col < 2; col ++) {
            UIImageView *view = [self generateImageView];
            view.frame = CGRectMake(margin + col * (size + margin),
                                    margin + row * (size + margin) + 64,
                                    size,
                                    size);
            [self.view addSubview:view];
        }
    }
}

- (UIImageView *)generateImageView {
    UIImageView *view = [[UIImageView alloc] init];
    UIColor *color = [UIColor colorWithRed:arc4random_uniform(255) / 255.0 green:arc4random_uniform(255) / 255.0 blue:arc4random_uniform(255) / 255.0 alpha:1.0];
    view.image = [self generateImageWithColor:color];
    view.userInteractionEnabled = YES;
    UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(onImageViewTap:)];
    [view addGestureRecognizer:tap];
    return view;
}

- (UIImage*)generateImageWithColor:(UIColor*)color {
    CGRect rect = CGRectMake(0.0f, 0.0f, 1.0f, 1.0f);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextFillRect(context, rect);
    UIImage *theImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return theImage;
}