從圖中可以看出，這里的 rect 不受控制，只根據 UIImageView 的大小有關

- (void)viewDidLoad {
    [super viewDidLoad];
 
    UIImageView * imgView = [[UIImageView alloc] initWithImage:[self viewImageFromColor:[UIColor redColor]
                                                                                     rect:CGRectMake(0, 0, 20, 20)]];
    imgView.frame = CGRectMake(100, 100, 200, 200);
    imgView.backgroundColor = [UIColor cyanColor];
    [self.view addSubview:imgView];
}

- (UIImage *)viewImageFromColor:(UIColor *)color rect:(CGRect)rect{
    
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [color CGColor]);
    CGContextFillRect(context, rect);
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    
    return image;
}

image.png