CALayer

UIView内部会自动创建一个图层就是CALayer对象，通过UIView的layer属性可以访问到这个层；当UIView需要显示到屏幕上，会调用drawRect: 方法进行绘图，并且会将所有内容绘制在自己的图层上，绘图完毕后，系统会将图层拷贝到屏幕上，于是就完成了UIView的显示；

- (void)myRedView

{

RedView *red = [[RedView alloc] initWithFrame:CGRectMake(100, 100, 200, 200)];

red.backgroundColor = [UIColor whiteColor];

red.layer.cornerRadius = 50;

red.layer.borderWidth = 5;

red.layer.borderColor = [UIColor orangeColor].CGColor;

red.layer.contents = (id)[UIImage imageNamed:@"=0"].CGImage;

red.layer.masksToBounds = YES;

red.layer.shadowColor = [UIColor blackColor].CGColor;

red.layer.shadowOffset = CGSizeMake(20, 15);

red.layer.shadowOpacity = 0.5;

[self.view addSubview:red];

}