-(void)viewMethod{    //1: 要明白uiview内部是这样实现    CALayer *layer1= [CALayer layer];    layer.delegate=self;    [layer1 setNeedsDisplay];    [self.layer addSublayer:layer];}- (void)drawRect:(CGRect)rect{    //contex和ctx是相同的，所以在view画的东西都在上下文中。  CGContextRef context=  UIGraphicsGetCurrentContext();    // Drawing code}-(void)drawLayer:(CALayer *)layer inContext:(CGContextRef)ctx{        [self drawRect:layer.frame];    //然后拿到 drawRect画的东西，放在图层上。}