ios Setting the RoundCorner does not work after override drawRect,but normal UIView is ok

I wrote an example about Wave Animation.The animation is ok,but I don't understand why the custom UIView needs to add "self.layer.masksToBounds = YES" to have the round Corner.

1. This is a custom UIView. I have rewritten its drawRect. If i don't set "masksToBounds" to YES, the round corner disappear.

   - (instancetype)initWithFrame:(CGRect)frame
   {
       self = [super initWithFrame:frame];
       if (self) {
           self.frame = CGRectMake(10, 40, 300, 300);
           self.layer.cornerRadius = 150;
           self.backgroundColor = [UIColor greenColor];
           self.layer.borderColor = [UIColor blueColor].CGColor;
           self.layer.borderWidth = 2;
           //        self.layer.masksToBounds = YES;  //if write this line,the round corner appear
           self.x_move = 0;
           self.y_move = 300;
       }
       return self;
   }
   
   - (void)drawRect:(CGRect)rect {
   
       CGContextRef context = UIGraphicsGetCurrentContext();
       CGMutablePathRef path = CGPathCreateMutable();
   
       CGContextSetLineWidth(context, 1);
       CGContextSetFillColorWithColor(context, [UIColor grayColor].CGColor);
       CGPathMoveToPoint(path, NULL, 0, 0);
   
       for(float i=0; i<=300; i++){
           float x=i;
           float y = 5 * sin( 0.05 * x+ self.x_move) + self.y_move;
           CGPathAddLineToPoint(path, nil, x, y);
       }
   
       CGPathAddLineToPoint(path, nil, 300, 0);
       CGPathAddLineToPoint(path, nil, 0, 0);
       CGContextAddPath(context, path);
       CGContextFillPath(context);
       CGContextDrawPath(context, kCGPathStroke);
       CGPathRelease(path);
   }
   
   - (void)startAnimation {
       if (self.waveTimer == nil) {
           self.waveTimer = [NSTimer scheduledTimerWithTimeInterval:0.1 target:self selector:@selector(refresh) userInfo:nil repeats:YES];
       }
   }
   
   - (void)refresh {
       self.x_move += 0.3;
       self.y_move -= 0.2;
       if(self.y_move - 100 < 0.00001){
           [self.waveTimer invalidate];
       }else{
           [self setNeedsDisplay];
       }
   }
   

ViewController:

self.wave = [[waveAnimation alloc] init];
[self.wave startAnimation];
[self.view addSubview:self.wave];

2. This is a normal UIView. Its "masksToBunds" is NO, but its round corner shows normal. Compared with the examples above, why one should add, one does not need.

   UIView *view = [[UIView alloc] initWithFrame:CGRectMake(20, 60, 100, 100)];
   view.backgroundColor = [UIColor greenColor];
   view.layer.cornerRadius = 50;
   view.layer.borderWidth = 2;
   view.layer.borderColor = [UIColor blackColor].CGColor;
   [self.view addSubview:view];