IPhone : drawRect method get called but the UIView cannot display

ModeButtonView is a subclass of UIView, and is already added as a subview of viewController.view:

@interface ModeButtonView ()

@property(nonatomic, strong) MonthModeButtonView *monthModeButton;

@end

-(instancetype)initWithFrame:(CGRect)frame{

self = [super initWithFrame:frame];
if (!self) {
    return nil;
}

self.backgroundColor = [UIColor colorWithRed:0.192 green:0.663      blue:0.647 alpha:1];

self.monthModeButton = [[MonthModeButtonView alloc] initWithFrame:CGRectMake(0.0f, 0.0f, 0.5 * self.bounds.size.width, 0.5 * self.bounds.size.height)];

self.monthModeButton.center = self.center;
[self addSubview:self.monthModeButton];

return self;

}

The MonthModeButtonView is also a subclass of UIView, and it is added as a subview of ModeButtonView. Its drawRect method is as follows:

• 
  

  (void)drawRect:(CGRect)rect {

  
  

  NSLog(@"drawRect get called");

  
  

  CGMutablePathRef path = CGPathCreateMutable();

  
  

  CGPathMoveToPoint(path, NULL, rect.origin.x, rect.origin.y);

  
  

  CGPathAddLineToPoint(path, NULL, rect.size.width, rect.size.height);

  
  

  CGPathMoveToPoint(path, NULL, rect.size.width, rect.origin.y);

  
  

  CGPathAddLineToPoint(path, NULL, rect.origin.x, rect.size.height);

  
  

  CGContextRef currentContext = UIGraphicsGetCurrentContext();

  
  

  CGContextAddPath(currentContext, path);

  
  

  [[UIColor redColor] setStroke];

  
  

  CGContextDrawPath(currentContext, kCGPathStroke);

  
  

  CGPathRelease(path);

  
  

}

I run the program, the ModeButtonView is displayed well on the screen, and the drawRect method of MonthModeButtonView also get called, however, it cannot be displayed on the screen, while I have added it as a subview of ModeButtonView.

However, if I put the code of MonthModeButtonView's drawRect method into ModeButtonView's drawRect method directly, it get displayed. It seems the problem happens because MonthModeButtonView is a subview of MoodeButtonView, but why? Any idea about that? Thanks.