drawRect method get called but the UIView cannot display - ios

Question

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?