Why does drawRect: work without calling [super drawrect:rect]?

Question

I\'m overriding drawRect: in one of my views and it works even without calling [super drawrect:rect]. How does that work?

- (void)drawRect:(CGRect)rect{  
    C         


        

Answer 1

Basically, it works because the machinery that sets up the graphics context for drawing, etc, doesn't live in the UIView implementation of -drawRect: (which is empty).

That machinery does live somewhere, of course, let's pretend in every UIView there's some internal method like this-- names of methods, functions, and properties are invented to protect the innocent:

- (void)_drawRectInternal:(CGRect)rect
{
    // Setup for drawing this view.
    _UIGraphicsContextPushTransform(self._computedTransform);

    // Call this object's implementation of draw (may be empty, or overridden).
    [self drawRect:rect];

    // draw our subviews: 
    for (UIView * subview in [self subviews]) {
        [subview _drawRectInternal:rect];
    }

    // Teardown for this view.
    _UIGraphicsContextPopTransform();
}

This is a cleaner way of building the frameworks than relying on a subclasser to do the right thing in a common override, where getting it wrong would have result in very puzzling bad behavior. (And this wouldn't really be possible at all since the draw might need a setup and a teardown step.)