setNeedsDisplay does not trigger drawRect in subviews as expected

Question

I'm struggling with setNeedsDisplay. I thought it was supposed to trigger calls of drawRect: for the view for which it is called and the hierarchy below that if it's within the view's bounds, but I'm not finding that to be the case. Here is my setup:

From the application delegate, I create a view whose size is a square that covers essentially the whole screen real estate. This view is called TrollCalendarView. There is not much that happens with TrollCalendarView except for a rotation triggered by the compass.

There are 7 subviews of TrollCalendarView called PlatformView intended to contain 2D draw objects arranged around the center of TrollCalendarView in a 7-sided arrangement. So when the iPad is rotated, these 7 views rotate such that they are always oriented with the cardinal directions.

Each of the PlatformView subviews contains 3 subviews called Tower. Each tower contains 2D draw objects implemented in drawRect:.

So, in summary, I have TrollCalendarView with empty drawRect:, and subviews PlatformView and Platformview -> Tower that each have drawRect implementations. Additionally, Tower lies within the bounds of Platform, and Platform lies within the bounds of TrollCalendarView.

In TrollCalendarView I've added a swipe recognizer. When I swipe happens, a property is updated, and I call [self setNeedsDisplay] but nothing seems to happen. I added NSLog entries to drawRect: method in each of these views, and only the TrollCalendarView drawRect: method is called. Ironically, that is the one view whose drawRect method will be empty.

There is no xib file.

What do I need to do to ensure the drawRect method in the other subviews is called? Is there documentation somewhere that describes all the nuances that could affect this?

Answer 1

I'm struggling with setNeedsDisplay. I thought it was supposed to trigger calls of drawRect for the view for which it is called and the hierarchy below that if it's within the view's bounds

No, that is not the case. Where did you get that idea?

-setNeedsDisplay: applies only to the view to which it is sent. If you need to invalidate other views, you need to add some code to send -setNeedsDisplay: to them, too. That's all there is to it.

Answer 2

I think this is an optimization in the framework; if your subviews don't need to draw again, then this is a major performance improvement. Realize that almost anything animatable does not require drawrect (moving, scaling, etc).

If you know that all of your subviews should be redrawn (and not simply moved), then override setNeedsDisplay in your main view and do like this:

-(void) setNeedsDisplay {
    [self.subviews makeObjectsPerformSelector:@selector(setNeedsDisplay)];
    [super setNeedsDisplay];
}

I have tested this, and it causes all subviews to be redrawn as well. Please note that you will earn efficiency karma points if you somehow filter your subviews and make sure you only send that to subviews which actually need redrawn... and even more if you can figure out how not to need to redraw them. :-)