Question

I ran into the strangest of problems: sometimes adding a sublayer to a CALayer does not work ([layer addSublayer:child]), i.e. the sublayer count remains at 0 and nothing is drawn to the screen.

Sometimes it does work and the child layer shows up on the screen as I would have expected. When it does not, usually performing a clean build fixes it, but not always. I also experimented with running two instances of my app from the Terminal, usually the second instance shows the problematic behavior.

Here is the (abbreviated) code:

   NSLog(@"Retain count before: %d", [childLayer retainCount]);
   // [parentLayer insertSublayer:childLayer atIndex:0]; // same problem here
   [parentLayer childLayer];
   NSLog(@"Retain count after: %d layer: %@", [childLayer retainCount], childLayer);
   NSLog(@"Sublayercount: %d", [[parentLayer sublayers] count]);

The child layer is a basic layer with the background set to a RGB color.

Debug output:

layer: <CALayer: 0x1982b0> X: 50.00 Y: -90.00 width: 200.00 height: 200.00
Retain count before: 2
Retain count after: 2 layer: <CALayer: 0x1982b0>
Sublayercount: 0

When it does work, the above output shows the retain count being increased correctly to 3 and the sublayercount to 1.

Is there something special about CALayers? Does addSublayer rely on something else/perform validity checks of the sublayer?

Can anybody suggest a way to debug this? Is there a way how I can step into addSublayer with the debugger?

Thanks! Mark.

Answer 1

I finally figured it out. Sometimes it helps to just talk (well, post) about the issue.

It turned out to be a problem with the way I initialized the custom view that holds the layers. There was a race condition between the awakeFromNib calls to the app controller and the view. The app controller would load the custom layers, but sometimes the custom view was not yet initialized.

The solution was to move the initialization code for the view in the app controller from awakeFromNib to appliationDidFinishLaunching.