Question

I’m using CALayers to draw to a UITableViewCell. I’m trying to figure out how layers are ordered with the content of the UITableViewCell. For instance:

1. I add labels to the UITableViewCell in my cellForRow:atIndexPath method
2. In the drawRect method of UITableViewCell I draw some content using the current context
3. Also, in drawRect I add a few sublayers

So what would be the order of these elements. I know I have zPosition on the CALayers but I’m not sure if they are always on top of any subviews of the UITableViewCell. And I’m not sure where the content that is drawn in drawRect comes in the order. Any help or links to documentation would be great. I have read through the Core Animation Programming Guide and didn’t see anywhere where this would be answered.

Answer 1

1. Your UITableViewCell (or derived class) derives from UIView. It owns one CALayer which you can access via the layer property. This layer takes up the whole area of the view. The UILabels you add to your cell view will be children of your cell’s UIView and their CALayer will be a child of the cell’s CALayer. Child layers are drawn in front of their parents.
2. When you draw anything in UIView’s drawRect method, the CALayer the UIView owns will become backed by a bitmap. Your UILabels will obscure this bitmap partially or fully depending on how big they are.
3. Any child CALayers you add to your cell’s layer later will be siblings of the UILabel’s layers and since you added them later they will be added under them by default (there are versions of the method that adds them which can control ordering though).

Note that drawing in a UITableViewCell and using child views at the same time isn’t recommended for performance reasons. Having more than 4 child views/layers isn’t recommended for the same reason.