Adding a UILabel's layer to another layer (Sep

I'm trying to understand why I can't add a UILabel's layer as a sublayer to another layer in a separate UIView object.

- (void)addNumber:(NSInteger)number toLayer:(CALayer *)layer {

  UILabel *numberLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds))];
  [numberLabel setFont:[UIFont boldSystemFontOfSize:12]];
  [numberLabel setText:[NSString stringWithFormat:@"%d", number]];
  /* if I change the BackgroundColor to an opaque color it renders as a solid black rect.
   * No matter what color I choose
   * Setting it as clear then it is transparent
   */
  [numberLabel setBackgroundColor:[UIColor clearColor]];
  [numberLabel setTextAlignment:NSTextAlignmentCenter];
  [numberLabel setTextColor:[UIColor blackColor]];

  CALayer *numberLayer = numberLabel.layer;

  /* However creating a CATextLayer is successful
  CALayer *numberLayer = [CATextLayer layer];
  [numberLayer setFont:(__bridge CFTypeRef)([UIFont boldSystemFontOfSize:12])];
  [numberLayer setBounds:CGRectMake(0, 0, CGRectGetMidX(self.bounds), CGRectGetMidY(self.bounds))];
  [numberLayer setString:[NSString stringWithFormat:@"%d", number]];
  [numberLayer setAlignmentMode:kCAAlignmentCenter];
  [numberLayer setForegroundColor:[[UIColor whiteColor] CGColor]];
  */

  [numberLayer setPosition:CGPointMake(CGRectGetMidX(layer.bounds),
                                        CGRectGetMidY(layer.bounds) + CGRectGetMidY(numberLayer.bounds))];  
  [layer addSublayer:numberLayer];
}

However, if I was to create a CATextLayer instead, it works fine. (see commented out code)

My understanding is that every UIView subclass is backed by a root CALayer. Should I not be able to add that root CALayer to the sublayer hierarchy of another CALayer ?

Thank you for your help

回答1:

numberLayer is a pointer to numberLabel.layer, so it's a single instance - a single instance of a layer or UIView can only be a child to one parent, not multiple.

回答2:

CALayer conforms to NSCoding protocol, so you can encode existing CALayer instance, then create new CALayer instance by decoding data from the first one.