将UILabel的图层添加到另一个图层（单独的UIView）

问题描述

我试图理解为什么我不能将UILabel的图层作为子图层添加到单独的UIView对象中的另一个图层。

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];
}

但是，如果我要创建一个CATextLayer，它可以正常工作。 （请参阅注释掉的代码）

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

我的理解是，每个UIView子类都由根CALayer支持。
我是否应该不能将该根CALayer添加到另一个CALayer的子层层次结构中？

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 ?

感谢您的帮助

推荐答案

numberLayer是指向numberLabel.layer的指针，因此它是单个实例-图层或UIView的单个实例只能是一个父级的子级，而不是多个。

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.

这篇关于将UILabel的图层添加到另一个图层（单独的UIView）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！