如果 :(为什么没有赋值？）(If : (Why is there no value assigned?))

 在你的问题中有两个问题： 
 
 
 
 上面的代码不起作用。 为什么？ 
 
 
 起初，我认为文本没有出现，因为它是用白色绘制的。 虽然它可能仍然如此，但这不是唯一的问题。 
 
 您将文本绘制到（r）中的矩形与您正在绘制的图层的边界相关。即，如果要绘制图层的左上角，则矩形的原点应为（0， 0）。 上面的代码仍然没有工作。 
 
 如果你检查CGSize drawInRect：withFont：... returns（渲染文本的大小），你可以看到渲染文本的大小是0。 这表示矩形绘制的高度为100是不够的。 增加矩形的高度解决了这个问题，现在文本是可见的。 如果你在图层中绘制一些文本，也许最明显的矩形是图层的边界（ CGRect r = [layer bounds]; ）。 
 
 我做了什么来实现它的工作： 
 
 
 
 改变了文字颜色 
 
 
 将r原点更改为（0,0） 
 
 
 r高度增加 
 
 
 
 
 有什么不同 [...] ？ 
 
 
 首先，你要将文本绘制到图层的图形上下文中（drawLayer：inContext：应该做什么），但在秒示例中，您将向图层的层次结构中添加另一图层，而不是在图层中绘制图层。 每当您的图层重新绘制时，都会添加一个新的文本图层。 所以实际上没有任何东西被绘制到图层上下文中。 
 
 接下来发生的事情是，在图层自己绘制之后（空），图层的子图层将自己绘制出来，并且它们的内容将在图层顶部进行组合。 这就是为什么你会看到文字图层的文字。 
 
 如果您添加这样的文本图层，您可以通过不必进行任何手动绘图来提高代码效率。 这就要求文本图层在某个早期点（只有一次）被添加到图层中，并且您不需要执行任何自定义绘图（它通常很慢）。 

There are two questions in you question:
 
 
 
The above code is not working. Why?
 
 
At first I thought that the text didn't show up because it was drawn with white on white. While it may still have been so, that is not the only problem. 
 
The rectangle that you are drawing your text into (r) is relative to the bounds of the layer you are drawing in. I.e. if you want to draw in the top left corner of the layer, the origin of the rect should be (0, 0). The above code is still not working though. 
 
If you inspect the CGSize that drawInRect:withFont:... returns (the size of the rendered text) you can see that the size of the rendered text is 0 wide. This is an indication that the height of 100 for the rectangle to draw in is not enough. Increasing the height of the rectangle solves this and now the text is visible. If you are drawing some text inside a layer, maybe the most sensible rectangle to draw in is the bounds of the layer (CGRect r = [layer bounds];). 
 
What I did to make it work:
 
 
 
Changed text color
 
 
Changed origin of r to (0,0)
 
 
Increased height of r
 
 
 
 
What is the difference [...] ?
 
 
In the first you are drawing text into the graphics context of the layer (what drawLayer:inContext: is supposed to do) but in the seconds example you are adding another layer to the hierarchy of the layer instead of drawing inside it. Every time your layer will redraw, a new text layer will be added to it. So nothing is actually drawn into the layers graphics context.
 
What happens next, after your layer has drawn itself (empty), is that the sublayers of your layers draw themselves and their content is composed on top of your layer. That is why you see the text of the text layer.
 
If you add a text layer like this you can make your code more efficient by not having to do any manual drawing. This requires that the text layer is added to your layer at some earlier point (only once) and that you don't do any custom drawing (it's often slow).