Positioning a Sublayer

Figure 3-1

Layer coordinate systems and positioning are similar to those of views. A layer’s own internal coordinate system is expressed by itsbounds, just like a view; its size is its bounds size, and its bounds origin is the internal coordinate at its top left.
However, a sublayer’s position within its superlayer is not described by its center, like a view; a layer does not have a center. Instead, a sublayer’s position within its superlayer is defined by a combination of two properties, its position and its anchorPoint. Think of the sublayer as pinned to its superlayer; then you have to say both where the pin passes through the sublayer and where it passes through the superlayer. (I didn’t make up that analogy, but it’s pretty apt.)
position
A point expressed in the superlayer’s coordinate system.
anchorPoint
Where the position point is located, with respect to the layer’s own bounds. It is a pair of floating-point numbers (a CGPoint) describing a fraction (or multiple) of the layer’s own bounds width and bounds height. Thus, for example, {0,0} is the top left of the layer’s bounds, and {1,1} is the bottom right of the layer’s bounds.
If the anchorPoint is {0.5,0.5} (the default), the position property works like a view’s center property. A view’s center is thus a special case of a layer’s position. This is quite typical of the relationship between view properties and layer properties; the view prop‐ erties are often a simpler, more convenient, and less powerful version of the layer prop‐ erties.
A layer’s position and anchorPoint are orthogonal (independent); changing one does not change the other. Therefore, changing either of them without changing the other changes where the layer is drawn within its superlayer.
For example, in Figure 3-1, the most important point in the circle is its center; all the other objects need to be positioned with respect to it. Therefore they all have the same position: the center of the circle. But they differ in their anchorPoint. For example, the arrow’s anchorPoint is {0.5,0.8}, the middle of the shaft, near the end. On the other hand, the anchorPoint of a cardinal point letter is more like {0.5,3}, well outside the letter’s bounds, so as to place the letter near the edge of the circle.
A layer’s frame is a purely derived property. When you get the frame, it is calculated from the bounds size along with the position and anchorPoint. When you set the frame, you set the bounds size and position. In general, you should regard the frame as a convenient façade and no more. Nevertheless, it is convenient! For example, to position a sublayer so that it exactly overlaps its superlayer, you can just set the sublayer’s frame to the superlayer’s bounds.
A layer created in code (as opposed to a view’s underlying layer) has a frame and bounds of {{0,0},{0,0}} and will not be visible on the screen even when you add it to a superlayer that is on the screen. Be sure to give your layer a nonzero width and height if you want to be able to see it. Creating a layer and adding it to a superlayer and then wondering why it isn’t appearing in the interface is a common begin‐ ner error.

翻译

layer的坐标系统和放置类似于Views。一个layer自身内的坐标系通过他们的bounds表现出来。他的size就是bounds的size，他的bounds的origin是它内部的左上角的点。
然而，一个子layer的在父layer的Center是不被描述的，就像一个View；layer没有Center。取而代之的是，一个子layer的坐标在其父类中被定义为两个属性的组合，他的位置和他的锚点。想一下把子layer定位在父layer；然后你必须说所有定位穿过子layer的位置和它穿过父layer的位置。

• Position： 在父layer坐标系的点。
• anchorPoint：关于layer自己bounds 的点。这是一对浮点数（CGPoint）描述layer自身的bounds的Width或者Height的一个分数（或多个分数）。因此，例如，{0，0}是layer的左上角，{1，1}是右下角。
  一个layer的坐标和描点是各自独立的；改变其中一个不会影响到另一个。因此，改变其中任何一个不会影响到已经绘制好的layer。
  例如，在Figure 3-1，最重要的点是圆的中心点；其他所有的对象需要依靠他被安置。因此，他们都有一个共同的Position：圆心。但是他们的anchorPoint（锚点）不同。例如，箭头的anchroPoint是{0.5，0.8}，中间的轴，靠近圆心。另一方面，一个字母的主要点更像{0.5，3}，在轴的bounds外面，以便轴靠近圆的边缘。
  layer的frame是纯衍生的属性。当你获得frame，他是通过bounds的size和Position、anchorPoint计算出来的。当你给frame赋值的时候，你是给bounds的size和Position赋值了。通常，你只需要留意frame作为一个设置界面外观的就行了。
  通过代码创建一个layer（相对于View的基本layer）有一个frame和bo'un'ds{{0,0},{0,0}}，他将不会显示在屏幕上即使当你把它添加到一个显示在屏幕的父layer上。确定给你的layer设置一个非零的Width和Height，如果你想看到他。创建一个layer，将他添加到父layer，好奇他为什么不显示是每一个初学者共同的错误。