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。
最后编辑于
©著作权归作者所有,转载或内容合作请联系作者
  • 序言:七十年代末,一起剥皮案震惊了整个滨河市,随后出现的几起案子,更是在滨河造成了极大的恐慌,老刑警刘岩,带你破解...
    沈念sama阅读 210,914评论 6 490
  • 序言:滨河连续发生了三起死亡事件,死亡现场离奇诡异,居然都是意外死亡,警方通过查阅死者的电脑和手机,发现死者居然都...
    沈念sama阅读 89,935评论 2 383
  • 文/潘晓璐 我一进店门,熙熙楼的掌柜王于贵愁眉苦脸地迎上来,“玉大人,你说我怎么就摊上这事。” “怎么了?”我有些...
    开封第一讲书人阅读 156,531评论 0 345
  • 文/不坏的土叔 我叫张陵,是天一观的道长。 经常有香客问我,道长,这世上最难降的妖魔是什么? 我笑而不...
    开封第一讲书人阅读 56,309评论 1 282
  • 正文 为了忘掉前任,我火速办了婚礼,结果婚礼上,老公的妹妹穿的比我还像新娘。我一直安慰自己,他们只是感情好,可当我...
    茶点故事阅读 65,381评论 5 384
  • 文/花漫 我一把揭开白布。 她就那样静静地躺着,像睡着了一般。 火红的嫁衣衬着肌肤如雪。 梳的纹丝不乱的头发上,一...
    开封第一讲书人阅读 49,730评论 1 289
  • 那天,我揣着相机与录音,去河边找鬼。 笑死,一个胖子当着我的面吹牛,可吹牛的内容都是我干的。 我是一名探鬼主播,决...
    沈念sama阅读 38,882评论 3 404
  • 文/苍兰香墨 我猛地睁开眼,长吁一口气:“原来是场噩梦啊……” “哼!你这毒妇竟也来了?” 一声冷哼从身侧响起,我...
    开封第一讲书人阅读 37,643评论 0 266
  • 序言:老挝万荣一对情侣失踪,失踪者是张志新(化名)和其女友刘颖,没想到半个月后,有当地人在树林里发现了一具尸体,经...
    沈念sama阅读 44,095评论 1 303
  • 正文 独居荒郊野岭守林人离奇死亡,尸身上长有42处带血的脓包…… 初始之章·张勋 以下内容为张勋视角 年9月15日...
    茶点故事阅读 36,448评论 2 325
  • 正文 我和宋清朗相恋三年,在试婚纱的时候发现自己被绿了。 大学时的朋友给我发了我未婚夫和他白月光在一起吃饭的照片。...
    茶点故事阅读 38,566评论 1 339
  • 序言:一个原本活蹦乱跳的男人离奇死亡,死状恐怖,灵堂内的尸体忽然破棺而出,到底是诈尸还是另有隐情,我是刑警宁泽,带...
    沈念sama阅读 34,253评论 4 328
  • 正文 年R本政府宣布,位于F岛的核电站,受9级特大地震影响,放射性物质发生泄漏。R本人自食恶果不足惜,却给世界环境...
    茶点故事阅读 39,829评论 3 312
  • 文/蒙蒙 一、第九天 我趴在偏房一处隐蔽的房顶上张望。 院中可真热闹,春花似锦、人声如沸。这庄子的主人今日做“春日...
    开封第一讲书人阅读 30,715评论 0 21
  • 文/苍兰香墨 我抬头看了看天上的太阳。三九已至,却和暖如春,着一层夹袄步出监牢的瞬间,已是汗流浃背。 一阵脚步声响...
    开封第一讲书人阅读 31,945评论 1 264
  • 我被黑心中介骗来泰国打工, 没想到刚下飞机就差点儿被人妖公主榨干…… 1. 我叫王不留,地道东北人。 一个月前我还...
    沈念sama阅读 46,248评论 2 360
  • 正文 我出身青楼,却偏偏与公主长得像,于是被迫代替她去往敌国和亲。 传闻我的和亲对象是个残疾皇子,可洞房花烛夜当晚...
    茶点故事阅读 43,440评论 2 348

推荐阅读更多精彩内容