iOS 8 Auto Layout界面自动布局系列4-使用VFL添加布局约束

来源：互联网发布：docker源码分析 pdf 编辑：程序博客网时间：2024/05/15 16:36

原文来自于：http://blog.csdn.net/pucker/article/details/45093483

本系列第一篇文章介绍了自动布局的基本原理，第二篇通过一个简单的例子演示了如何使用IB以可视化方式创建自动布局约束，第三篇使用代码直接创建NSLayoutConstraint实例来定义自动布局约束。本篇文章在第三篇文章的基础上，使用Visual Format Language（暂且翻译为可视化格式语言，简称VFL）创建约束。
在第三篇文章中，我们仅仅创建了4个视图，就需要创建将近20个NSLayoutConstraint实例，而且每次创建NSLayoutConstraint实例时都需要传入7个参数（firstItem.firstAttribute {==,<=,>=} secondItem.secondAttribute * multiplier + constant），非常繁琐而且容易出错。在实际项目中，视图的层次会更复杂，约束的数量就会成倍增加，有没有办法既直观又简单地创建约束？那你不妨试试VFL，这也是这篇文章的主题。

VFL的使用非常简单直观，不过既然是一门语言，必然就有其语法要求。。。
又要学语法。。。你是逗我吗？
好吧，我们还是以之前的例子边做边讲吧。

打开Xcode，新建项目，选择iOS -> Application -> Single View Application。项目命名为AutoLayoutByVFL，语言任意选择（本篇文章使用的是Objective-C），设备选择Universal。下载苹果Logo图片apple.jpg，并将其拖入项目中。文件下载地址：
http://yunpan.cn/cfmJB82dfSwf6（提取码：4049）

界面上方用来显示苹果Logo图片的是一个UIImageView，其具有如下4个约束：
- logoImageView左侧与父视图左侧对齐
- logoImageView右侧与父视图右侧对齐
- logoImageView顶部与父视图顶部对齐
- logoImageView高度为父视图高度一半

将ViewController类的viewDidLoad方法修改如下：

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">- (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>)viewDidLoad{    [<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> viewDidLoad];    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIImageView</span>* logoImageView = [[<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIImageView</span> alloc] initWithImage:[<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIImage</span> imageNamed:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"apple.jpg"</span>]];    logoImageView<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    logoImageView<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.contentMode</span> = UIViewContentModeScaleAspectFit;    [<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.view</span> addSubview:logoImageView];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，logoImageView左侧与父视图左侧对齐，logoImageView右侧与父视图右侧对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|-0-[logoImageView]-0-|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:@{@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"logoImageView"</span>: logoImageView}];    [NSLayoutConstraint activateConstraints:hConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//垂直方向上，logoImageView顶部与父视图顶部对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* vConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"V:|-0-[logoImageView]"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:@{@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"logoImageView"</span>: logoImageView}];    [NSLayoutConstraint activateConstraints:vConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//logoImageView高度为父视图高度一半</span>    NSLayoutConstraint* heightConstraint = [NSLayoutConstraint constraintWithItem:logoImageView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.view</span> attribute:NSLayoutAttributeHeight multiplier:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.5</span>f constant:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f];    heightConstraint<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.active</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">YES</span>;}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li></ul>

viewDidLoad方法首先调用了NSLayoutConstraint类的一个静态方法constraintsWithVisualFormat:…..，根据传入的VFL字符串生成若干约束，该方法定义如下：

<code class="hljs css has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">+ (<span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">NSArray</span> *)<span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">constraintsWithVisualFormat</span><span class="hljs-pseudo" style="color: rgb(0, 0, 0); box-sizing: border-box;">:(NSString</span> *)<span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">format</span> <span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">options</span><span class="hljs-pseudo" style="color: rgb(0, 0, 0); box-sizing: border-box;">:(NSLayoutFormatOptions)opts</span> <span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">metrics</span><span class="hljs-pseudo" style="color: rgb(0, 0, 0); box-sizing: border-box;">:(NSDictionary</span> *)<span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">metrics</span> <span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">views</span><span class="hljs-pseudo" style="color: rgb(0, 0, 0); box-sizing: border-box;">:(NSDictionary</span> *)<span class="hljs-tag" style="color: rgb(0, 0, 0); box-sizing: border-box;">views</span>;</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li></ul>

参数format是一个符合VFL语法的字符串。上述代码中的H:表示本VFL字符串描述的是水平方向的约束，与之相对的是V:表示垂直方向。如果VFL字符串没有指明H:还是V:，则默认为水平方向。|表示父视图。VFL要求所有视图的名字必须放在中括号之内，[logoImageView]指代的就是logoImageView。-0-表示的是间距值为0。
所以@”H:|-0-[logoImageView]-0-|”表示在水平方向上，logoImageView左侧与其父视图左侧的间距为0，logoImageView右侧与其父视图右侧的间距为0。@”V:|-0-[logoImageView]”表示在垂直方向上，logoImageView顶部与其父视图顶部的间距为0。
另外说一句，-0-可以不写，也就是说如果间距为0则不用明确写出，所以@”H:|-0-[logoImageView]-0-|”可以精简为@”H:|[logoImageView]|”，@”V:|-0-[logoImageView]”可以精简为@”V:|[logoImageView]”，是不是很直观？
参数opts表示布局的对齐方式与方向：

<code class="hljs haskell has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-title" style="box-sizing: border-box;">typedef</span> <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NS_OPTIONS</span>(<span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSUInteger</span>, <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatOptions</span>){    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllLeft</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeLeft</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllRight</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeRight</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllTop</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeTop</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllBottom</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeBottom</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllLeading</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeLeading</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllTrailing</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeTrailing</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllCenterX</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeCenterX</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllCenterY</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeCenterY</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllBaseline</span> = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeBaseline</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllLastBaseline</span> = <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllBaseline</span>,    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignAllFirstBaseline</span> <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NS_ENUM_AVAILABLE_IOS</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>_0) = (<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutAttributeFirstBaseline</span>),    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatAlignmentMask</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0xFFFF</span>,    /* choose only one <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">of</span> these three */    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatDirectionLeadingToTrailing</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>, // <span class="hljs-default" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">default</span></span>    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatDirectionLeftToRight</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>,    <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatDirectionRightToLeft</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>,      <span class="hljs-type" style="box-sizing: border-box; color: rgb(102, 0, 102);">NSLayoutFormatDirectionMask</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0x3</span> << <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">16</span>,  };</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li></ul>

参数metrics是一个字典。其中的键（Key）是字符串，出现在VFL语句中；值（Value）是NSNumber对象。在解析VFL字符串时，编译器将自动把键替换为其对应的值，目的是使VFL更容易明白和理解。
另外，在解析VFL时，UIKit需要知道VFL字符串中的视图名称究竟对应哪个真实的视图，视图映射字典参数views就用来提供这个信息。其中的键（Key）是字符串，出现在VFL语句中；值（Value）是UIView对象。上述代码中的@{@”logoImageView”: logoImageView}表示的就是字符串@”logoImageView”对应视图logoImageView。
不过有个很遗憾的事实要告诉你，VFL并不能表达所有的约束，例如“logoImageView高度为父视图高度一半”这样的具有比例关系的约束，就无法使用VFL表达出来，所以这时我们只能直接创建NSLayoutConstraint实例了，就像上面的代码那样。
接着我们添加UIScrollView，在viewDidLoad方法中添加如下代码：

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIScrollView</span>* scrollView = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIScrollView</span> new];    scrollView<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    scrollView<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.backgroundColor</span> = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIColor</span> blueColor]; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//为了方便查看效果，暂时将scrollView背景色设置为蓝色</span>    [<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.view</span> addSubview:scrollView];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，scrollView左侧与父视图左侧对齐，scrollView右侧与父视图右侧对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hScrollViewConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|[scrollView]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(scrollView)];    [NSLayoutConstraint activateConstraints:hScrollViewConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//垂直方向上，scrollView顶部与logoImageView底部对齐，scrollView底部与父视图底部对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* vScrollViewConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"V:[logoImageView][scrollView]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(logoImageView, scrollView)];    [NSLayoutConstraint activateConstraints:vScrollViewConstraintArray];</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li></ul>

需要额外说明的是，这里调用了一个NSDictionaryOfVariableBindings宏，它能够方便我们构建字典参数。简单说来，NSDictionaryOfVariableBindings(scrollView)就等于@{@”scrollView”: scrollView}，NSDictionaryOfVariableBindings(logoImageView, scrollView)就等于@{@”logoImageView”: logoImageView, @”scrollView”: scrollView}。
另外，在垂直方向上，我们可以把之前的@”V:|[logoImageView]”与刚才的@”V:[logoImageView][scrollView]|”合并为一句@”V:|[logoImageView][scrollView]|”，就不需要分别创建heightConstraint与vScrollViewConstraintArray了，达到进一步精简的目的。

接着我们添加scrollView中的两个UILabel对象，在viewDidLoad方法中添加如下代码：

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span>* nameLabel = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span> new];    nameLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    nameLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.text</span> = @<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"苹果公司"</span>;    nameLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.backgroundColor</span> = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIColor</span> greenColor];    [scrollView addSubview:nameLabel];    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span>* descriptionLabel = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span> new];    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.text</span> = @<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"苹果公司（Apple Inc. ）是美国的一家高科技公司。由史蒂夫·乔布斯、斯蒂夫·沃兹尼亚克和罗·韦恩(Ron Wayne)等三人于1976年4月1日创立，并命名为美国苹果电脑公司（Apple Computer Inc. ）， 2007年1月9日更名为苹果公司，总部位于加利福尼亚州的库比蒂诺。\n苹果公司创立之初主要开发和销售的个人电脑，截至2014年致力于设计、开发和销售消费电子、计算机软件、在线服务和个人计算机。苹果的Apple II于1970年代助长了个人电脑革命，其后的Macintosh接力于1980年代持续发展。该公司硬件产品主要是Mac电脑系列、iPod媒体播放器、iPhone智能手机和iPad平板电脑；在线服务包括iCloud、iTunes Store和App Store；消费软件包括OS X和iOS操作系统、iTunes多媒体浏览器、Safari网络浏览器，还有iLife和iWork创意和生产力套件。苹果公司在高科技企业中以创新而闻名世界。\n苹果公司1980年12月12日公开招股上市，2012年创下6235亿美元的市值记录，截至2014年6月，苹果公司已经连续三年成为全球市值最大公司。苹果公司在2014年世界500强排行榜中排名第15名。2013年9月30日，在宏盟集团的“全球最佳品牌”报告中，苹果公司超过可口可乐成为世界最有价值品牌。2014年，苹果品牌超越谷歌（Google），成为世界最具价值品牌 。"</span>;    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.numberOfLines</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.backgroundColor</span> = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIColor</span> yellowColor];    [scrollView addSubview:descriptionLabel];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，nameLabel左侧与父视图左侧对齐，nameLabel右侧与父视图右侧对齐，nameLabel宽度与logoImageView宽度相同</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hNameLabelConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|[nameLabel(==logoImageView)]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(nameLabel, logoImageView)];    [NSLayoutConstraint activateConstraints:hNameLabelConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，descriptionLabel左侧与父视图左侧对齐，descriptionLabel右侧与父视图右侧对齐，descriptionLabel宽度与logoImageView宽度相同</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hDescriptionLabelConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|[descriptionLabel(==logoImageView)]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(descriptionLabel, logoImageView)];    [NSLayoutConstraint activateConstraints:hDescriptionLabelConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//垂直方向上，nameLabel顶部与父视图顶部对齐，nameLabel高度为20，nameLabel底部与descriptionLabel顶部对齐，descriptionLabel底部与父视图底部对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* vLabelConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"V:|[nameLabel(20)][descriptionLabel]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(nameLabel, descriptionLabel)];    [NSLayoutConstraint activateConstraints:vLabelConstraintArray];</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li></ul>

其中@”H:|[nameLabel(==logoImageView)]|”表示nameLabel宽度与logoImageView宽度相等，@”V:|[nameLabel(20)][descriptionLabel]|”表示nameLabel的宽度为20。到此我们就完成了这个例子，在此附上全部代码：

<code class="hljs objectivec has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">- (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span>)viewDidLoad{    [<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> viewDidLoad];    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIImageView</span>* logoImageView = [[<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIImageView</span> alloc] initWithImage:[<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIImage</span> imageNamed:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"apple.jpg"</span>]];    logoImageView<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    logoImageView<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.contentMode</span> = UIViewContentModeScaleAspectFit;    [<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.view</span> addSubview:logoImageView];    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIScrollView</span>* scrollView = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIScrollView</span> new];    scrollView<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    [<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.view</span> addSubview:scrollView];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，logoImageView左侧与父视图左侧对齐，logoImageView右侧与父视图右侧对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|[logoImageView]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:@{@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"logoImageView"</span>: logoImageView}];    [NSLayoutConstraint activateConstraints:hConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//垂直方向上，logoImageView顶部与父视图顶部对齐，logoImageView底部与scrollView顶部对齐，scrollView底部与父视图底部对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* vConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"V:|[logoImageView][scrollView]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:@{@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"logoImageView"</span>: logoImageView, @<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"scrollView"</span>: scrollView}];    [NSLayoutConstraint activateConstraints:vConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//logoImageView高度为父视图高度一半</span>    NSLayoutConstraint* heightConstraint = [NSLayoutConstraint constraintWithItem:logoImageView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">self</span><span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.view</span> attribute:NSLayoutAttributeHeight multiplier:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.5</span>f constant:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0.0</span>f];    heightConstraint<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.active</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">YES</span>;    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，scrollView左侧与父视图左侧对齐，scrollView右侧与父视图右侧对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hScrollViewConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|[scrollView]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(scrollView)];    [NSLayoutConstraint activateConstraints:hScrollViewConstraintArray];    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span>* nameLabel = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span> new];    nameLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    nameLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.text</span> = @<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"苹果公司"</span>;    nameLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.backgroundColor</span> = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIColor</span> greenColor];    [scrollView addSubview:nameLabel];    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span>* descriptionLabel = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UILabel</span> new];    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.translatesAutoresizingMaskIntoConstraints</span> = <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">NO</span>;    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.text</span> = @<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"苹果公司（Apple Inc. ）是美国的一家高科技公司。由史蒂夫·乔布斯、斯蒂夫·沃兹尼亚克和罗·韦恩(Ron Wayne)等三人于1976年4月1日创立，并命名为美国苹果电脑公司（Apple Computer Inc. ）， 2007年1月9日更名为苹果公司，总部位于加利福尼亚州的库比蒂诺。\n苹果公司创立之初主要开发和销售的个人电脑，截至2014年致力于设计、开发和销售消费电子、计算机软件、在线服务和个人计算机。苹果的Apple II于1970年代助长了个人电脑革命，其后的Macintosh接力于1980年代持续发展。该公司硬件产品主要是Mac电脑系列、iPod媒体播放器、iPhone智能手机和iPad平板电脑；在线服务包括iCloud、iTunes Store和App Store；消费软件包括OS X和iOS操作系统、iTunes多媒体浏览器、Safari网络浏览器，还有iLife和iWork创意和生产力套件。苹果公司在高科技企业中以创新而闻名世界。\n苹果公司1980年12月12日公开招股上市，2012年创下6235亿美元的市值记录，截至2014年6月，苹果公司已经连续三年成为全球市值最大公司。苹果公司在2014年世界500强排行榜中排名第15名。2013年9月30日，在宏盟集团的“全球最佳品牌”报告中，苹果公司超过可口可乐成为世界最有价值品牌。2014年，苹果品牌超越谷歌（Google），成为世界最具价值品牌 。"</span>;    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.numberOfLines</span> = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>;    descriptionLabel<span class="hljs-variable" style="color: rgb(102, 0, 102); box-sizing: border-box;">.backgroundColor</span> = [<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">UIColor</span> yellowColor];    [scrollView addSubview:descriptionLabel];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，nameLabel左侧与父视图左侧对齐，nameLabel右侧与父视图右侧对齐，nameLabel宽度与logoImageView宽度相同</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hNameLabelConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|[nameLabel(==logoImageView)]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(nameLabel, logoImageView)];    [NSLayoutConstraint activateConstraints:hNameLabelConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//水平方向上，descriptionLabel左侧与父视图左侧对齐，descriptionLabel右侧与父视图右侧对齐，descriptionLabel宽度与logoImageView宽度相同</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* hDescriptionLabelConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"H:|[descriptionLabel(==logoImageView)]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(descriptionLabel, logoImageView)];    [NSLayoutConstraint activateConstraints:hDescriptionLabelConstraintArray];    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//垂直方向上，nameLabel顶部与父视图顶部对齐，nameLabel高度为20，nameLabel底部与descriptionLabel顶部对齐，descriptionLabel底部与父视图底部对齐</span>    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">NSArray</span>* vLabelConstraintArray = [NSLayoutConstraint constraintsWithVisualFormat:@<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"V:|[nameLabel(20)][descriptionLabel]|"</span> options:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> metrics:<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">nil</span> views:NSDictionaryOfVariableBindings(nameLabel, descriptionLabel)];    [NSLayoutConstraint activateConstraints:vLabelConstraintArray];}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li><li style="box-sizing: border-box; padding: 0px 5px;">34</li><li style="box-sizing: border-box; padding: 0px 5px;">35</li><li style="box-sizing: border-box; padding: 0px 5px;">36</li><li style="box-sizing: border-box; padding: 0px 5px;">37</li><li style="box-sizing: border-box; padding: 0px 5px;">38</li><li style="box-sizing: border-box; padding: 0px 5px;">39</li><li style="box-sizing: border-box; padding: 0px 5px;">40</li><li style="box-sizing: border-box; padding: 0px 5px;">41</li><li style="box-sizing: border-box; padding: 0px 5px;">42</li><li style="box-sizing: border-box; padding: 0px 5px;">43</li><li style="box-sizing: border-box; padding: 0px 5px;">44</li><li style="box-sizing: border-box; padding: 0px 5px;">45</li><li style="box-sizing: border-box; padding: 0px 5px;">46</li><li style="box-sizing: border-box; padding: 0px 5px;">47</li><li style="box-sizing: border-box; padding: 0px 5px;">48</li><li style="box-sizing: border-box; padding: 0px 5px;">49</li><li style="box-sizing: border-box; padding: 0px 5px;">50</li><li style="box-sizing: border-box; padding: 0px 5px;">51</li><li style="box-sizing: border-box; padding: 0px 5px;">52</li><li style="box-sizing: border-box; padding: 0px 5px;">53</li><li style="box-sizing: border-box; padding: 0px 5px;">54</li></ul>

与第三篇文章中逐个去创建NSLayoutConstraint对象相比，是不是简单直观了不少？程序最终项目文件链接：
http://yunpan.cn/cVE8i8WmnpJwv（提取码：d348）
本篇文章我们初步了解了VFL的基本使用方式，关于VFL的具体语法格式请参看苹果的《Auto Layout Guide》，链接：
https://developer.apple.com/library/ios/documentation/UserExperience/Conceptual/AutolayoutPG/Introduction/Introduction.html#//apple_ref/doc/uid/TP40010853
在本系列后续文章中，我将继续介绍自动布局中的动画实现、事件处理、调试，以及iOS 8中新添加的Size Class的使用，敬请期待吧。

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