AutoLayout不管是在StoryBorad还是在xib中都相对来说比较简单，VFL(Visual fromat language)可视化语言基本上用到的比较少，在xCode4时候自动布局的概念还没有，直接使用VFL会很方便，可视化语言依赖于oc运行时创建对应的约束，如果IBOutlet发生改变有的时候会造成莫名其妙的Bug。xCode5之后可视化语言用到的场景相对较少，但是作为一个工作的辅助还是可以稍微了解下。
基础知识
在StotyBoard中添加一个标签一个按钮，不适用自动布局，简单的控制它们之间的水平距离为80，如下图所示:
视图中添加约束:
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NSLayoutConstraint *labelContraint=[NSLayoutConstraint constraintWithItem:self.changeButton attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.descriptionLabel attribute:NSLayoutAttributeRight multiplier:1.0 constant:60];
[self.view addConstraint:labelContraint];
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这个只是视图约束的一种方式，下面这种方式才是本文的主角:
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//使用可视化语言添加约束NSDictionary *viewDictionary=NSDictionaryOfVariableBindings(_descriptionLabel,_changeButton);
NSArray *visualConstraint=[NSLayoutConstraint constraintsWithVisualFormat:@"[_descriptionLabel]-60-[_changeButton]" options:0 metrics:nil views:viewDictionary];
[self.view addConstraints:visualConstraint];
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这里面用到的constraintsWithVisualFormat方法,具体参数说明如下:
format:参数是vfl语句，语句的基本元素下面会详细解释一下；
opts:枚举参数，默认写0；
metrics:字典，当在format中使用了动态数据，会根据字典去匹配，接下来会具体有例子；
views：字典，传入需要用到的视图集合；
具体format需要参考一下表达式的意思:
水平方向 H:
垂直方向 V:
Views [需要定义的视图]
SuperView |
关系 >=,==,<=
间隙 -
视图内部约束 （）
Demo实战
通过VFL控制手动添加的标签的位置，具体效果如下:
代码实现如下:
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UILabel *link=[[UILabel alloc]init];link.text=@"http://www.cnblogs.com/xiaofeixiang";
link.translatesAutoresizingMaskIntoConstraints=NO;
[link setBackgroundColor:[UIColor greenColor]];[self.view addSubview:link];
NSArray *horizontal=[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-40-[link]-20-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(link)];
NSArray *vertical=[NSLayoutConstraint constraintsWithVisualFormat:@"V:[_descriptionLabel]-100-[link(>=30)]" options:0 metrics:nil views:NSDictionaryOfVariableBindings(link,_descriptionLabel)];
[self.view addConstraints:horizontal];
[self.view addConstraints:vertical];
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第一个约束是控制标签距离父视图左右之间的距离，第二个控制标签和”博客园-FlyElephant"之间的垂直距离为100.当然如果你想通过字典控制垂直之间的距离可以按照下面这么做:
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NSArray *vertical=[NSLayoutConstraint constraintsWithVisualFormat:@"V:[_descriptionLabel]-Vertical-[link(>=30)]" options:0 metrics:@{@"Vertical":@200} views:NSDictionaryOfVariableBindings(link,_descriptionLabel)];
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最后的结果:
友情提示在添加约束的时候不要和StoryBoard中的冲突，如果添加的水平约束StoryBoard中也有的话，就会出现下面这种情况:
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2015-07-01 10:54:13.537 VFLDemo[2358:60863] Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints)
( "<NSLayoutConstraint:0x7fc5e3732860 H:[UILabel:0x7fc5e372ef30'\U535a\U5ba2\U56ed-FlyElephant']-(15)-[UIButton:0x7fc5e372d550'\U7fa4:228407086']>",
"<NSLayoutConstraint:0x7fc5e37344e0 H:[UILabel:0x7fc5e372ef30'\U535a\U5ba2\U56ed-FlyElephant']-(60)-[UIButton:0x7fc5e372d550'\U7fa4:228407086']>"
)Will attempt to recover by breaking constraint<NSLayoutConstraint:0x7fc5e37344e0 H:[UILabel:0x7fc5e372ef30'博客园-FlyElephant']-(60)-[UIButton:0x7fc5e372d550'群:228407086']>
Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful. |