追求Masonry

Autolayout就像一个知情达理，善解人意的好姑娘，可惜长相有点不堪入目，所以追求者寥寥无几。所幸遇到了化妆大师cloudkite，给她来了一个完美的化妆，从此丑小鸭Autolayout变成了美天鹅Masonry。前几日有幸一见，果然名不虚传，长相甜美，还善解人意。我果断放弃了Frame，开始追求Masonry

初识Masonry

初见

UIEdgeInsets edge = UIEdgeInsetsMake(10, 10, 10, 10);  CGSize superSize = view.superview.frame.size;  CGFloat width = superSize.width - edge.left - edge.right;  CGFloat heitht = superSize.height - edge.top - edge.bottom;  view.frame = CGRectMake(edge.left, edge.top, width, heitht);

逻辑比较复杂，阅读的时候还得想半天才能想明白，这个frame到底要表达的是什么意思。而且关键的是父View的大小如果改变，还需要再次重新设置Frame。看着Frame这黄脸婆，心里一阵别扭...我们来看看充满青春活力的小鲜肉Masonry是怎么样的:

UIEdgeInsets edge = UIEdgeInsetsMake(10, 10, 10, 10);  [view mas_makeConstraints:^(MASConstraintMaker *make) {      make.edges.equalTo(view.superview).insets(edge);  }];

使用mas_makeConstraints在block中对View添加约束。view相对父View的边距为edge。代码简单，逻辑一目了然。而且还能跟父View一起调整。简直是perfect，初见Masonry，惊为天人

细品

cloudkite给Autolayout披上一层漂亮的外衣之后，将其称为Masonry，但Masonry的本质还是Autolayout。

Autolayout是什么呢？Autolayout就是给View添加一堆约束，让View在布局的时候通过约束计算出Frame，然后进行布局(Autolayout更多内容见 Autolayout的第一次亲密接触 )。make.edges.equalTo(view.superview).insets(edge);就是添加约束的过程。

对于一个约束。他实际表示的是一个不等或者相等关系

用Masonry创建一个完整的约束应该是这样的

//view1的左边距离父View左边10个点：  [view1 mas_makeConstraints:^(MASConstraintMaker *make) {      make.left.equalTo(view1.superview.mas_left).multipliedBy(1).offset(10);  }];

Attribute

MASConstraintMaker

上面的表达式中，我们可以看到，make是MASConstraintMaker类型。MASConstraintMaker给我们提供了22种Attribute类型

//Basic Attribute  @property (nonatomic, strong, readonly) MASConstraint *left;  @property (nonatomic, strong, readonly) MASConstraint *top;  @property (nonatomic, strong, readonly) MASConstraint *right;  @property (nonatomic, strong, readonly) MASConstraint *bottom;  @property (nonatomic, strong, readonly) MASConstraint *leading;  @property (nonatomic, strong, readonly) MASConstraint *trailing;  @property (nonatomic, strong, readonly) MASConstraint *width;  @property (nonatomic, strong, readonly) MASConstraint *height;  @property (nonatomic, strong, readonly) MASConstraint *centerX;  @property (nonatomic, strong, readonly) MASConstraint *centerY;  @property (nonatomic, strong, readonly) MASConstraint *baseline;    //Margin Attribute  @property (nonatomic, strong, readonly) MASConstraint *leftMargin;  @property (nonatomic, strong, readonly) MASConstraint *rightMargin;  @property (nonatomic, strong, readonly) MASConstraint *topMargin;  @property (nonatomic, strong, readonly) MASConstraint *bottomMargin;  @property (nonatomic, strong, readonly) MASConstraint *leadingMargin;  @property (nonatomic, strong, readonly) MASConstraint *trailingMargin;  @property (nonatomic, strong, readonly) MASConstraint *centerXWithinMargins;  @property (nonatomic, strong, readonly) MASConstraint *centerYWithinMargins;    //Convenient Attribute  @property (nonatomic, strong, readonly) MASConstraint *edges;  @property (nonatomic, strong, readonly) MASConstraint *size;  @property (nonatomic, strong, readonly) MASConstraint *center;

Attribute总体来说分为三大类

1. Basic Attribute: 基本属性，支持到iOS6，一般使用得比较多
2. Margin Attribute: 边缘相关属性，支持到iOS8。由于版本要求比较高，一般用得比较少。Margin相关的详细内容请参考 iOS8上关于UIView的Margin新增了3个APIs
3. Convenient Attribute: 便捷属性，为了使用方便而特意新增的属性。Autolayout本身没有对应的相关属性

Convenient Attribute实际是基本属性的组合。比如：edges表示left, right, top, bottom。下面的两个代码实际的意义是一样的

//Convenient Attribute  make.edges.insets(edge);    //Basic Attribute  make.left.right.top.bottom.insets(edge);

MASConstraint

前面我们看到MASConstraintMaker中所有的Attribute都是MASConstraint类型。对于多个Attribute一起写的表达式：

make.left.right.top.bottom.insets(edge);

//Basic Attribute  @property (nonatomic, strong, readonly) MASConstraint *left;  @property (nonatomic, strong, readonly) MASConstraint *top;  @property (nonatomic, strong, readonly) MASConstraint *right;  @property (nonatomic, strong, readonly) MASConstraint *bottom;  @property (nonatomic, strong, readonly) MASConstraint *leading;  @property (nonatomic, strong, readonly) MASConstraint *trailing;  @property (nonatomic, strong, readonly) MASConstraint *width;  @property (nonatomic, strong, readonly) MASConstraint *height;  @property (nonatomic, strong, readonly) MASConstraint *centerX;  @property (nonatomic, strong, readonly) MASConstraint *centerY;  @property (nonatomic, strong, readonly) MASConstraint *baseline;    //Margin Attribute  @property (nonatomic, strong, readonly) MASConstraint *leftMargin;  @property (nonatomic, strong, readonly) MASConstraint *rightMargin;  @property (nonatomic, strong, readonly) MASConstraint *topMargin;  @property (nonatomic, strong, readonly) MASConstraint *bottomMargin;  @property (nonatomic, strong, readonly) MASConstraint *leadingMargin;  @property (nonatomic, strong, readonly) MASConstraint *trailingMargin;  @property (nonatomic, strong, readonly) MASConstraint *centerXWithinMargins;  @property (nonatomic, strong, readonly) MASConstraint *centerYWithinMargins;

细看一下，MASConstraint中的Attribute和MASConstraintMaker完全一样。只是MASConstraintMaker中多了3种Convenient Attribute。

两者Attribute的一致，大大的提升了使用的方便性。使用过程中我们不用再去区分当前属性是MASConstraint还是MASConstraintMaker类型。(事实上没研究他的类型之前，我都不知道他们分别属于2种不同类的属性)

Relationship

约束表示的是2个item之间的关系，在Autolayout中一共定义了3种关系：=, >=, <=，对应到Masonry中:

- (MASConstraint * (^)(id attr))equalTo;  - (MASConstraint * (^)(id attr))greaterThanOrEqualTo;  - (MASConstraint * (^)(id attr))lessThanOrEqualTo;

相等关系我们一般用的多。那么不相等关系我们什么时候用呢？

[label mas_makeConstraints:^(MASConstraintMaker *make) {      make.left.offset(0);      make.centerY.offset(0);      make.width.lessThanOrEqualTo(label.superview);  }];

multiplier

multiplier表示Attribute前面的乘数。Masonry提供了2种添加multiplier的方法

//    Sets the NSLayoutConstraint multiplier property  - (MASConstraint * (^)(CGFloat multiplier))multipliedBy;    //    Sets the NSLayoutConstraint multiplier to 1.0/dividedBy  - (MASConstraint * (^)(CGFloat divider))dividedBy;

Constant

Masonry提供了4种设置constant的方法

//Modifies the NSLayoutConstraint constant,only affects MASConstraints in which the first item's NSLayoutAttribute is one of the following NSLayoutAttributeTop, NSLayoutAttributeLeft, NSLayoutAttributeBottom, NSLayoutAttributeRight  - (MASConstraint * (^)(MASEdgeInsets insets))insets;    //Modifies the NSLayoutConstraint constant,only affects MASConstraints in which the first item's NSLayoutAttribute is one of the following NSLayoutAttributeWidth, NSLayoutAttributeHeight  - (MASConstraint * (^)(CGSize offset))sizeOffset;    //Modifies the NSLayoutConstraint constant, only affects MASConstraints in which the first item's NSLayoutAttribute is one of the following NSLayoutAttributeCenterX, NSLayoutAttributeCenterY  - (MASConstraint * (^)(CGPoint offset))centerOffset;    //Modifies the NSLayoutConstraint constant  - (MASConstraint * (^)(CGFloat offset))offset;

其实一般情况下，我只使用offset....

小技巧

1. 如果等式2边的Attribute是一样的，我们可以省略等式右边的Attribute
2. 如果是等于关系，并且右边的view是父View。连equalTo也可以省略
3. 如果equalTo里面传的是NSValue类型，效果跟设置offset是一样的
4. 如果offset为0，其实也是可以省略的...

下面所有代码实际效果是一样的：

// 完整的  make.left.equalTo(view1.superview.mas_left).offset(0);    //省略Attribute的  make.left.equalTo(view1.superview).offset(0);    //省略equalTo的  make.left.offset(0);    //使用equalTo替代offset的  make.left.equalTo(@0);    //终极大招，省略所有的... 可惜会有warning  make.left;

不过对于make.left，编译器会报一个警告：你用getter方法获取回来的值未使用，所以不应该使用"."语法

对于这个警告我们可以将返回值转为空消除：

(void)make.left;

不过终究又变得麻烦了，又要多写6个字母，愁人...

设置或更新约束

对于约束的设置，Masonry提供了3种方法，分别为设置约束、更新约束、重写设置约束

// 设置约束      - (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *make))block;    // 更新约束  - (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block;    // 重新设置约束  - (NSArray *)mas_remakeConstraints:(void(^)(MASConstraintMaker *make))block;

批量设置约束

假设有View1，view2，view3三个View，我们想要他们的宽高都等于CGSizeMake(100, 50)。我们可以对他们进行批量设置：

NSValue *sizeValue = [NSValue valueWithCGSize:CGSizeMake(100, 50)];  [@[view1,view2,view3] mas_makeConstraints:^(MASConstraintMaker *make) {      make.size.equalTo(sizeValue);  }];

由于我们还要设置view的top,left等位置约束。那可不可以在设置位置的mas_makeConstraints里面批量设置宽高呢？实际是可以的！

//advance set  [view1 mas_makeConstraints:^(MASConstraintMaker *make) {      (void)make.top.left;      make.size.equalTo(@[view2,view3,sizeValue]);  }];

不过需要注意的是。设置约束的时候，view一定是已经被addSubview的(详情参考 Autolayout的第一次亲密接触 )，否则会抛异常。所以我们一般在最后一个view上加批量约束

Priority

我们知道约束是有优先级的，Masonry给我们提供了4个设置优先级的接口:

 //    Sets the NSLayoutConstraint priority to a float or MASLayoutPriority  - (MASConstraint * (^)(MASLayoutPriority priority))priority;    //    Sets the NSLayoutConstraint priority to MASLayoutPriorityLow  - (MASConstraint * (^)())priorityLow;    //    Sets the NSLayoutConstraint priority to MASLayoutPriorityMedium  - (MASConstraint * (^)())priorityMedium;    //    Sets the NSLayoutConstraint priority to MASLayoutPriorityHigh  - (MASConstraint * (^)())priorityHigh;

需要注意的是，使用priorityLow、priorityMedium、priorityHigh的时候。不是.priorityHigh，而是.priorityHigh()

key

当约束冲突发生的时候，我们经常为找不到是哪个View冲突的而烦恼，这一堆View是个什么东西呀？

"<MASLayoutConstraint:0x7f8de483fb10 UIView:0x7f8de2f53870.left == UIView:0x7f8de2f586c0.left>",  "<MASLayoutConstraint:0x7f8de4818b50 UIView:0x7f8de2f53870.right == UIView:0x7f8de2f586c0.right>",  "<MASLayoutConstraint:0x7f8de4818870 UIView:0x7f8de2f53870.width == 100>",  "<NSLayoutConstraint:0x7f8de4847e90 UIView:0x7f8de2f586c0.width == 375>"    Will attempt to recover by breaking constraint   <MASLayoutConstraint:0x7f8de4818870 UIView:0x7f8de2f53870.width == 100>

这时候我们可以设置View的key:

self.view.mas_key = @"self.view";  view1.mas_key = @"view1";

设置之后再看一下，哈哈，现在好多了。可以清晰的知道是哪个view了

"<MASLayoutConstraint:0x7fcd98d17c40 UIView:view1.left == UIView:self.view.left>",  "<MASLayoutConstraint:0x7fcd98d2b2c0 UIView:view1.right == UIView:self.view.right>",  "<MASLayoutConstraint:0x7fcd98d2adb0 UIView:view1.width == 100>",  "<NSLayoutConstraint:0x7fcd98e42050 UIView:self.view.width == 375>"    Will attempt to recover by breaking constraint   <MASLayoutConstraint:0x7fcd98d2adb0 UIView:view1.width == 100>

大家可能会觉得这样一个一个设置，多麻烦啊！别着急，Masonry提供了批量设置的宏MASAttachKeys
只需要一句代码即可全部设置：

MASAttachKeys(self.view,view1);

拨开Masonry的衣服

Masonry的基本使用方法介绍完了，那么我们来看看Masonry的内部到底有些什么东西？

结构

Masonry一共有十三个类，我将这13个类分为5个模块：

Help

Help模块主要是一些辅助的类。

NSLayoutConstraint+MASDebugAdditions：这个类的主要作用是重写NSLayoutConstraint的description函数。让约束发生冲突的时候，更易读。如果View或者constraint设置了Key，直接用key的值显示到description中。如果没有设置，显示View或者constraint的指针。

Shorthand

对于系统原有类(NSArray,UIView)的扩展。Masonry的category方法和属性都加有mas_前缀。这也是Apple建议的做法，避免跟系统原有方法冲突。但是有时候我们可能想用的更方便，不想写mas_前缀(没办法，我就是这么懒...)

在NSArray+MASShorthandAdditions和View+MASShorthandAdditions中定义了不带mas_前缀的扩展。这些扩展根据你是否定义了MAS_SHORTHAND宏来确定是否编译。所以你只需要定义MAS_SHORTHAND宏，就可以方便的使用不带mas_前缀的方法，比如：-[view makeConstraints:]

Public

Public模块主要是对外暴露的方法。使用者使用Masonry可以直接接触到。

Core

Core模块就是Masonry的核心部分，Masonry的大部分功能都在这4个类里实现

Property

此模块主要封装了一些MASConstraint持有的属性。为了使用更方便，或者扩展功能

实现

当我们给View添加一个约束的时候到底发生了什么？

[view1 mas_makeConstraints:^(MASConstraintMaker *make) {      make.left.top.equalTo(view1.superview).offset(20);  }];

我们首先来看make.left.top.equalTo(view1.superview).offset(20);

一、执行"make.left"

MASConstraintMaker类中有一个属性constraints专门用来存储constraint

@property (nonatomic, strong) NSMutableArray *constraints;

当执行make.left的时候, 会将相应的MASConstraint添加到constraints数组中

- (MASConstraint *)left {      return [self addConstraintWithLayoutAttribute:NSLayoutAttributeLeft];  }    - (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {      return [self constraint:nil addConstraintWithLayoutAttribute:layoutAttribute];  }        //核心方法  - (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {      MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];      MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];        //调用make.left.top时走入这里将原来的ViewConstraint替换成MASCompositeConstraint      if ([constraint isKindOfClass:MASViewConstraint.class]) {          //replace with composite constraint          NSArray *children = @[constraint, newConstraint];          MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];          compositeConstraint.delegate = self;          [self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];          return compositeConstraint;      }        // 调用make.left的时候走入这里，将constraint加入到self.constraints中      if (!constraint) {          newConstraint.delegate = self;          [self.constraints addObject:newConstraint];      }      return newConstraint;  }

对MASConstraintMaker调用Attribute的get方法，最终都会走到-constraint:addConstraintWithLayoutAttribute:中，在这个方法中，通过对应的Attribute生成MASViewConstraint。然后将MASViewConstraint加入到constraints中

二、执行".top"

make.left返回的是MASConstraint类型。所以make.left.top是对MASViewConstraint类型调用top方法。

- (MASConstraint *)top {      return [self addConstraintWithLayoutAttribute:NSLayoutAttributeTop];  }  - (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {      NSAssert(!self.hasLayoutRelation, @"Attributes should be chained before defining the constraint relation");        return [self.delegate constraint:self addConstraintWithLayoutAttribute:layoutAttribute];  }

当执行-addConstraintWithLayoutAttribute的时候，ViewConstraint通过delegate又调回到MASConstraintMaker的-constraint:addConstraintWithLayoutAttribute:中。

在MASConstraintMaker的-constraint:addConstraintWithLayoutAttribute:里，将原来constraints中的MASViewConstraint替换成MASCompositeConstraint。MASCompositeConstraint持有top,left 2个属性。对MASCompositeConstraint做操作时候，其内部的所有属性都会执行相应的操作

三、执行".equalTo(view1.superview)"

- (MASConstraint * (^)(id))equalTo {      return ^id(id attribute) {          return self.equalToWithRelation(attribute, NSLayoutRelationEqual);      };  }    - (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {      return ^id(id attribute, NSLayoutRelation relation) {          if ([attribute isKindOfClass:NSArray.class]) {              NSAssert(!self.hasLayoutRelation, @"Redefinition of constraint relation");              NSMutableArray *children = NSMutableArray.new;              for (id attr in attribute) {                  MASViewConstraint *viewConstraint = [self copy];                  viewConstraint.secondViewAttribute = attr;                  [children addObject:viewConstraint];              }              MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];              compositeConstraint.delegate = self.delegate;              [self.delegate constraint:self shouldBeReplacedWithConstraint:compositeConstraint];              return compositeConstraint;          } else {              NSAssert(!self.hasLayoutRelation || self.layoutRelation == relation && [attribute isKindOfClass:NSValue.class], @"Redefinition of constraint relation");              self.layoutRelation = relation;              self.secondViewAttribute = attribute;              return self;          }      };  }

当执行Relationship的方法时，都会走到-equalToWithRelation中。在这个方法里面主要是给realationship和secondViewAttribute赋值：

1. 如果不是数组，直接对realationship和secondViewAttribute赋值
2. 如果是数组，如：.equalTo(@[view1.mas_left,view2.mas_left])，逻辑上肯定不能是不等关系(>=,<=)，所以realationship不用赋值，使用默认值(=)。copy出多个viewConstraint，将secondViewAttribute赋值。然后用多个viewConstraint组成的compositeConstraint替换调原来的viewConstraint。

四、执行".offset(10)"

- (MASConstraint * (^)(CGFloat))offset {      return ^id(CGFloat offset){          self.offset = offset;          return self;      };  }    - (void)setOffset:(CGFloat)offset {      self.layoutConstant = offset;  }

offset(10)会将10传入到ViewConstraint中，用layoutConstant属性将其存起来。（offset主要影响的是约束里面的constant）

五、mas_makeConstraints

看完了make.left.top.equalTo(view1.superview).offset(20);，我们再看看mas_makeConstraints中到底做了什么？

- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {      self.translatesAutoresizingMaskIntoConstraints = NO;      MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];      block(constraintMaker);      return [constraintMaker install];  }

mas_makeConstraints方法很简单，

1. 将self.translatesAutoresizingMaskIntoConstraints至为NO。translatesAutoresizingMaskIntoConstraints表示是否将设置的Frame转化为约束。当自己设置约束的时候需要将其置为NO
2. 创建出MASConstraintMaker对象
3. 通过block抛出到外面设值
4. constraintMaker install

上面的代码我们知道，关键的地方还是在于constraintMaker install

六、constraintMaker install

- (NSArray *)install {      if (self.removeExisting) {          NSArray *installedConstraints = [MASViewConstraint installedConstraintsForView:self.view];          for (MASConstraint *constraint in installedConstraints) {              [constraint uninstall];          }      }      NSArray *constraints = self.constraints.copy;      for (MASConstraint *constraint in constraints) {          constraint.updateExisting = self.updateExisting;          [constraint install];      }      [self.constraints removeAllObjects];      return constraints;  }

1. 如果需要removeExisting，就把已有的约束remove掉，当调用mas_remakeConstraints的时候会将removeExisting值置为YES
2. 遍历constraints，调用[constraint install]
3. 清空constraints，这里的constraintMaker只是一个零时属性，只是一个工具类，不需要存储。所以用完之后就可以将constraints清空

其实真正关键的地方在[constraint install]

七、constraint install

- (void)install {      // 1. 已经installed的将不做任何操作      if (self.hasBeenInstalled) {          return;      }        //2. 从ViewAttribute中剥离出item和attribute      MAS_VIEW *firstLayoutItem = self.firstViewAttribute.item;      NSLayoutAttribute firstLayoutAttribute = self.firstViewAttribute.layoutAttribute;      MAS_VIEW *secondLayoutItem = self.secondViewAttribute.item;      NSLayoutAttribute secondLayoutAttribute = self.secondViewAttribute.layoutAttribute;        //3. 如果没有secondViewAttribute，默认secondItem为其父View，secontAttribute等于firstLayoutAttribute。      if (!self.firstViewAttribute.isSizeAttribute && !self.secondViewAttribute) {          secondLayoutItem = self.firstViewAttribute.view.superview;          secondLayoutAttribute = firstLayoutAttribute;      }        //4. 创建真正用于Autolayout的约束layoutConstraint      MASLayoutConstraint *layoutConstraint          = [MASLayoutConstraint constraintWithItem:firstLayoutItem                                          attribute:firstLayoutAttribute                                          relatedBy:self.layoutRelation                                             toItem:secondLayoutItem                                          attribute:secondLayoutAttribute                                         multiplier:self.layoutMultiplier                                           constant:self.layoutConstant];      //5. 将priority和key赋值      layoutConstraint.priority = self.layoutPriority;      layoutConstraint.mas_key = self.mas_key;        //6. 找到要添加约束的installView      if (self.secondViewAttribute.view) {          MAS_VIEW *closestCommonSuperview = [self.firstViewAttribute.view mas_closestCommonSuperview:self.secondViewAttribute.view];          NSAssert(closestCommonSuperview,                   @"couldn't find a common superview for %@ and %@",                   self.firstViewAttribute.view, self.secondViewAttribute.view);          self.installedView = closestCommonSuperview;      } else if (self.firstViewAttribute.isSizeAttribute) {          self.installedView = self.firstViewAttribute.view;      } else {          self.installedView = self.firstViewAttribute.view.superview;      }        //7. 添加或更新约束      MASLayoutConstraint *existingConstraint = nil;      if (self.updateExisting) {          existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];      }      if (existingConstraint) {          // just update the constant          existingConstraint.constant = layoutConstraint.constant;          self.layoutConstraint = existingConstraint;      } else {          [self.installedView addConstraint:layoutConstraint];          self.layoutConstraint = layoutConstraint;          [firstLayoutItem.mas_installedConstraints addObject:self];      }  }

1. 如果已经installed就不做任何操作
2. 从ViewAttribute中剥离出item和attribute。前面我们介绍过MASViewAttribute的类主要是将item和attribute2个属性封装在了一起。
3. secondViewAttribute的值来自于.equalTo(item.attribute)中的item.attribute。当我们写下类似make.left.offset(10);约束的时候，是没有secondViewAttribute的，这时候默认secondItem为其父View，secontAttribute等于firstLayoutAttribute。这就解释了为什么可以这样写make.left.offset(10);
4. 创建真正用于Autolayout的约束layoutConstraint
5. 将priority和key赋值
6. 找到要添加约束的installView。如果是2个View之间的约束，需要寻找这2个View最接近的共同父View。添加约束
7. 添加或更新约束。当调用mas_updateConstraints的时候updateExisting=YES。这时候会查找是否有已经存在的约束。有就更新，没有就添加。如果是mas_makeConstraints或mas_remakeConstraints，则直接添加

Extension

仅仅将代码结构和基本实现过程解析了一下，更多实现细节还需要大家自己去阅读源码

说实话，Masonry的代码写得真漂亮，不管是代码格式规范，还是设计模式。看起来简直是一种享受。建议大家阅读。

Autolayout的第一次亲密接触 也更新了一些东西。没阅读过或者阅读时间比较早的朋友可以再看看~

Reference