Masonry 源码学习

有三种方式来使用代码创建约束。

• use layout anchors
• use the NSLayoutConstraint class
• use the Visual Format Language

Masonry是对NSLayoutConstraint的封装

先来看看NSLayoutConstraint的创建：

+ (instancetype)constraintWithItem:(id)view1  
                                          attribute:(NSLayoutAttribute)attr1  
                                        relatedBy:(NSLayoutRelation)relation  
                                             toItem:(id)view2  
                                          attribute:(NSLayoutAttribute)attr2  
                                         multiplier:(CGFloat)multiplier  
                                          constant:(CGFloat)c;

我们把它分成5个部分来看：

• firstViewAttribute部分，包含view1和attr1
• layoutRelation部分，relation
• secondAttribute部分，包含view2和attr2
• layoutMultiplier部分，multiplier
• layoutConstant部分，c

NSLayoutConstraint对象应该添加到哪个view上面呢？
这里分两种情况。

• 当secondAttribute的view2存在，并且是UIView类型的对象时，约束添加到view1与view2的closestCommonSuperview（最近的公共superView）
• 当secondAttribute的view2存在，但是不是UIView类型的对象时
  • 当firstViewAttribute的attr1为size属性（width或者height）时，约束添加到firstViewAttribute的view1上。
  • 当firstViewAttribute的attr1不是size属性，约束添加到firstViewAttribute.view.superView上。

---

使用Masonry链式语法的创建约束：

//第一种
make.edge.equalTo(superView).inset(UIEdgeInsetMake(10,10,10,10));
//第二种
make.left.right.height.bottom.equalTo(superView).inset(UIEdgeInsetMake(10,10,10,10));
//第三种
make.left.equalTo(superView).offset(@10);
make.right.equalTo(superView).offset(@-10);
make.top.equalTo(superView).offset(@10);
make.bottom.equalTo(superView).offset(@-10);

上面用三种方式创建了相同的约束效果。

我们先从第三种来看Masonry的实现

[view mas_makeConstraints:^(MASConstraintMaker *make) {
       make.left.equalTo(superView).offset(@10);
}];

mas_makeConstraints为UIView的MASAdditions类别所添加的方法：

#define MAS_VIEW UIView
@interface MAS_VIEW (MASAdditions)
- (NSArray *)mas_makeConstraints:(void(NS_NOESCAPE ^)(MASConstraintMaker *make))block;
...
@end

实现：

@implementation MAS_VIEW (MASAdditions)

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

步骤分析：

• 创建了一个MASConstraintMaker对象，拥有对象view（弱引用）。
• 在block中配置constraintMaker对象。
• [constraintMaker install]。

install方法实现：

- (NSArray *)install {
    ...
    NSArray *constraints = self.constraints.copy;
    for (MASConstraint *constraint in constraints) {
        ...
        [constraint install];
    }
    [self.constraints removeAllObjects];
    return constraints;
}

是将MASConstraintMaker匿名类别中的属性@property (nonatomic, strong) NSMutableArray *constraints;遍历一遍执行install方法。constraints是MASConstraint类型的对象数组。
可以想到在block中配置constraintMaker，其本质就是在constraintMaker的constraints中添加MASConstraint对象。

现在来看看make.left.equalTo(superView).offset(@10);是如何创建MASConstraint对象并添加入constraintMaker的constraints中。
MASConstraintMaker类定义了所有的约束：

@interface MASConstraintMaker : NSObject

@property (nonatomic, strong, readonly) MASConstraint *left;
...

当使用make.left时，会执行left的getter方法：

- (MASConstraint *)left {
    return [self addConstraintWithLayoutAttribute:NSLayoutAttributeLeft];
}
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    return [self constraint:nil addConstraintWithLayoutAttribute:layoutAttribute];
}
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
    MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
    if ([constraint isKindOfClass:MASViewConstraint.class]) {
        ...
    }
    if (!constraint) {
        newConstraint.delegate = self;
        [self.constraints addObject:newConstraint];
    }
    return newConstraint;
}

可以left的getter方法执行步骤如下：

• 创建一个MASViewAttribute对象viewAttribute，view为当前view，layoutAttribute:为NSLayoutAttributeLeft。
• 创建一个MASViewConstraint对象newConstraint，firstViewAttribute为viewAttribute。
• newConstraint的代理设为self。
• 将newConstraint放入constraints中。

那其实mark.left就已经创建了MASViewConstraint对象（继承自MASConstraint），并添加入maker的constraints中。

这时候很好奇make.left后面的.equalTo(superView).offset(@10)又是做了什么？
可以看出make.left返回的是MASConstraint类型对象，所以.equalTo(superView)操作是在MASConstraint对象上的。

现在来看看MASConstraint类，类的声明如下：

@interface MASConstraint : NSObject

// Chaining Support  就是用这个类来实现链式语法滴
...
- (MASConstraint * (^)(id attr))equalTo;
- (MASConstraint * (^)(CGFloat offset))offset;
..
- (MASConstraint *)left;
- (MASConstraint *)right;
...
@end

链式语法能实现的关键之处就是返回本身，分情况说：

• 当里面的属性使用需要提供参数（如equalTo(superview)，使用equalTo需要提供superView这个参数）时，定义的就是一个(MASConstraint * (^)(id attr))类型的属性，这样就实现了make.left.equalTo(superView)后返回的又是一个MASConstraint，于是就链式调用啦。
• 当里面的属性可以直接使用(如make.left.right.heigt，make.left返回MASConstarint后直接使用了.right.height..)，则是直接定义成MASConstraint类型的属性，于是就链式调用啦。

MASConstraint类是提供链式语法支持的，具体的约束属性设置是通过它的子类MASViewConstraint和MASCompositeConstraint。

MASViewConstraint和MASCompositeConstraint的关系：
其中MASCompositeConstraint是用来管理MASViewConstraint集合的，如make.left.right.top.bottom其实是创建了4个MASViewConstraint，其中firstAttribute的layoutAttribute属性分别是NSLayoutAttributeLeft,NSLayoutAttributeRight,NSLayoutAttributeTop,NSLayoutAttributeBottom，然后将这4个MASViewConstraint放入MASCompositeConstraint的@property (nonatomic, strong) NSMutableArray *childConstraints;中统一管理，所以当调用make.left.right.top.bottom.equalTo(superView)时，循环childConstraints调用equalTo(superView)。

所以具体的约束属性设置(NSLayoutConstraint的创建和添加)是由MASViewConstraint完成的。

MASViewConstraint是如何工作的？
MASViewConstraint类定义如下：

@interface MASViewConstraint : MASConstraint 
@property (nonatomic, strong, readonly) MASViewAttribute *firstViewAttribute;
@property (nonatomic, strong, readonly) MASViewAttribute *secondViewAttribute;

- (id)initWithFirstViewAttribute:(MASViewAttribute *)firstViewAttribute;
+ (NSArray *)installedConstraintsForView:(MAS_VIEW *)view;
@end

MASViewConstraint类需要用firstViewAttribute初始化，也就是说它一生成就拥有了firstViewAttribute。
匿名闭包中的定义：

@interface MASViewConstraint ()

@property (nonatomic, strong, readwrite) MASViewAttribute *secondViewAttribute;
@property (nonatomic, weak) MAS_VIEW *installedView;
@property (nonatomic, weak) MASLayoutConstraint *layoutConstraint;
@property (nonatomic, assign) NSLayoutRelation layoutRelation;
@property (nonatomic, assign) MASLayoutPriority layoutPriority;
@property (nonatomic, assign) CGFloat layoutMultiplier;
@property (nonatomic, assign) CGFloat layoutConstant;
@property (nonatomic, assign) BOOL hasLayoutRelation;
@property (nonatomic, strong) id mas_key;
@property (nonatomic, assign) BOOL useAnimator;

@end

看到这里就大概知道MASViewConstraint是用来干什么的了，可以使用firstViewAttribute和secondViewAttribute，layoutMultiplier，layoutPriority，layoutConstant，layoutRelation来创建一个NSLayoutConstraint，赋值给layoutConstraint，并把这个layoutConstraint添加到installedView中。
所以make.left后面的.equalTo(superView)和.offset(@10)就是对当前MASViewConstraint的layoutRelation，secondViewAttribute、layoutConstant的赋值。看下是否是这样：

//基类MASConstraint
- (MASConstraint * (^)(id))equalTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
    };
}
- (MASConstraint * (^)(CGFloat))offset {
    return ^id(CGFloat offset){
        self.offset = offset;
        return self;
    };
}
//抽象方法，必须子类实现
- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation { MASMethodNotImplemented(); }

//子类MASViewConstraint的实现
- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {
    return ^id(id attribute, NSLayoutRelation relation) {
        if ([attribute isKindOfClass:NSArray.class]) {
         ...
        } else {
            NSAssert(!self.hasLayoutRelation || self.layoutRelation == relation && [attribute isKindOfClass:NSValue.class], @"Redefinition of constraint relation");
            self.layoutRelation = relation;
            self.secondViewAttribute = attribute;
            return self;
        }
    };
}

- (void)setOffset:(CGFloat)offset {
    self.layoutConstant = offset;
}

以上都是对单一的Constraint操作原理，也是最简单的操作。

我们现在从第二种来看Masonry的实现

[view mas_makeConstraints:^(MASConstraintMaker *make) { 
make.left.right.top.bottom.equalTo(superView).inset(UIEdgeInsetsMake(10,10,10,10));
}];

make.left前面已经分析过，现在看看.right干了什么。

//MASConstraint中right的getter方法
- (MASConstraint *)right {
    return [self addConstraintWithLayoutAttribute:NSLayoutAttributeRight];
}
//抽象方法，需要子类实现
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute __unused)layoutAttribute {
    MASMethodNotImplemented();
}

//子类MASViewConstraint的实现
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    NSAssert(!self.hasLayoutRelation, @"Attributes should be chained before defining the constraint relation");

    return [self.delegate constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
}

呵呵哒，交给delegate去做啦。

MASConstraintDelegate
定义：

@protocol MASConstraintDelegate 
- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint;
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute;
@end

理解代理的作用先要理解容器的概念。在Masonry里存在两种容器

• 一种是MASConstraintMaker，里面有constraints，这个容器是用来放入使用make来创建的多个约束。

  [greenView makeConstraints:^(MASConstraintMaker *make) {
  //这里使用make写了6行，所以有6个MASConstraint在make的constraint里面
        make.top.greaterThanOrEqualTo(superview).offset(padding);
        make.left.equalTo(superview).offset(padding);
        make.bottom.equalTo(blueView.top).offset(-padding);
        make.right.equalTo(redView.left).offset(-padding);
  
        make.width.height.equalTo(redView);
        make.height.equalTo(blueView).multipliedBy(0.3);
    }];

• 一种是MASCompositeConstraint，里面有childConstraints

  [greenView makeConstraints:^(MASConstraintMaker *make) {
  //这里使用make写了1行，所以只有1个MASConstraint（MASCompositeConstraint）在make的constraint里面
  //MASCompositeConstraint里面的childConstraints有4个MASViewConstraint，分别是:
  //view.top equalTo superview.top
  //view.right equaoTo superview.right
  //view.bottom equalTo superview.bottom
  //view.right equalTo superview.right
        make.top.right.bottom.left.equalTo(superview);
    }];

下面来看这句代码在容器中的演变过程：

make.top.right.bottom.left.equalTo(superview)

• make.top
  • 生成对象A:MASViewConstraint(view.top)
  • 将A的delegate设为make
  • 将A放入make的constraints中，此时make.constraints = [A]
  • 返回A
• make.top.right
  • 生成对象B:MASViewConstraint(view.right)
  • 使用A和B生成MASCompositeConstraint对象C，将A，B的delegate设为C
  • 将C在make.constraints中替换A，此时make.constraints = [C]，C.childConstraints = [A,B]
  • 返回C
• make.top.right.bottom
  • 生成对象D:MASViewConstraint(view.bottom)，D的delegate为C
  • 将D放入C.childConstraints中，此时C.childConstraints = [A,B,D]
  • 返回C
• make.top.right.bottom.left
  • 生成对象E:MASViewConstraint(view.left)，E的delegate为C
  • 将E放入C.childConstraints中，此时C.childConstraints = [A,B,D,E]
  • 返回C
• make.top.right.bottom.left.equalTo(superview)
  • 会依次调用A,B,D,E的equalTo(superView)

在上面的过程中可以看到：

• 对make.constraints的添加和替换元素的操作
• 对MASCompositeConstraint对象的添加元素的操作（当.equalTo(@[view1,view2])时就有替换操作了，在里面没体现出）。
• 每个constraint的delegate为它的父容器，因为需要父容器来执行添加和替换约束的操作。

理解上面的部分，代码就很好理解了：
基类MASConstraint的代码：

#pragma mark - Chaining

- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute __unused)layoutAttribute {
    MASMethodNotImplemented();
}

- (MASConstraint *)left {
    return [self addConstraintWithLayoutAttribute:NSLayoutAttributeLeft];
}
- (MASConstraint *)top {
    return [self addConstraintWithLayoutAttribute:NSLayoutAttributeTop];
}
...
- (MASConstraint * (^)(id))equalTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
    };
}
...

MASViewConstraint的实现

#pragma mark - attribute chaining
//添加操作
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    NSAssert(!self.hasLayoutRelation, @"Attributes should be chained before defining the constraint relation");

    return [self.delegate constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
}

//里面有替换操作
- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {
    return ^id(id attribute, NSLayoutRelation relation) {
        if ([attribute isKindOfClass:NSArray.class]) {
            //对应.equalTo(@[view1,view2])这种写法
            NSAssert(!self.hasLayoutRelation, @"Redefinition of constraint relation");
            NSMutableArray *children = NSMutableArray.new;
            for (id attr in attribute) {
                MASViewConstraint *viewConstraint = [self copy];
                viewConstraint.layoutRelation = relation;
                viewConstraint.secondViewAttribute = attr;
                [children addObject:viewConstraint];
            }
            MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
            compositeConstraint.delegate = self.delegate;
            //使用delegate的shouldBeReplacedWithConstraint方法
            [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;
        }
    };
}

MASCompositeConstraint的实现

#pragma mark - attribute chaining

- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    [self constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
    return self;
}
#pragma mark - MASConstraintDelegate

- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint {
    NSUInteger index = [self.childConstraints indexOfObject:constraint];
    NSAssert(index != NSNotFound, @"Could not find constraint %@", constraint);
    [self.childConstraints replaceObjectAtIndex:index withObject:replacementConstraint];
}

- (MASConstraint *)constraint:(MASConstraint __unused *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    id strongDelegate = self.delegate;
    MASConstraint *newConstraint = [strongDelegate constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
    newConstraint.delegate = self;
    [self.childConstraints addObject:newConstraint];
    return newConstraint;
}

MASConstraintMaker的实现

#pragma mark - MASConstraintDelegate

- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint {
    NSUInteger index = [self.constraints indexOfObject:constraint];
    NSAssert(index != NSNotFound, @"Could not find constraint %@", constraint);
    [self.constraints replaceObjectAtIndex:index withObject:replacementConstraint];
}

- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
    MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
    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;
    }
    if (!constraint) {
        newConstraint.delegate = self;
        [self.constraints addObject:newConstraint];
    }
    return newConstraint;
}

从第一种写法来看Masnory的实现
实现代码：

- (MASConstraint *)addConstraintWithAttributes:(MASAttribute)attrs {
    __unused MASAttribute anyAttribute = (MASAttributeLeft | MASAttributeRight | MASAttributeTop | MASAttributeBottom | MASAttributeLeading
                                          | MASAttributeTrailing | MASAttributeWidth | MASAttributeHeight | MASAttributeCenterX
                                          | MASAttributeCenterY | MASAttributeBaseline
#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000) || (__MAC_OS_X_VERSION_MIN_REQUIRED >= 101100)
                                          | MASAttributeFirstBaseline | MASAttributeLastBaseline
#endif
#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000)
                                          | MASAttributeLeftMargin | MASAttributeRightMargin | MASAttributeTopMargin | MASAttributeBottomMargin
                                          | MASAttributeLeadingMargin | MASAttributeTrailingMargin | MASAttributeCenterXWithinMargins
                                          | MASAttributeCenterYWithinMargins
#endif
                                          );

    NSAssert((attrs & anyAttribute) != 0, @"You didn't pass any attribute to make.attributes(...)");

    NSMutableArray *attributes = [NSMutableArray array];

    if (attrs & MASAttributeLeft) [attributes addObject:self.view.mas_left];
    if (attrs & MASAttributeRight) [attributes addObject:self.view.mas_right];
    if (attrs & MASAttributeTop) [attributes addObject:self.view.mas_top];
    if (attrs & MASAttributeBottom) [attributes addObject:self.view.mas_bottom];
    if (attrs & MASAttributeLeading) [attributes addObject:self.view.mas_leading];
    if (attrs & MASAttributeTrailing) [attributes addObject:self.view.mas_trailing];
    if (attrs & MASAttributeWidth) [attributes addObject:self.view.mas_width];
    if (attrs & MASAttributeHeight) [attributes addObject:self.view.mas_height];
    if (attrs & MASAttributeCenterX) [attributes addObject:self.view.mas_centerX];
    if (attrs & MASAttributeCenterY) [attributes addObject:self.view.mas_centerY];
    if (attrs & MASAttributeBaseline) [attributes addObject:self.view.mas_baseline];

#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000) || (__MAC_OS_X_VERSION_MIN_REQUIRED >= 101100)

    if (attrs & MASAttributeFirstBaseline) [attributes addObject:self.view.mas_firstBaseline];
    if (attrs & MASAttributeLastBaseline) [attributes addObject:self.view.mas_lastBaseline];

#endif

#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000)

    if (attrs & MASAttributeLeftMargin) [attributes addObject:self.view.mas_leftMargin];
    if (attrs & MASAttributeRightMargin) [attributes addObject:self.view.mas_rightMargin];
    if (attrs & MASAttributeTopMargin) [attributes addObject:self.view.mas_topMargin];
    if (attrs & MASAttributeBottomMargin) [attributes addObject:self.view.mas_bottomMargin];
    if (attrs & MASAttributeLeadingMargin) [attributes addObject:self.view.mas_leadingMargin];
    if (attrs & MASAttributeTrailingMargin) [attributes addObject:self.view.mas_trailingMargin];
    if (attrs & MASAttributeCenterXWithinMargins) [attributes addObject:self.view.mas_centerXWithinMargins];
    if (attrs & MASAttributeCenterYWithinMargins) [attributes addObject:self.view.mas_centerYWithinMargins];

#endif

    NSMutableArray *children = [NSMutableArray arrayWithCapacity:attributes.count];

    for (MASViewAttribute *a in attributes) {
        [children addObject:[[MASViewConstraint alloc] initWithFirstViewAttribute:a]];
    }

    MASCompositeConstraint *constraint = [[MASCompositeConstraint alloc] initWithChildren:children];
    constraint.delegate = self;
    [self.constraints addObject:constraint];
    return constraint;
}

很暴力通俗的代码，不用解释了...
所以make.edge就是：

- (MASConstraint *)edges {
    return [self addConstraintWithAttributes:MASAttributeTop | MASAttributeLeft | MASAttributeRight | MASAttributeBottom];
}

也可以在外部使用:

@property (nonatomic, strong, readonly) MASConstraint *(^attributes)(MASAttribute attrs);

写法：

//与make.edge是相同的
make.attributes(MASAttributeTop | MASAttributeLeft | MASAttributeRight | MASAttributeBottom);

---

Masnory如何make,reMake,updateConstraint
直接看代码

@implementation MAS_VIEW (MASAdditions)

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

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

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

在普通的mas_makeConstraints三部曲：创建maker，配置maker，install maker中，mas_updateConstraints将maker的updateExisting设为YES。mas_remakeConstraints将maker的removeExisting设为YES。
来看看updateExisting和removeExisting在install方法中如何操控的：

- (NSArray *)install {
    if (self.removeExisting) {
//获得view的所有生效的约束（MASViewConstraint类型，封装了NSLayoutConstraint），然后uninstall
        NSArray *installedConstraints = [MASViewConstraint installedConstraintsForView:self.view];
        for (MASConstraint *constraint in installedConstraints) {
            [constraint uninstall];
        }
    }
    NSArray *constraints = self.constraints.copy;
    for (MASConstraint *constraint in constraints) {
//标记updateExisting为YES的时候，在constraint的install里面会去找是否有相同的约束
        constraint.updateExisting = self.updateExisting;
        [constraint install];
    }
    [self.constraints removeAllObjects];
    return constraints;
}

关于MASConstraint的install，在MASCompositeConstraint中是循环childContraints的install，在MASViewContraint中是用当前的所有属性创建layoutConstraint并添加到installView上，installVIew如何确定，NSLayoutConstraint对象应该添加到哪个view上面呢？已经说了。代码很好理解，不贴了。

至于设置了updateExisting的install流程的改变：

• 找到相同的约束，何为相同，只有constant不同，其它都要相同。

  -(MASLayoutConstraint *)layoutConstraintSimilarTo:(MASLayoutConstraint *)layoutConstraint {
  
    for (NSLayoutConstraint *existingConstraint in self.installedView.constraints.reverseObjectEnumerator) {
        if (![existingConstraint isKindOfClass:MASLayoutConstraint.class]) continue;
        if (existingConstraint.firstItem != layoutConstraint.firstItem) continue;
        if (existingConstraint.secondItem != layoutConstraint.secondItem) continue;
        if (existingConstraint.firstAttribute != layoutConstraint.firstAttribute) continue;
        if (existingConstraint.secondAttribute != layoutConstraint.secondAttribute) continue;
        if (existingConstraint.relation != layoutConstraint.relation) continue;
        if (existingConstraint.multiplier != layoutConstraint.multiplier) continue;
        if (existingConstraint.priority != layoutConstraint.priority) continue;
  
        return (id)existingConstraint;
    }
    return nil;
  }

• 如果找到了就改变已经返回的constraint的constant，并把这个constraint赋值给layoutConstraint就OVER了。