核心数据结构

1. MASViewAttribute
   MASViewAttribute存储了UIView及其布局属性（NSLayoutAttribute），用于约束等式的左部分或右部分。

@interface MASViewAttribute : NSObject

@property (nonatomic, weak, readonly) MAS_VIEW *view;
@property (nonatomic, weak, readonly) id item;
@property (nonatomic, assign, readonly) NSLayoutAttribute layoutAttribute;

@end

2. MASConstraint,
   MASConstraint可以表示单个布局约束(MASViewConstraint)，也可以表示多个布局约束(MASCompositeConstraint)。MASConstraint支持用链式语法创建新的MASConstraint或者修改MASConstraint自身的一些属性。

3.MASViewConstraint继承自MASConstraint，表示单个布局约束。

@interface MASViewConstraint : MASConstraint <NSCopying>

@property (nonatomic, strong, readonly) MASViewAttribute *firstViewAttribute;
@property (nonatomic, strong, readonly) MASViewAttribute *secondViewAttribute;

@property (nonatomic, assign) NSLayoutRelation layoutRelation;
@property (nonatomic, assign) MASLayoutPriority layoutPriority;
@property (nonatomic, assign) CGFloat layoutMultiplier;
@property (nonatomic, assign) CGFloat layoutConstant;

@end

4.MASCompositeConstraint继承自MASConstraint，表示多个布局约束，其实现就是MASViewConstraint的数组容器。

@interface MASCompositeConstraint () <MASConstraintDelegate>

@property (nonatomic, strong) NSMutableArray *childConstraints;

@end

4.MASConstraintMaker
MASConstraintMaker用来创建并存储MASConstraint。

@interface MASConstraintMaker () <MASConstraintDelegate>

@property (nonatomic, weak) MAS_VIEW *view;
@property (nonatomic, strong) NSMutableArray *constraints;

@end

使用分析

示例代码：

    [superView addSubview:someView];
    [someView mas_makeConstraints:^(MASConstraintMaker *make) {  //c1
        make.left.right.top.equalTo(superView).offset(100);      //c2
        make.height.equalTo(@100);                               //c3
    }];

c1语句的内部实现如下

- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {
    self.translatesAutoresizingMaskIntoConstraints = NO;
//创建MASConstraintMaker对象
    MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];
//调用传入的block，block内部会创建若干个MASConstraint，并添加到constraintMaker的constraints数组里。
    block(constraintMaker);
//安装constraints数组中的约束，即创建若干个NSLayoutContraint,并添加到合适的UIView上。
    return [constraintMaker install];
}

c2语句调用的一系列方法内部实现如下，可以发现以下方法返回的对象是MASConstraint对象或者是Block对象（block返回的对象是MASConstraint对象），这正是链式编程的思想，可以通过点‘.’来连接不同的函数调用 。

@implementation MASConstraintMaker

- (MASConstraint *)left {
    return [self addConstraintWithLayoutAttribute:NSLayoutAttributeLeft];
}

end

@implementation MASConstraint

#pragma mark - Chaining

- (MASConstraint *)right {
    return [self addConstraintWithLayoutAttribute:NSLayoutAttributeRight];
}

- (MASConstraint *)top {
    return [self addConstraintWithLayoutAttribute:NSLayoutAttributeTop];
}

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

#pragma mark - NSLayoutRelation proxies

- (MASConstraint * (^)(id))equalTo {
    return ^id(id attribute) {
        return self.equalToWithRelation(attribute, NSLayoutRelationEqual);
    };
}

#pragma mark - NSLayoutConstraint constant proxies
- (MASConstraint * (^)(CGFloat))offset {
    return ^id(CGFloat offset){
        self.offset = offset;
        return self;
    };
}
@end

按照上面的内部实现，我们可以将示例代码展开整理为：

- (void)createViews {
    [superView addSubview:someView];
    [someView mas_makeConstraints:^(MASConstraintMaker *make) {
        //创建一个视图约束leftViewConstraint，并将约束添加到maker.constraints中，最后返回leftViewConstraint。
        MASConstraint *leftViewConstraint = make.left;
        //创建一个视图约束rightViewConstraint，一个组合约束compositeConstraint,将leftViewConstraint和rightViewConstraint加到compositeConstraint1.childConstraints中，再将maker.constraints中的leftViewConstraint替换成compositeConstraint，最后返回compositeConstraint。
        MASConstraint *compositeConstraint = leftViewConstraint.right;
        //创建一个视图约束topViewConstraint,将其添加到compositeConstraint.childConstraints，最后返回compositeConstraint。
        compositeConstraint = compositeConstraint.top;
        //设置compositeConstraint.childConstraints中的每个视图约束的secondViewAttribute。
        compositeConstraint = compositeConstraint.equalTo(superView);
        //设置compositeConstraint.childConstraints中的每个视图约束的layoutConstant。
        compositeConstraint = compositeConstraint.offset(100);
        

        //创建一个视图约束heightViewConstraint，并将约束添加到maker.constraints中，最后返回heightViewConstraint。
        MASConstraint *heightViewConstraint = make.height;
        //设置heightViewConstraint的layoutConstant，并返回heightViewConstraint。
        heightViewConstraint = heightViewConstraint.equalTo(@100);
        
        //最后maker.constraints中有两个元素：compositeConstraint和heightViewConstraint。
    }];
}