一、封裝結(jié)構(gòu)

MASConstraint 約束
MASCompositeConstraint 一組約束//
MASViewConstraint 一個約束//囊括整個約束計算公式的所有數(shù)據(jù)（firstViewAttribute、secondViewAttribute、常量值、關(guān)系、優(yōu)先級、比例值。用于鏈式操作的“delegate”）。

MASConstraintMaker 創(chuàng)建約束的工廠
MASViewAttribute 封裝 View+約束， 等式兩邊的計算因子（結(jié)構(gòu)相當于元組類型）

View+MASAdditions 入口（ 創(chuàng)建計算MASViewAttribute的工廠 ）
ViewController+MASAdditions 入口（ 創(chuàng)建MASViewAttribute的工廠 ）

NSArray+MASAdditions 對數(shù)組中的每一個view做相同的約束操作。 批量操作。還提供了一套統(tǒng)一布局的方法。（固定大小，固定間距）
MASUtilities 用來將任何類型（數(shù)值類型，對象類型）， 轉(zhuǎn)化為對象類型

二、鏈式

1、

make.height.inset(10);//無效。因為layoutAttribute是NSLayoutAttributeHeight
make.height.mas_equalTo(10);

2、

make.height.mas_equalTo(10); // 如果是NSValue類型。給firstView設置
make.height.mas_equalTo(self.view);// 如果是View類型， 創(chuàng)建secondViewAttribute。并且屬性NSLayoutAttribute與firstViewAttribute相同
make.height.mas_equalTo(self.view.mas_height); //如果是計算因子類型。則直接賦值。

3，

鏈式如何實現(xiàn)的？

MASCompositeConstraint（一組約束） 代理。
MASConstraintMaker （創(chuàng)建約束的工廠） 代理。

//操作1
make.top.offset(0);
make.bottom.offset(0);
make.size.sizeOffset(CGSizeZero);

//鏈式1
make.top.left.right.offset(0);

//鏈式2
make.edges.top.left.offset(0);

//鏈式3
make.top.equalTo(@20).equalTo(@30);

// 不支持這種鏈式： 后面的-10會覆蓋前面的10
make.top.left.equalTo(@10).bottom.right.equalTo(@(-10));

    MASLayoutConstraint *existingConstraint = nil;
    if (self.updateExisting) {
        existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];
    }
    if (existingConstraint) {
        // just update the constant
        existingConstraint.constant = layoutConstraint.constant;
        self.layoutConstraint = existingConstraint;
    }////////

//不支持重復添加約束
//equalTo(v2.mas_left)第二個約束，重復添加。
//NSAssert(!self.hasLayoutRelation || self.layoutRelation == relation && [attribute isKindOfClass:NSValue.class], @"Redefinition of constraint relation");

make.top.left.equalTo(v2.mas_top).equalTo(v2.mas_left).equalTo(@0);

// 不支持這種鏈式
make.center.size.edges

masConstraint

#pragma mark - attribute chaining

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

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

masCompositeConstraint

#pragma mark - attribute chaining

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

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


- (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];
}

maker

- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    return [self constraint:nil addConstraintWithLayoutAttribute:layoutAttribute];
}

- (MASConstraint *)addConstraintWithAttributes:(MASAttribute)attrs {
    /////
    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;

}


- (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;
}

- (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];
}

4、設置布局值

嘗試用不支持的值設置布局常量.attempting to set layout constant with unsupported value
NSNumber，CGPoint，CGSize，UIEdgeInsets

嘗試添加不支持的屬性attempting to add unsupported attribute
UIView、NSValue、MASViewAttribute

offset只是改變約束常量值
equalTo會增加約束關(guān)系（secondeViewAttribute），如果是NSValue類型（與offset效果相似），根據(jù)約束的布局屬性NSLayoutAttribute是否匹配，設置對應的常量值。

//相同
make.left.equalTo(@20);
make.left.offset(20);
//相同
make.left.equalTo(v2.mas_right).offset(20);
make.left.equalTo(v2.mas_right).equalTo(@20);

make.top.inset(0).insets(UIEdgeInsetsZero).offset(0).sizeOffset(CGSizeZero).centerOffset(CGPointZero).valueOffset([NSValue valueWithCGPoint:CGPointZero]).valueOffset( [NSValue value:&CGPointZero withObjCType:@encode(typeof((CGPointZero)))]);

//NSLayoutAttributeTop,NSLayoutAttributeLeft ， 會造成崩潰
Constraint improperly relates anchors of incompatible types

make.top.equalTo(v2.mas_left);//崩潰
make.top.equalTo(@[v2.mas_top,v2.mas_left]);//崩潰

如果是第一個View不是大小約束，且第二個Item是nil，則第二個Item自動設置為第一個View的父View，屬性關(guān)系與第一個View的屬性相同。

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