OC 常用的約束框架是Masonry，而swift常用的是SnapKit，不過今天就只看看Masonry。

先看個例子：

[view mas_makeConstraints:^(MASConstraintMaker *make) {
    make.left.offset(0);
    make.top.offset(0);
    make.right.offset(0);
    make.height.offset(300);
}];

[lable mas_updateConstraints:^(MASConstraintMaker *make) {
    make.top.offset(600);
    make.left.offset(100);
}];

[button mas_makeConstraints:^(MASConstraintMaker *make) {
    make.top.equalTo(view.mas_bottom).offset(10);
    make.left.equalTo(view);
    make.height.offset(50);
    make.width.offset(50);
}];

• 源碼

1. 首先來看mas_makeConstraints，內(nèi)部初始化MASConstraintMaker，然后用block送出去操作：

#define MAS_VIEW UIView
@implementation MAS_VIEW (MASAdditions)

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

2. 然后先回到外面來，看一下make.left（right,top,height等等都一樣），最終MASConstraintMaker的操作都會加到約束數(shù)組中：

@implementation MASConstraintMaker

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

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

- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    ...
    if (!constraint) {
        newConstraint.delegate = self;
        [self.constraints addObject:newConstraint];//添加到數(shù)組
    }
    return newConstraint;
}

3. 添加到約束數(shù)組之后返回了MASConstraint，接著來看看equalTo，offset：

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

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

• equalTo，offset其實都是block形式，所以調(diào)用時可以用()而不用[]，而且都返回MASConstraint，所以可以連續(xù)調(diào)用，緊接著make.left使用鏈式語法。

4. block(constraintMaker)執(zhí)行完后，就進行[constraintMaker install]：

@implementation MASConstraintMaker

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

會先判斷self.removeExisting移除舊約束，再遍歷約束數(shù)組進行[constraint install]：

- (void)install {
    ...
    //MASLayoutConstraint 繼承于 NSLayoutConstraint
    MASLayoutConstraint *layoutConstraint
        = [MASLayoutConstraint constraintWithItem:firstLayoutItem
                                        attribute:firstLayoutAttribute
                                        relatedBy:self.layoutRelation
                                           toItem:secondLayoutItem
                                        attribute:secondLayoutAttribute
                                       multiplier:self.layoutMultiplier
                                         constant:self.layoutConstant];
    
    layoutConstraint.priority = self.layoutPriority;
    layoutConstraint.mas_key = self.mas_key;
    //找出約束添加到哪個view上，并賦值給self.installedView
    //self.firstViewAttribute.view是要約束的view，self.secondViewAttribute.view是作為參考的view
    if (self.secondViewAttribute.view) {
        MAS_VIEW *closestCommonSuperview = [self.firstViewAttribute.view mas_closestCommonSuperview:self.secondViewAttribute.view];
        self.installedView = closestCommonSuperview;//第一個和第二個view共同最近的一個superView
    } else if (self.firstViewAttribute.isSizeAttribute) {
        self.installedView = self.firstViewAttribute.view;//如果約束寬高，則自己添加約束
    } else {
        self.installedView = self.firstViewAttribute.view.superview;//默認父控件
    }

    MASLayoutConstraint *existingConstraint = nil;
    if (self.updateExisting) {
        existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];
    }
    if (existingConstraint) {
        // just update the constant
        existingConstraint.constant = layoutConstraint.constant;//更新
        self.layoutConstraint = existingConstraint;
    } else {
        [self.installedView addConstraint:layoutConstraint];//添加約束
        self.layoutConstraint = layoutConstraint;
        [firstLayoutItem.mas_installedConstraints addObject:self];
    }
}

添加約束的控件默認是父控件，如果固定寬高則為自身，如果有self.secondViewAttribute.view則找出共同父控件，最終判斷self.updateExisting給控件添加約束或者更新約束。

5. 另外，上面出現(xiàn)的self.updateExisting會在mas_updateConstraints里設(shè)置，self.removeExisting會在mas_remakeConstraints里設(shè)置：

- (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];
}

• 總結(jié)

mas_makeConstraints會返回該View的約束數(shù)組，通過block傳送MASConstraintMaker（負責管理約束）添加約束，block完畢后，
MASConstraintMaker調(diào)用install，然后會遍歷約束數(shù)組，每一個MASViewConstraint都調(diào)用install，最終會調(diào)用原生的方法添加約束。