Masonry框架详细解析（三） —— MASConstraintMaker工厂类（一）

版本记录

版本号	时间
V1.0	2018.07.17

前言

我们做APP界面，也就是布局UI，那么关于布局，我们有很多方法，苹果也都提供了支持，市场上我们用的并不是系统提供原生的layout，对于OC语言一般都是使用一个第三方的布局框架 —— Masonry。接下来几篇我们就一起深入看一下这个框架。感兴趣的看上面几篇文章。
1. Masonry框架详细解析（一） —— 基本概览（一）
2. Masonry框架详细解析（二） —— 基本结构API和约束入口（一）

API

首先我们看一下API接口。

#import "MASConstraint.h"
#import "MASUtilities.h"

typedef NS_OPTIONS(NSInteger, MASAttribute) {
    MASAttributeLeft = 1 << NSLayoutAttributeLeft,
    MASAttributeRight = 1 << NSLayoutAttributeRight,
    MASAttributeTop = 1 << NSLayoutAttributeTop,
    MASAttributeBottom = 1 << NSLayoutAttributeBottom,
    MASAttributeLeading = 1 << NSLayoutAttributeLeading,
    MASAttributeTrailing = 1 << NSLayoutAttributeTrailing,
    MASAttributeWidth = 1 << NSLayoutAttributeWidth,
    MASAttributeHeight = 1 << NSLayoutAttributeHeight,
    MASAttributeCenterX = 1 << NSLayoutAttributeCenterX,
    MASAttributeCenterY = 1 << NSLayoutAttributeCenterY,
    MASAttributeBaseline = 1 << NSLayoutAttributeBaseline,
    
#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000) || (__MAC_OS_X_VERSION_MIN_REQUIRED >= 101100)
    
    MASAttributeFirstBaseline = 1 << NSLayoutAttributeFirstBaseline,
    MASAttributeLastBaseline = 1 << NSLayoutAttributeLastBaseline,
    
#endif
    
#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000)
    
    MASAttributeLeftMargin = 1 << NSLayoutAttributeLeftMargin,
    MASAttributeRightMargin = 1 << NSLayoutAttributeRightMargin,
    MASAttributeTopMargin = 1 << NSLayoutAttributeTopMargin,
    MASAttributeBottomMargin = 1 << NSLayoutAttributeBottomMargin,
    MASAttributeLeadingMargin = 1 << NSLayoutAttributeLeadingMargin,
    MASAttributeTrailingMargin = 1 << NSLayoutAttributeTrailingMargin,
    MASAttributeCenterXWithinMargins = 1 << NSLayoutAttributeCenterXWithinMargins,
    MASAttributeCenterYWithinMargins = 1 << NSLayoutAttributeCenterYWithinMargins,

#endif
    
};

/**
 *  Provides factory methods for creating MASConstraints.
 *  Constraints are collected until they are ready to be installed
 *
 */
@interface MASConstraintMaker : NSObject

/**
 *  The following properties return a new MASViewConstraint
 *  with the first item set to the makers associated view and the appropriate MASViewAttribute
 */
@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;

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

@property (nonatomic, strong, readonly) MASConstraint *firstBaseline;
@property (nonatomic, strong, readonly) MASConstraint *lastBaseline;

#endif

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

@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;

#endif

/**
 *  Returns a block which creates a new MASCompositeConstraint with the first item set
 *  to the makers associated view and children corresponding to the set bits in the
 *  MASAttribute parameter. Combine multiple attributes via binary-or.
 */
@property (nonatomic, strong, readonly) MASConstraint *(^attributes)(MASAttribute attrs);

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeEdges
 *  which generates the appropriate MASViewConstraint children (top, left, bottom, right)
 *  with the first item set to the makers associated view
 */
@property (nonatomic, strong, readonly) MASConstraint *edges;

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeSize
 *  which generates the appropriate MASViewConstraint children (width, height)
 *  with the first item set to the makers associated view
 */
@property (nonatomic, strong, readonly) MASConstraint *size;

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeCenter
 *  which generates the appropriate MASViewConstraint children (centerX, centerY)
 *  with the first item set to the makers associated view
 */
@property (nonatomic, strong, readonly) MASConstraint *center;

/**
 *  Whether or not to check for an existing constraint instead of adding constraint
 */
@property (nonatomic, assign) BOOL updateExisting;

/**
 *  Whether or not to remove existing constraints prior to installing
 */
@property (nonatomic, assign) BOOL removeExisting;

/**
 *  initialises the maker with a default view
 *
 *  @param  view    any MASConstraint are created with this view as the first item
 *
 *  @return a new MASConstraintMaker
 */
- (id)initWithView:(MAS_VIEW *)view;

/**
 *  Calls install method on any MASConstraints which have been created by this maker
 *
 *  @return an array of all the installed MASConstraints
 */
- (NSArray *)install;

- (MASConstraint * (^)(dispatch_block_t))group;

@end

这里MASConstraintMaker就是工厂方法，用于生成约束。具体负责创建MASConstraint类型的对象（依赖于MASConstraint接口，而不依赖于具体实现）。mas_makeConstraints方法中的Block的参数就是MASConstraintMaker的对象。用户可以通过该Block回调过来的MASConstraintMaker对象给View指定要添加的约束以及该约束的值。该工厂中的.m文件中的constraints属性数组就记录了该工厂创建的所有MASConstraint对象。

#define MAS_VIEW UIView

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

1. MASAttribute枚举

typedef NS_OPTIONS(NSInteger, MASAttribute) {
    MASAttributeLeft = 1 << NSLayoutAttributeLeft,
    MASAttributeRight = 1 << NSLayoutAttributeRight,
    MASAttributeTop = 1 << NSLayoutAttributeTop,
    MASAttributeBottom = 1 << NSLayoutAttributeBottom,
    MASAttributeLeading = 1 << NSLayoutAttributeLeading,
    MASAttributeTrailing = 1 << NSLayoutAttributeTrailing,
    MASAttributeWidth = 1 << NSLayoutAttributeWidth,
    MASAttributeHeight = 1 << NSLayoutAttributeHeight,
    MASAttributeCenterX = 1 << NSLayoutAttributeCenterX,
    MASAttributeCenterY = 1 << NSLayoutAttributeCenterY,
    MASAttributeBaseline = 1 << NSLayoutAttributeBaseline,
    
#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000) || (__MAC_OS_X_VERSION_MIN_REQUIRED >= 101100)
    
    MASAttributeFirstBaseline = 1 << NSLayoutAttributeFirstBaseline,
    MASAttributeLastBaseline = 1 << NSLayoutAttributeLastBaseline,
    
#endif
    
#if (__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000) || (__TV_OS_VERSION_MIN_REQUIRED >= 9000)
    
    MASAttributeLeftMargin = 1 << NSLayoutAttributeLeftMargin,
    MASAttributeRightMargin = 1 << NSLayoutAttributeRightMargin,
    MASAttributeTopMargin = 1 << NSLayoutAttributeTopMargin,
    MASAttributeBottomMargin = 1 << NSLayoutAttributeBottomMargin,
    MASAttributeLeadingMargin = 1 << NSLayoutAttributeLeadingMargin,
    MASAttributeTrailingMargin = 1 << NSLayoutAttributeTrailingMargin,
    MASAttributeCenterXWithinMargins = 1 << NSLayoutAttributeCenterXWithinMargins,
    MASAttributeCenterYWithinMargins = 1 << NSLayoutAttributeCenterYWithinMargins,

#endif

该枚举其实就是对NSLayoutAttribute枚举的一种封装。

typedef NS_ENUM(NSInteger, NSLayoutAttribute) {
    NSLayoutAttributeLeft = 1,
    NSLayoutAttributeRight,
    NSLayoutAttributeTop,
    NSLayoutAttributeBottom,
    NSLayoutAttributeLeading,
    NSLayoutAttributeTrailing,
    NSLayoutAttributeWidth,
    NSLayoutAttributeHeight,
    NSLayoutAttributeCenterX,
    NSLayoutAttributeCenterY,
    NSLayoutAttributeLastBaseline,
    NSLayoutAttributeBaseline NS_SWIFT_UNAVAILABLE("Use 'lastBaseline' instead") = NSLayoutAttributeLastBaseline,
    NSLayoutAttributeFirstBaseline NS_ENUM_AVAILABLE_IOS(8_0),
    
    
    NSLayoutAttributeLeftMargin NS_ENUM_AVAILABLE_IOS(8_0),
    NSLayoutAttributeRightMargin NS_ENUM_AVAILABLE_IOS(8_0),
    NSLayoutAttributeTopMargin NS_ENUM_AVAILABLE_IOS(8_0),
    NSLayoutAttributeBottomMargin NS_ENUM_AVAILABLE_IOS(8_0),
    NSLayoutAttributeLeadingMargin NS_ENUM_AVAILABLE_IOS(8_0),
    NSLayoutAttributeTrailingMargin NS_ENUM_AVAILABLE_IOS(8_0),
    NSLayoutAttributeCenterXWithinMargins NS_ENUM_AVAILABLE_IOS(8_0),
    NSLayoutAttributeCenterYWithinMargins NS_ENUM_AVAILABLE_IOS(8_0),
    
    NSLayoutAttributeNotAnAttribute = 0
};

这里还对版本进行了判断，一般是以iOS8.0作为分割线进行判断。

__IPHONE_OS_VERSION_MIN_REQUIRED >= 80000
NS_ENUM_AVAILABLE_IOS(8_0)

2. 几个MASConstraint属性

/**
 *  The following properties return a new MASViewConstraint
 *  with the first item set to the makers associated view and the appropriate MASViewAttribute
 */
@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;

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

@property (nonatomic, strong, readonly) MASConstraint *firstBaseline;
@property (nonatomic, strong, readonly) MASConstraint *lastBaseline;

#endif

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

@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;

#endif

/**
 *  Returns a block which creates a new MASCompositeConstraint with the first item set
 *  to the makers associated view and children corresponding to the set bits in the
 *  MASAttribute parameter. Combine multiple attributes via binary-or.
 */
@property (nonatomic, strong, readonly) MASConstraint *(^attributes)(MASAttribute attrs);

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeEdges
 *  which generates the appropriate MASViewConstraint children (top, left, bottom, right)
 *  with the first item set to the makers associated view
 */
@property (nonatomic, strong, readonly) MASConstraint *edges;

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeSize
 *  which generates the appropriate MASViewConstraint children (width, height)
 *  with the first item set to the makers associated view
 */
@property (nonatomic, strong, readonly) MASConstraint *size;

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeCenter
 *  which generates the appropriate MASViewConstraint children (centerX, centerY)
 *  with the first item set to the makers associated view
 */
@property (nonatomic, strong, readonly) MASConstraint *center;

这里属性的定义也是分版本的，这里有几点说明：

1）布局属性定义

这里几个属性都是MASConstraint类型的，以iOS8.0为界限进行定义。下面就以left属性为例看一下实现。

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

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

这里，不管是left还是right或者center都会调用方法- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute，然后就是添加约束。这里依赖了两个类，分别是MASViewAttribute和MASViewConstraint。

主要看一下下面这个方法，这个走的MASConstraint+Private.h文件中的代理MASConstraintDelegate，该代理中有两个方法。

@protocol MASConstraintDelegate <NSObject>

/**
 *  Notifies the delegate when the constraint needs to be replaced with another constraint. For example
 *  A MASViewConstraint may turn into a MASCompositeConstraint when an array is passed to one of the equality blocks
 */
- (void)constraint:(MASConstraint *)constraint shouldBeReplacedWithConstraint:(MASConstraint *)replacementConstraint;

- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute;

@end

maker工厂方法中都实现了这个代理方法。

//代理方法实现
- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {
    //实例化MASViewAttribute
    MASViewAttribute *viewAttribute = [[MASViewAttribute alloc] initWithView:self.view layoutAttribute:layoutAttribute];
    //实例化MASViewConstraint
    MASViewConstraint *newConstraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:viewAttribute];
    //判断传入的参数是不是MASViewConstraint类型，如果是进入if分支
    if ([constraint isKindOfClass:MASViewConstraint.class]) {
        //replace with composite constraint
        NSArray *children = @[constraint, newConstraint];
         //MASCompositeConstraint实例化并设置代理
        MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];
        compositeConstraint.delegate = self;
        //调用代理方法进行约束替换
        [self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];
        return compositeConstraint;
    }
    //如果constraint为nil，那么就设置代理，并将新的约束添加到属性数组constraints里面
    if (!constraint) {
        newConstraint.delegate = self;
        [self.constraints addObject:newConstraint];
    }
    return newConstraint;
}

这里需要说明MASViewConstraint是MASConstraint的子类。通过NSLayoutAttribute创建一个MASConstraint并将创建的约束加入到数组constraints中，如果设置的是size、center等复合型的约束，则创建包含多个子约束的MASCompositeConstraint。

@interface MASViewConstraint : MASConstraint <NSCopying>

前面方法中addConstraintWithLayoutAttribute:传入的constraint参数为nil，那么在上面走代理方法的时候就走第二个if，也就是向数组里面添加约束。

这里顺带看一下另外一个MASConstraintDelegate代理方法

- (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];
}

这个代理方法很清晰了，就是当constraint不为nil时候，就将constraint替换为replacementConstraint。

2）attributes 的block

/**
 *  Returns a block which creates a new MASCompositeConstraint with the first item set
 *  to the makers associated view and children corresponding to the set bits in the
 *  MASAttribute parameter. Combine multiple attributes via binary-or.
 */
@property (nonatomic, strong, readonly) MASConstraint *(^attributes)(MASAttribute attrs);

该只读属性返回一个block，该块使用第一个item集创建新的MASCompositeConstraint与makers关联的视图和与MASAttribute参数中的设置位对应的子项。通过binary-or组合多个属性。

看一下这个block的实现

- (MASConstraint *(^)(MASAttribute))attributes {
    return ^(MASAttribute attrs){
        return [self addConstraintWithAttributes:attrs];
    };
}

该block是一个带参数和带返回值的block，传入的是MASAttribute类型的参数返回的是MASConstraint，最后都是调用下面方法进行具体的实现。

- (MASConstraint *)addConstraintWithAttributes:(MASAttribute)attrs {
      // 这里是列举出所有可能的MASAttribute类型的布局
    __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
                                          );
    //断言，如果传入的布局参数不再上面列举出的布局类型中就抛出异常
    NSAssert((attrs & anyAttribute) != 0, @"You didn't pass any attribute to make.attributes(...)");
    
    //定义一个可变数据，并判断attrs的类型，并将相应的布局添加到可变数组中
    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
      
     // 定义另外一个局部可变数组
     NSMutableArray *children = [NSMutableArray arrayWithCapacity:attributes.count];
    
    //遍历上面的attributes可变数组，用其中的元素实例化MASViewConstraint对象并添加到新的children数组中
    for (MASViewAttribute *a in attributes) {
        [children addObject:[[MASViewConstraint alloc] initWithFirstViewAttribute:a]];
    }
   
   //根据数组children实例化MASCompositeConstraint对象，并作为返回值返回
    MASCompositeConstraint *constraint = [[MASCompositeConstraint alloc] initWithChildren:children];
    constraint.delegate = self;
    [self.constraints addObject:constraint];
    return constraint;
}

3）edges、size和center

主要对应下面三个属性

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeEdges
 *  which generates the appropriate MASViewConstraint children (top, left, bottom, right)
 *  with the first item set to the makers associated view
 */

//根据MASCompositeConstraintTypeEdges类型创建一个MASCompositeConstraint对象，
//产生适合的MASViewConstraint子对象 (top, left, bottom, right)
@property (nonatomic, strong, readonly) MASConstraint *edges;

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeSize
 *  which generates the appropriate MASViewConstraint children (width, height)
 *  with the first item set to the makers associated view
 */

//根据MASCompositeConstraintTypeSize类型创建一个MASCompositeConstraint对象，
//产生适合的MASViewConstraint子对象 (width, height)
@property (nonatomic, strong, readonly) MASConstraint *size;

/**
 *  Creates a MASCompositeConstraint with type MASCompositeConstraintTypeCenter
 *  which generates the appropriate MASViewConstraint children (centerX, centerY)
 *  with the first item set to the makers associated view
 */

//根据MASCompositeConstraintTypeCenter类型创建一个MASCompositeConstraint对象，
//产生适合的MASViewConstraint子对象 (centerX, centerY)
@property (nonatomic, strong, readonly) MASConstraint *center;

下面我们以edge为例说明一下实现情况。

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

然后调用上面的方法- (MASConstraint *)addConstraintWithAttributes:(MASAttribute)attrs进行处理。

4）updateExisting

/**
 *  Whether or not to check for an existing constraint instead of adding constraint
 */

//是否去检查一个存在的约束，而不是添加约束
@property (nonatomic, assign) BOOL updateExisting;

下面我们看一下在maker工厂方法中的使用，如下所示：

- (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;
}

在maker工厂方法里面，遍历属性数组self.constraints，并调用MASConstraint的install方法安装约束。在将设置好添加到constraints数组中的约束都设置好后就要开始把约束加到View上起作用，方法为install，逻辑如下：

如果是remake，也就是removeExisting =YES时，将已经添加上的约束全部uninstall，把本次maker里的所有创建的contraint设置好updateExisting，然后一个一个的install。

- (void)install { MASMethodNotImplemented(); }

#define MASMethodNotImplemented() \
    @throw [NSException exceptionWithName:NSInternalInconsistencyException \
                                   reason:[NSString stringWithFormat:@"You must override %@ in a subclass.", NSStringFromSelector(_cmd)] \
                                 userInfo:nil]

5）removeExisting

/**
 *  Whether or not to remove existing constraints prior to installing
 */

//在安装约束前是否移除约束
@property (nonatomic, assign) BOOL removeExisting;

同样在MASConstraintMaker的install方法里面，取出当前view安装的所有约束，并调用uninstall方法卸载约束。

if (self.removeExisting) {
    NSArray *installedConstraints = [MASViewConstraint installedConstraintsForView:self.view];
    for (MASConstraint *constraint in installedConstraints) {
        [constraint uninstall];
    }
}

- (void)uninstall { MASMethodNotImplemented(); }

6）group合并约束

该方法可以将多个maker中的多个约束合并为一个约束。

- (MASConstraint * (^)(dispatch_block_t))group;

下面是其实现

- (MASConstraint *(^)(dispatch_block_t group))group {
    return ^id(dispatch_block_t group) {
        NSInteger previousCount = self.constraints.count;
        group();

        NSArray *children = [self.constraints subarrayWithRange:NSMakeRange(previousCount, self.constraints.count - previousCount)];
        MASCompositeConstraint *constraint = [[MASCompositeConstraint alloc] initWithChildren:children];
        constraint.delegate = self;
        return constraint;
    };
}

参考文章

1. Masonry研究3 MASConstraintMaker
2. Masonry介绍与使用实践(快速上手Autolayout)

后记

本篇主要讲述了MASConstraintMaker工厂类，感兴趣的给个赞或者关注~~~~

人面猴
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Masonry框架详细解析（三） —— MASConstraintMaker工厂类（一）

版本记录

前言

API

1. MASAttribute枚举

2. 几个MASConstraint属性

参考文章

后记

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