浅谈
这篇文章是 Masonry 框架源码的解析和笔记。学习Masonry之前,先了解这个框架设计的初衷—传统的利用系统API进行纯代码布局的不足。然后,根据Masonry常见的几个链式语法中,顺藤摸瓜地了解Masonry的调用栈。最后,学习并思考这个框架用到的设计模式和链式编程思想。
1. 之前的不足:系统API纯代码布局
- 系统给的自动布局(AutoLayout)的API
+(instancetype)constraintWithItem:(id)view1attribute:(NSLayoutAttribute)attr1relatedBy:(NSLayoutRelation)relationtoItem:(nullable id)view2attribute:(NSLayoutAttribute)attr2multiplier:(CGFloat)multiplierconstant:(CGFloat)c;
- 传统代码中使用系统API进行布局
- (void)viewDidLoad {[super viewDidLoad];// Do any additional setup after loading the view, typically from a nib.self.view.backgroundColor = [UIColor yellowColor];UIView *subView = [[UIView alloc] init];subView.backgroundColor = [UIColor redColor];// 在设置约束前,先将子视图添加进来[self.view addSubview:subView];// 使用autoLayout约束,禁止将AutoresizingMask转换为约束[subView setTranslatesAutoresizingMaskIntoConstraints:NO];// 设置subView相对于VIEW的上左下右各40像素NSLayoutConstraint *constraintTop = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:40];NSLayoutConstraint *constraintLeft = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:40];// 由于iOS坐标系的原点在左上角,所以设置下,右边距使用负值NSLayoutConstraint *constraintBottom = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-40];NSLayoutConstraint *constraintRight = [NSLayoutConstraint constraintWithItem:subView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-40];// 将四条约束加进数组中NSArray *array = [NSArray arrayWithObjects:constraintTop, constraintLeft, constraintBottom, constraintRight, nil];// 把约束条件设置到父视图的Contraints中[self.view addConstraints:array];
}
可见,系统传统的代码布局有点繁琐。为了简化上述传统布局代码,被广泛应用的第三方框架 Masonry 对AutoLayout 进行了封装,Swift版则是 SnapKit。这篇文章就是针对 Masonry 源代码的解析与学习笔记。在这之前,如下图所示,是 Masonry 源代码的结构图:
2. 顺藤摸瓜:Masonry链式语法的调用栈解析
2.1 mas_makeConstraints
:外部调用
- 调用例子
#import "Masonry.h"
[self.containerView addSubview:self.bannerView];
[self.bannerView mas_makeConstraints:^(MASConstraintMaker *make) {make.leading.equalTo(self.containerView.mas_leading);make.top.equalTo(self.containerView.mas_top);make.trailing.equalTo(self.containerView.mas_trailing);make.height.equalTo(@(kViewWidth(131.0)));
}];
2.2 mas_makeConstraints
:实现原理,通过导入的头文件分析
- Masonry.h
#import <Foundation/Foundation.h>//! Project version number for Masonry.
FOUNDATION_EXPORT double MasonryVersionNumber;//! Project version string for Masonry.
FOUNDATION_EXPORT const unsigned char MasonryVersionString[];#import "MASUtilities.h"
#import "View+MASAdditions.h"
#import "View+MASShorthandAdditions.h"
#import "ViewController+MASAdditions.h"
#import "NSArray+MASAdditions.h"
#import "NSArray+MASShorthandAdditions.h"
#import "MASConstraint.h"
#import "MASCompositeConstraint.h"
#import "MASViewAttribute.h"
#import "MASViewConstraint.h"
#import "MASConstraintMaker.h"
#import "MASLayoutConstraint.h"
#import "NSLayoutConstraint+MASDebugAdditions.h"
其中View+MASAdditions
分类为UIView
添加了mas_makeConstraints
方法
- View+MASAdditions.m
- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {self.translatesAutoresizingMaskIntoConstraints = NO;MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];block(constraintMaker);return [constraintMaker install];
}
- View+MASAdditions.m
- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block {self.translatesAutoresizingMaskIntoConstraints = NO;MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self];block(constraintMaker);return [constraintMaker install];
}
- MASConstraintMaker.m
@interface MASConstraintMaker () <MASConstraintDelegate>@property (nonatomic, weak) MAS_VIEW *view;
@property (nonatomic, strong) NSMutableArray *constraints;@end
- (id)initWithView:(MAS_VIEW *)view {self = [super init];if (!self) return nil;self.view = view;self.constraints = NSMutableArray.new;return self;
}
2.3 .top
:通过MASConstraintMaker
类源码分析
先分析设置 第一个约束属性 的情况(且唯一一个):例如
make.top.equalTo(self.containerView.mas_top);
2.3.1 MASConstraintMaker
的分析
- MASConstraintMaker.m
- (MASConstraint *)top {return [self addConstraintWithLayoutAttribute:NSLayoutAttributeTop];
}- (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]) {//replace with composite constraintNSArray *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;
}
该方法返回的newConstraint
是一个MASViewConstraint
类的示例,而MASViewConstraint
类又是MASConstraint
的子类,返回类型写成MASConstraint
没毛病。
代码较多,暂时可以只先看if (!constraint)
里面的代码。可见,最后设置 newConstraint
对象代理为self
(即 MASConstraintMaker
),并添加到一开始准备好的 self.constraints
数组中,返回。
其中,设置 MASViewConstraint
类 newConstraint
对象的 MASConstraintDelegate
代理为self
(即 MASConstraintMaker
),其作用就是为了能够同时设置多个约束属性!即链式语法。
- MASConstraint+Private.h
@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
2.3.2 MASConstraintMaker的继续分析
第2.3.1节的MASConstraintMaker.m
代码中,先是初始化了 MASViewAttribute
对象并保存了 view、item
以及 NSLayoutAttribute
三个属性。
- MASViewAttribute.m
- (id)initWithView:(MAS_VIEW *)view layoutAttribute:(NSLayoutAttribute)layoutAttribute {self = [self initWithView:view item:view layoutAttribute:layoutAttribute];return self;
}- (id)initWithView:(MAS_VIEW *)view item:(id)item layoutAttribute:(NSLayoutAttribute)layoutAttribute {self = [super init];if (!self) return nil;_view = view;_item = item;_layoutAttribute = layoutAttribute;return self;
}
然后又初始化了 MASViewConstraint
对象,内部配置了些默认参数并保存了如上的第一个约束参数 MASViewAttribute
。
- MASViewConstraint.m
- (id)initWithFirstViewAttribute:(MASViewAttribute *)firstViewAttribute {self = [super init];if (!self) return nil;_firstViewAttribute = firstViewAttribute;self.layoutPriority = MASLayoutPriorityRequired;self.layoutMultiplier = 1;return self;
}
2.4 .equalTo :通过基类MASConstraint及其子类MASViewConstraint分析
第一个约束属性 设置完后,走到.equalTo
时,前面返回已经是一个 MASViewConstraint
(继承自MASConstraint
) 对象了,因而调用的是在基类MASConstraint
中声明并实现的block
属性getter
方法。
- MASConstraint.m
- (MASConstraint * (^)(id))equalTo {return ^id(id attribute) {return self.equalToWithRelation(attribute, NSLayoutRelationEqual);};
}
其中,基类 MASConstraint
仅仅声明,并没有实现equalToWithRelation
抽象方法。但是,如2.3节中的链式语法.top
,该方法返回的newConstraint
实际是其子类–MASViewConstraint
类的实例,故而可调用子类MASViewConstraint
实现的equalToWithRelation
方法:
- MASViewConstraint.m
- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {return ^id(id attribute, NSLayoutRelation relation) {if ([attribute isKindOfClass:NSArray.class]) {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;[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;}};
}
代码较多,暂时可先看else {
里面的代码。
(1) self.layoutRelation = relation;
首先是 self.layoutRelation
保存了约束关系且重写了 set
方法,在里面用 self.hasLayoutRelation
这个 BOOL
标识已经有约束关系。
- MASViewConstraint.m
- (void)setLayoutRelation:(NSLayoutRelation)layoutRelation {_layoutRelation = layoutRelation;self.hasLayoutRelation = YES;
}
(2) self.secondViewAttribute = attribute;
然后同样是重写了 self.secondViewAttribute
的 set
方法,这里会根据不同的情况做不同的操作。
- (void)setSecondViewAttribute:(id)secondViewAttribute {if ([secondViewAttribute isKindOfClass:NSValue.class]) {[self setLayoutConstantWithValue:secondViewAttribute];} else if ([secondViewAttribute isKindOfClass:MAS_VIEW.class]) {_secondViewAttribute = [[MASViewAttribute alloc] initWithView:secondViewAttribute layoutAttribute:self.firstViewAttribute.layoutAttribute];} else if ([secondViewAttribute isKindOfClass:MASViewAttribute.class]) {MASViewAttribute *attr = secondViewAttribute;if (attr.layoutAttribute == NSLayoutAttributeNotAnAttribute) {_secondViewAttribute = [[MASViewAttribute alloc] initWithView:attr.view item:attr.item layoutAttribute:self.firstViewAttribute.layoutAttribute];;} else {_secondViewAttribute = secondViewAttribute;}} else {NSAssert(NO, @"attempting to add unsupported attribute: %@", secondViewAttribute);}
}
其中,第1种情况对应的是:
make.height.equalTo(@20.0f)
传入 NSValue
的时, 会直接设置 constraint
的 offset
, centerOffset
, sizeOffset
, 或者 insets
。调用栈如下:
//MASViewConstraint.m
if ([secondViewAttribute isKindOfClass:NSValue.class]) {[self setLayoutConstantWithValue:secondViewAttribute];
}
//MASConstraint.m
- (void)setLayoutConstantWithValue:(NSValue *)value {if ([value isKindOfClass:NSNumber.class]) {self.offset = [(NSNumber *)value doubleValue];} else if (strcmp(value.objCType, @encode(CGPoint)) == 0) {CGPoint point;[value getValue:&point];self.centerOffset = point;} else if (strcmp(value.objCType, @encode(CGSize)) == 0) {CGSize size;[value getValue:&size];self.sizeOffset = size;} else if (strcmp(value.objCType, @encode(MASEdgeInsets)) == 0) {MASEdgeInsets insets;[value getValue:&insets];self.insets = insets;} else {NSAssert(NO, @"attempting to set layout constant with unsupported value: %@", value);}
}
//MASViewConstraint.m
- (void)setOffset:(CGFloat)offset {self.layoutConstant = offset;
}
//MASViewConstraint.m
- (void)setLayoutConstant:(CGFloat)layoutConstant {_layoutConstant = layoutConstant;
#if TARGET_OS_MAC && !(TARGET_OS_IPHONE || TARGET_OS_TV)if (self.useAnimator) {[self.layoutConstraint.animator setConstant:layoutConstant];} else {self.layoutConstraint.constant = layoutConstant;}
#elseself.layoutConstraint.constant = layoutConstant;
#endif
}
第2种情况,一般是直接传入一个视图:
make.top.equalTo(self)
这时, 就会初始化一个 layoutAttribute
属性与 firstViewArribute
相同的 MASViewAttribute
, 上面的代码就会使视图与 view 顶部对齐。
第3种情况,会传入一个视图的 MASViewAttribute:
make.top.equalTo(view.mas_bottom);
使用这种写法时, 一般是因为约束的方向不同. 这行代码会使视图的顶部与 view 的底部对齐。
2.5 .height.width
:Masonry的链式语法特性
调用例子
make.height.width.equalTo(@20);
其中,.height
设置第一个约束属性时,调用的是 MASConstraintMaker.m
中的 .height
, addConstraintWithLayoutAttribute
,以及- (MASConstraint *)constraint:(MASConstraint *)constraint addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute
。
- MASConstraintMaker.m
- (MASConstraint *)height {return [self addConstraintWithLayoutAttribute:NSLayoutAttributeHeight];
}- (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]) {//replace with composite constraintNSArray *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;
}
该方法调用栈返回的是一个MASViewConstraint
(父类是 MASConstraint
) 对象。
因此,通过 .width
设置第二个约束属性的时候,调用的先是基类 MASConstraint.m
中的.width
,然后调用由子类MASViewConstraint
实现的addConstraintWithLayoutAttribute
方法。这时候的调用栈为:
- MASConstraint.m
- (MASConstraint *)width {return [self addConstraintWithLayoutAttribute:NSLayoutAttributeWidth];
}- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute __unused)layoutAttribute {MASMethodNotImplemented();
}
- MASViewConstraint.m
- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute {NSAssert(!self.hasLayoutRelation, @"Attributes should be chained before defining the constraint relation");return [self.delegate constraint:self addConstraintWithLayoutAttribute:layoutAttribute];
}
这其中,self.delegate
是什么呢?如2.3.1节所述,MASConstraintMaker.m
中设置了 MASViewConstraint
类 newConstraint
对象的 MASConstraintDelegate
代理为“self”
(即 MASConstraintMaker
),其作用就是为了能够同时设置多个约束属性,即链式语法。所以,第二个设置约束属性跟第一个设置约束属性最终 调用的方法一样(都是MASConstraintMaker.m中实现的addConstraintWithLayoutAttribute
)。
- MASConstraintMaker.m
- (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 constraintNSArray *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;
}
当设置 第二次约束属性 并执行完之后,我们还可以发现 constraint
不为 nil
,而是一个 MASViewConstraint
对象 ,所以该方法调用栈返回的不是 MASViewConstraint
对象,而是 MASCompositeConstraint
这个对象了,下面我们来看看这个类。
2.6 约束的集合: MASCompositeConstraint
MASCompositeConstraint
是约束的集合,它里面有个私有的数组用来存放多个 MASViewAttribute
对象。
make.height.width.equalTo(@20)
当设置 第二个约束属性,走到 .width
时,最终走的是:
- MASConstraintMaker.m
- (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 constraintNSArray *children = @[constraint, newConstraint];MASCompositeConstraint *compositeConstraint = [[MASCompositeConstraint alloc] initWithChildren:children];compositeConstraint.delegate = self;[self constraint:constraint shouldBeReplacedWithConstraint:compositeConstraint];return compositeConstraint;}....
}
其中,可以成功的走进 if
判读里面,将 .height
.wight
两条约束 MASViewConstraint
对象塞到数组里,创建 MASCompositeConstraint
对象,并且同样设置了 delegate
,最后还把 self.constraints
里面事先添加好的约束 MASViewConstraint
对象替换成了 MASCompositeConstraint
对象。
#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];
}
另外,我们可以点击 MASCompositeConstraint
初始化方法里看看,它内部会通过 for
循环,把数组里面的所有 MASViewConstraint
对象同样设置了 delegate
- (id)initWithChildren:(NSArray *)children {self = [super init];if (!self) return nil;_childConstraints = [children mutableCopy];for (MASConstraint *constraint in _childConstraints) {constraint.delegate = self;}return self;
}
这么做的目的同时是为了能够继续链式调用,比如我们再设置第三个约束属性 .left
make.height.width.left.equalTo(@20);
这时候的调用栈如下:
- MASConstraint.m
- (MASConstraint *)left {return [self addConstraintWithLayoutAttribute:NSLayoutAttributeLeft];
}
- MASCompositeConstraint.m
- (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;
}
可以发现,这里又是通过 delegate 方式,调用 MASConstraintMaker
工厂类中的:
- MASConstraintMaker.m
- (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 constraintNSArray *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;
}
此时,注意到两个 if
体都没有走进去,既不像第一次,也不像第二次约束设置的时候。所以,这次仅仅是初始化了个 MASViewConstraint
对象就直接返回了,然后回到上个方法中添加到 MASCompositeConstraint
的私有数组 self.childConstraints
中返回备用。
关于三次 约束设置之后的 .equalTo(@20)
,因为执行完 .left
时,返回的是 MASCompositeConstraint
对象,到这一步的时候会有点变化,调用栈如下:
- MASConstraint.m
- (MASConstraint * (^)(id))equalTo {return ^id(id attribute) {return self.equalToWithRelation(attribute, NSLayoutRelationEqual);};
}
- MASCompositeConstraint.m
- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {return ^id(id attr, NSLayoutRelation relation) {for (MASConstraint *constraint in self.childConstraints.copy) {constraint.equalToWithRelation(attr, relation);}return self;};
}
可以发现,这里会循环之前准备好的私有数组 self.childConstraints
,调用 MASViewConstraint.m
的 equalToWithRelation
方法,和上面讲的一样了。
2.7 添加约束到视图
mas_makeConstraints
方法的最后会调用 [constraintMaker install]
方法来添加所有存储在 self.constraints
数组中的所有约束。
- MASConstraintMaker.m
- (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;
}
(1). 如果需要重新构建约束,也就是 调用 mas_remakeConstraints:
方法,会先取出视图的所有约束,然后通过一个 for
循环,调用 uninstall
来清空所有约束:
(2). 如果不需要重新构建约束,会取出 self.constraints
数组中准备好的约束,通过 for
循环,调用 install
来把约束添加到视图上。
关于 install
,是基类 MASConstraint
的抽象方法,方法体由MASViewConstraint
或 MASCompositeConstraint
实现。而 MASCompositeConstraint
的 install
方法体中其实也是调用的由MASViewConstraint
类实现的install
。
- MASConstraint.m
- (void)install { MASMethodNotImplemented(); }
- MASCompositeConstraint.m
- (void)install {for (MASConstraint *constraint in self.childConstraints) {constraint.updateExisting = self.updateExisting;[constraint install];}
}
- MASViewConstraint.m
这里代码较多,就不分开解析了,直接分为7步写到源码的注释中,如下所示:
- (void)install {//【1】如果约束以及存在并是 active 会直接返回。if (self.hasBeenInstalled) {return;}//【2】如果 self.layoutConstraint 响应了 isActive 方法并且不为空,会激活这条约束并添加到 mas_installedConstraints 数组中,最后返回。if ([self supportsActiveProperty] && self.layoutConstraint) {self.layoutConstraint.active = YES;[self.firstViewAttribute.view.mas_installedConstraints addObject:self];return;}//【3】这边是获取即将用于初始化 NSLayoutConstraint 的子类 MASLayoutConstraint 的几个属性。MAS_VIEW *firstLayoutItem = self.firstViewAttribute.item;NSLayoutAttribute firstLayoutAttribute = self.firstViewAttribute.layoutAttribute;MAS_VIEW *secondLayoutItem = self.secondViewAttribute.item;NSLayoutAttribute secondLayoutAttribute = self.secondViewAttribute.layoutAttribute;// alignment attributes must have a secondViewAttribute// therefore we assume that is refering to superview// eg make.left.equalTo(@10)//【4】这边是判断当前即将添加的约束是否是 size 类型的并且 self.secondViewAttribute 也就是约束的第二个参数是 nil,(eg make.left.equalTo(@10))会自动将约束添加到约束的第一个参数视图的 superview 上。if (!self.firstViewAttribute.isSizeAttribute && !self.secondViewAttribute) {secondLayoutItem = self.firstViewAttribute.view.superview;secondLayoutAttribute = firstLayoutAttribute;}//【5】然后就会初始化 NSLayoutConstraint 的子类 MASLayoutConstraint。MASLayoutConstraint *layoutConstraint= [MASLayoutConstraint constraintWithItem:firstLayoutItemattribute:firstLayoutAttributerelatedBy:self.layoutRelationtoItem:secondLayoutItemattribute:secondLayoutAttributemultiplier:self.layoutMultiplierconstant:self.layoutConstant];layoutConstraint.priority = self.layoutPriority;layoutConstraint.mas_key = self.mas_key;//【6】这段代码会先判断是否有约束第二个参数的视图,有的话会寻找约束第一个和第二参数视图的公共 Superview,相当于求两个数的最小公倍数;如果不满足第一个条件,会判断约束第一个参数是否是 size 类型的,是的话直接取到它的视图;最后都不满足会直接取到约束第一个参数视图父视图。if (self.secondViewAttribute.view) {MAS_VIEW *closestCommonSuperview = [self.firstViewAttribute.view mas_closestCommonSuperview:self.secondViewAttribute.view];NSAssert(closestCommonSuperview,@"couldn't find a common superview for %@ and %@",self.firstViewAttribute.view, self.secondViewAttribute.view);self.installedView = closestCommonSuperview;} else if (self.firstViewAttribute.isSizeAttribute) {self.installedView = self.firstViewAttribute.view;} else {self.installedView = self.firstViewAttribute.view.superview;}//【7】如果需要升级当前的约束就会获取原有的约束,并替换为新的约束,这样就不需要再次为 view 安装约束。如果原来的 view 中不存在可以升级的约束,那么就会在上一步寻找到的 installedView 上面添加约束。MASLayoutConstraint *existingConstraint = nil;if (self.updateExisting) {existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];}if (existingConstraint) {// just update the constantexistingConstraint.constant = layoutConstraint.constant;self.layoutConstraint = existingConstraint;} else {[self.installedView addConstraint:layoutConstraint];self.layoutConstraint = layoutConstraint;[firstLayoutItem.mas_installedConstraints addObject:self];}
}
其中第【6】步中的mas_closestCommonSuperview
方法,它会寻找 firstLayoutItem
和 secondLayoutItem
两个视图的公共 superview,
相当于求两个数的最小公倍数.
- View+MASAdditions.m
- (instancetype)mas_closestCommonSuperview:(MAS_VIEW *)view {MAS_VIEW *closestCommonSuperview = nil;MAS_VIEW *secondViewSuperview = view;while (!closestCommonSuperview && secondViewSuperview) {MAS_VIEW *firstViewSuperview = self;while (!closestCommonSuperview && firstViewSuperview) {if (secondViewSuperview == firstViewSuperview) {closestCommonSuperview = secondViewSuperview;}firstViewSuperview = firstViewSuperview.superview;}secondViewSuperview = secondViewSuperview.superview;}return closestCommonSuperview;
}
3. 顺藤再摸瓜:Masonry其它链式语法的调用栈解析(选读
3.1 make.edges.equalTo(view)
make.edges.equalTo(view)
我们再来看看这种写法,调用栈如下:
- MASConstraintMaker.m
- (MASConstraint *)edges {return [self addConstraintWithAttributes:MASAttributeTop | MASAttributeLeft | MASAttributeRight | MASAttributeBottom];
}
- (MASConstraint *)addConstraintWithAttributes:(MASAttribute)attrs {__unused MASAttribute anyAttribute = (MASAttributeLeft | MASAttributeRight | MASAttributeTop | MASAttributeBottom | MASAttributeLeading| MASAttributeTrailing | MASAttributeWidth | MASAttributeHeight | MASAttributeCenterX| MASAttributeCenterY | ......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];...... 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;
}
代码太多省略了一部分,可以发现这段代码作用就是返回一个包含多条约束的 MASCompositeConstraint
对象,接着后面的操作也都是一样的了。
3.2 make.edges.equalTo(UIEdgeInsetsMake(0.0f, 0.0f, 0.0f, 0.0f));
上面3.1中例子的写法还可以改成这样:
make.edges.equalTo(UIEdgeInsetsMake(0.0f, 0.0f, 0.0f, 0.0f));
这里的 equalTo 需要注意下,它是一个宏,定义在 MASConstraint.h 中:
- MASConstraint.h
#define mas_equalTo(...) equalTo(MASBoxValue((__VA_ARGS__)))
#define mas_greaterThanOrEqualTo(...) greaterThanOrEqualTo(MASBoxValue((__VA_ARGS__)))
#define mas_lessThanOrEqualTo(...) lessThanOrEqualTo(MASBoxValue((__VA_ARGS__)))
#define mas_offset(...) valueOffset(MASBoxValue((__VA_ARGS__)))
#ifdef MAS_SHORTHAND_GLOBALS
#define equalTo(...) mas_equalTo(__VA_ARGS__)
#define greaterThanOrEqualTo(...) mas_greaterThanOrEqualTo(__VA_ARGS__)
#define lessThanOrEqualTo(...) mas_lessThanOrEqualTo(__VA_ARGS__)
#define offset(...) mas_offset(__VA_ARGS__)
代入上述宏定义,前面的代码等效成:
make.edges.equalTo(MASBoxValue(UIEdgeInsetsMake(0.0f, 0.0f, 0.0f, 0.0f)));
可以发现,其实里面调用的是 MASBoxValue
这个宏,它将 C 和 Objective-C 语言中的一些基本数据结构比如说 double
CGPoint
CGSize
这些值用 NSValue
进行包装。
这里还支持直接调用 size、center 等,具体实现都差不多,就不熬述了:
make.center.equalTo(CGPointMake(0, 50));
make.size.equalTo(CGSizeMake(200, 100));
3.3 make.height.equalTo(@[redView, blueView])
make.height.equalTo(@[redView, blueView])
再来看看这种传数组的,在走到 .equalTo
时,最终会调用 MASViewConstraint.m
里面的 equalToWithRelation
方法
- MASConstraint.m
- (MASConstraint * (^)(id))equalTo {return ^id(id attribute) {return self.equalToWithRelation(attribute, NSLayoutRelationEqual);};
}
- MASViewConstraint.m
- (MASConstraint * (^)(id, NSLayoutRelation))equalToWithRelation {return ^id(id attribute, NSLayoutRelation relation) {if ([attribute isKindOfClass:NSArray.class]) {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;[self.delegate constraint:self shouldBeReplacedWithConstraint:compositeConstraint];return compositeConstraint;} else { .... }};
}
这边还是遍历数组,并且 MASViewConstraint
实现 NSCopying
协议,调用 [self copy]
会创建 MASViewConstraint
对象
- (id)copyWithZone:(NSZone __unused *)zone {MASViewConstraint *constraint = [[MASViewConstraint alloc] initWithFirstViewAttribute:self.firstViewAttribute];constraint.layoutConstant = self.layoutConstant;constraint.layoutRelation = self.layoutRelation;constraint.layoutPriority = self.layoutPriority;constraint.layoutMultiplier = self.layoutMultiplier;constraint.delegate = self.delegate;return constraint;
}
然后会根据传的数组里面的 Value 类型来做不同的操作,前面讲过就不熬述了:
- (void)setSecondViewAttribute:(id)secondViewAttribute {if ([secondViewAttribute isKindOfClass:NSValue.class]) {[self setLayoutConstantWithValue:secondViewAttribute];} else if ([secondViewAttribute isKindOfClass:MAS_VIEW.class]) {_secondViewAttribute = [[MASViewAttribute alloc] initWithView:secondViewAttribute layoutAttribute:self.firstViewAttribute.layoutAttribute];} else if ([secondViewAttribute isKindOfClass:MASViewAttribute.class]) {_secondViewAttribute = secondViewAttribute;} else {NSAssert(NO, @"attempting to add unsupported attribute: %@", secondViewAttribute);}
}
最后便是生成 MASCompositeConstraint
对象,并通过 delegate
方式,调用 MASConstraintMaker
的方法,替换 self.constraints
数组里的约束:
- (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. 举一反三:框架源码的学习启示
4.1 简化的设计模式:工厂类&工厂方法
MASConstraintMaker
类就是一个工厂类,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)。在UIView的View+MASAdditions
分类中就是调用的MASConstraintMaker
类中的一些方法。上述我们在使用Masonry给subView添加约束时,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constraints
属性数组就记录了该工厂创建的所有MASConstraint
对象。
MASConstraintMaker
之所以成为约束工厂类,因为MASConstraintMaker
赋值创建NSLayoutConstraint
对象,因为Masonry
将NSLayoutConstraint
类进一步封装成了MASViewConstraint
,所以MASConstraintMaker
是负责创建MASViewConstraint
的对象,并调用MASViewConstraint
对象的Install
方法将该约束添加到相应的视图中。
说了这么多,总结一下,如果你调用maker.top, maker.left
等等这些方法都会调用下方的工厂方法来创建相应的MASViewConstraint
对象,并记录在工厂对象的约束数组中。之所以能链式调用,就是讲当前的工厂对象(MASConstraintMaker)指定为MASViewConstraint
对象的代理,所以一个MASViewConstraint
对象就可以通过代理来调用工厂方法来创建另一个新的MASViewConstraint
对象了,此处用到了代理模式。
- Client:UIView,通过分类View+MASAdditions来扮演
- 工厂类:MASConstraintMaker
- 抽象产品:MASConstraint
- 具体产品:MASViewConstraint, MASCompositeConstraint
4.2 真正的设计模式:组合模式
换一种角度看,Masonry 并非单纯的工厂模式,而是采用了经典的 Composite 设计模式,可译作组合模式。
4.2.1 经典 组合模式 中的参与者:
Client
- 通过 Component 接口操纵组合部件的对象。
Component - 为组合中的对象声明接口。
- 在适当的情况下,实现所有类共有接口的缺省行为
- 声明一个接口用于访问和管理 Component 的子组件。
- 在递归结构中定义一个接口,用于访问一个父部件,并在合适的情况下实现它。
Leaf - 在组合中表示叶节点对象,叶节点没有子节点。
- 在组合中定义图元对象的行为。
Composite - 定义有子部件的那些部件的行为。
- 在 Composite 接口中实现与子部件有关的操作。
4.2.2 从 组合模式 的角度看,Masonry 框架中的角色分析:
UIView
,通过分类View+MASAdditions
来调用Masonry
Client
- MASConstraintMaker
Component - MASConstraint
Leaf - MASViewConstraint
Composite - MASCompositeConstraint
4.3 编程思想:链式编程
Objective-C是一门动态语言,它使用了一种动态的消息发送机制,即对象(object)或类(class)调用方法。而OC中的点语法则只能通过setter和getter方法作用于类的属性,而不能作用于某个方法。想实现链式语法,只能通过类似block属性的getter方法。
链式编程思想:核心思想为将block作为方法的返回值,且返回值的类型为调用者本身,并将该方法以setter的形式返回,这样就可以实现了连续调用,即为链式编程。