传送门:链式编程小Demo
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。
系统给的自动布局(AutoLayout)的API
+(instancetype)constraintWithItem:(id)view1
attribute:(NSLayoutAttribute)attr1
relatedBy:(NSLayoutRelation)relation
toItem:(nullable id)view2
attribute:(NSLayoutAttribute)attr2
multiplier:(CGFloat)multiplier
constant:(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:
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)));
}];
Masonry.h
#import //! 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];
}
MASConstraintMaker.m
@interface MASConstraintMaker () @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;
}
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);
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 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;
}
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 /**
* 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
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;
}
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。
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;
}
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;
} #else self.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。
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 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;
}
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 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;
}
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。
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 constraint NSArray *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 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 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;
}
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。
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: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; //【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 constant existingConstraint.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;
}
例子
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。
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doubleCGPointCGSize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));
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];
}
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。
角色分析
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
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,中文可译作组合模式。
通过 Component 接口操纵组合部件的对象。
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。
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。
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。
UIView,通过分类View+MASAdditions来调用Masonry
MASConstraintMaker
MASConstraint
MASViewConstraint
MASCompositeConstraint
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。
【举例】简单使用链式编程思想实现一个简单计算器的功能:
CaculateMaker.h
// CaculateMaker.h // ChainBlockTestApp #import #import @interface CaculateMaker : NSObject @property (nonatomic, assign) CGFloat result;
- (CaculateMaker *(^)(CGFloat num))add; @end
CaculateMaker.m
// CaculateMaker.m // ChainBlockTestApp #import "CaculateMaker.h" @implementation CaculateMaker - (CaculateMaker *(^)(CGFloat num))add;{ return ^CaculateMaker *(CGFloat num){
_result += num; return self;
};
} @end
ViewController.m
CaculateMaker *maker = [[CaculateMaker alloc] init];
maker.add(20).add(30);
通过上面Masonry布局可以看出,它为UIView写了一个category,拓展了mas_makeConstraints方法,并将MASConstraintMaker对象作为block的参数传递,在block的实现里完成UIView的布局,提现了函数式编程思想。
NSObject+Caculate.h
// NSObject+Caculate.h // ChainBlockTestApp #import #import #import "CaculateMaker.h" @interface NSObject (Caculate) - (CGFloat)caculate:(void (^)(CaculateMaker *make))block; @end
NSObject+Caculate.m
// NSObject+Caculate.m // ChainBlockTestApp #import "NSObject+Caculate.h" @implementation NSObject (Caculate) - (CGFloat)caculate:(void (^)(CaculateMaker *make))block;{
CaculateMaker *make = [[CaculateMaker alloc] init];
block(make); return make.result;
} @end
ViewController.m
CGFloat result = [NSObject caculate:^(CaculateMaker *maker) {
maker.add(10).add(20).add(30);
}]; NSLog(@"结果为:%.2f",result);
Masonry解析
工厂模式
组合模式
链式编程