iOS自动布局——Masonry详解

欢迎大家前往腾讯云+社区，获取更多腾讯海量技术实践干货哦~本文由鹅厂新鲜事儿发表于云+社区专栏作者：oceanlong | 腾讯 移动客户端开发工程师前言UI布局是整个前端体系里不可或缺的一环。代码的布局是设计语言与用户视觉感受沟通的桥梁，不论它看起来多么简单或是琐碎，但不得不承认，绝大部分软件开发的问题，都是界面问题。那么，如何高效的完成UI开发，也是软件行业一直在克服的问题。所以，软件界面开发的核心点即是：如何减少UI设计稿的建模难度和减少建模转化到代码的实现难度最初iOS提供了平面直角坐标系的方式，来解决布局问题，即所谓的手动布局。平面直角坐标系确实是一套完备在理论，这在数学上已经验证过了，只要我们的屏幕还是平面，它就肯定是有效的。但有效不一定高效，我们在日常的生活中，很少会用平面直角坐标系来向人描述位置关系。更多的是依靠相对位置。所幸，iOS为我们提供自动布局的方法，来解决这一困境。自动布局的基本理念其实说到本质，它和手动布局是一样的。对一个控件放在哪里，我们依然只关心它的(x, y, width, height)。但手动布局的方式是，一次性计算出这四个值，然后设置进去，完成布局。但当父控件或屏幕发生变化时，子控件的计算就要重新来过，非常麻烦。因此，在自动布局中，我们不再关心(x, y, width, height)的具体值，我们只关心(x, y, width, height)四个量对应的约束。约束那么何为约束呢？obj1.property1 =（obj2.property2 * multiplier）+ constant value子控件的某一个量一定与另一个控件的某一个量呈线性关系，这就是约束。那么，给(x, y, width, height)四个量，分别给一个约束，就可以确定一个控件的最终位置。 //创建左边约束 NSLayoutConstraint *leftLc = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:20]; [self.view addConstraint:leftLc];这一段代码即是：控件（blueView）的 x = rootView的x * 1.0 + 20这里一定要注意，这样的一条约束，涉及了子控件和父控件，所以这条约束一定要添加到父控件中。添加约束的规则：如果两个控件是父子控件，则添加到父控件中。如果两个控件不是父子控件，则添加到层级最近的共同父控件中。示例 //关闭Autoresizing blueView.translatesAutoresizingMaskIntoConstraints = NO; //创建左边约束 NSLayoutConstraint *leftLc = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:20]; [self.view addConstraint:leftLc]; //创建右边约束 NSLayoutConstraint *rightLc = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-20]; [self.view addConstraint:rightLc]; //创建底部约束 NSLayoutConstraint *bottomLc = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-20]; [self.view addConstraint:bottomLc]; //创建高度约束 NSLayoutConstraint *heightLc = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:50]; [blueView addConstraint: heightLc];我们注意到，自动布局其实工作分两步：创建视图的约束将约束添加到合适的位置约束关系从上面的描述中，已经非常清晰了。那么如何寻找约束添加的合适位置呢？到这里，我们只是解决了如何减少UI设计稿的建模难度的问题，显然，减少建模转化到代码的实现难度这个效果没能达成。关于如何解决减少建模转化到代码的实现难度的问题，开源库上面的代码，我们可以看到，虽然自动布局已经比手动布局优雅不少了，但它依然行数较多。每条约束大约都需要三行代码，面对复杂的页面，这样开发出来，会很难阅读。Masonry则为我们解决了这个问题。Masonry地址引入Masonry我们选择使用Cocoapods的方式。引入比较简单：我们先在工程目录下，创建Podfile文件:2.编辑Podfile其中，‘IosOcDemo’就是我们工程的名字，根据需要，我们自行替换。3.添加依赖完成后，执行指令pod install。CocoaPods就会为我们自动下载并添加依赖。实践这样的一个代码，用手动布局，我们大致的代码应该是这样：-(void)initBottomView{ self.bottomBarView = [[UIView alloc]initWithFrame:CGRectZero]; self.bottomButtons = [[NSMutableArray alloc]init]; _bottomBarView.backgroundColor = [UIColor yellowColor]; [self addSubview:_bottomBarView]; for(int i = 0 ; i < 3 ; i++) { UIButton *button = [[UIButton alloc]initWithFrame:CGRectZero]; button.backgroundColor = [UIColor redColor]; [_bottomButtons addObject:button]; [self addSubview:button]; }}-(void)layoutBottomView{ _bottomBarView.frame = CGRectMake(20, _viewHeight - 200, _viewWidth - 40, 200); for (int i = 0 ; i < 3; i++) { UIButton button = _bottomButtons[i]; CGFloat x = i * (_viewWidth - 40 - 20 * 4) / 3 + 20(i+1) + 20; CGFloat y = _viewHeight - 200; CGFloat width = (_viewWidth - 40 - 20 * 4) / 3; CGFloat height = 200; button.frame = CGRectMake(x, y, width, height); }}我们来看一下，在Masonry的帮助下，我们可以把刚刚的代码写成什么样的: -(void)initBottomView{ _bottomBarView = [[UIView alloc]initWithFrame:CGRectZero]; _bottomBarView.backgroundColor = [UIColor yellowColor]; _bottomBarView.translatesAutoresizingMaskIntoConstraints = NO; [self addSubview:_bottomBarView]; [_bottomBarView mas_makeConstraints:^(MASConstraintMaker make) { make.left.equalTo(self).with.offset(20); make.right.equalTo(self).with.offset(-20); make.height.mas_equalTo(200); make.bottom.equalTo(self); }]; _bottomButtons = [[NSMutableArray alloc]init]; for(int i = 0 ; i < 3 ; i++) { UIButton button = [[UIButton alloc]initWithFrame: CGRectZero]; button.backgroundColor = [UIColor redColor]; button.translatesAutoresizingMaskIntoConstraints = NO; [_bottomButtons addObject:button]; [_bottomBarView addSubview:button]; [button mas_makeConstraints:^(MASConstraintMaker make) { if (i == 0) { make.left.mas_equalTo(20); }else{ UIButton previousButton = _bottomButtons[i-1]; make.left.equalTo(previousButton.mas_right).with.offset(20); } make.top.mas_equalTo(_bottomBarView.mas_top); make.width.equalTo(_bottomBarView.mas_width).with.multipliedBy(1.0f/3).offset(-204/3); make.height.equalTo(_bottomBarView.mas_height); }]; }}我们可以看到在Masonry的封装下，代码变得非常简练易读，需要行数略有增加，但是计算过程减少了，我们能更加关注于多个UIView间的位置关系，这与当前的UI设计语言是契合的。所以Masonry能否让我们更直观地表达UI。源码解读Masonry的封装很有魅力，那么，我们可以简单地来看一下，它是如何封装的。我们再仔细看一下Masonry的API会发现，我们是直接在UIView上进行调用的。也就是说，Masonry对UIView进行了扩展。在View+MASUtilities.h中：#if TARGET_OS_IPHONE || TARGET_OS_TV #import <UIKit/UIKit.h> #define MAS_VIEW UIView #define MAS_VIEW_CONTROLLER UIViewController #define MASEdgeInsets UIEdgeInsets然后在View+MASAdditions.h中，我们看到了Masonry的扩展：#import “MASUtilities.h”#import “MASConstraintMaker.h”#import “MASViewAttribute.h”/ * Provides constraint maker block * and convience methods for creating MASViewAttribute which are view + NSLayoutAttribute pairs /@interface MAS_VIEW (MASAdditions)/ * following properties return a new MASViewAttribute with current view and appropriate NSLayoutAttribute */@property (nonatomic, strong, readonly) MASViewAttribute *mas_left;@property (nonatomic, strong, readonly) MASViewAttribute *mas_top;@property (nonatomic, strong, readonly) MASViewAttribute *mas_right;@property (nonatomic, strong, readonly) MASViewAttribute *mas_bottom;@property (nonatomic, strong, readonly) MASViewAttribute *mas_leading;@property (nonatomic, strong, readonly) MASViewAttribute *mas_trailing;@property (nonatomic, strong, readonly) MASViewAttribute *mas_width;@property (nonatomic, strong, readonly) MASViewAttribute *mas_height;@property (nonatomic, strong, readonly) MASViewAttribute *mas_centerX;@property (nonatomic, strong, readonly) MASViewAttribute *mas_centerY;@property (nonatomic, strong, readonly) MASViewAttribute mas_baseline;@property (nonatomic, strong, readonly) MASViewAttribute (^mas_attribute)(NSLayoutAttribute attr);…/ * Creates a MASConstraintMaker with the callee view. * Any constraints defined are added to the view or the appropriate superview once the block has finished executing * * @param block scope within which you can build up the constraints which you wish to apply to the view. * * @return Array of created MASConstraints */- (NSArray *)mas_makeConstraints:(void(NS_NOESCAPE ^)(MASConstraintMaker *make))block;一些，适配的代码，我省略了，先看核心代码。在刚刚的例子中，我们正是调用的mas_makeConstraints方法。- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block { self.translatesAutoresizingMaskIntoConstraints = NO; MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self]; block(constraintMaker); return [constraintMaker install];}mas_makeConstraints方法比较简单，只是封装了MASConstraintMaker初始化，设置约束和安装。这里的block就是我们刚刚在外层设置的约束的函数指针。也就是这一串：^(MASConstraintMaker *make) { make.left.equalTo(self.view).with.offset(10); make.right.equalTo(self.view).with.offset(-10); make.height.mas_equalTo(50); make.bottom.equalTo(self.view).with.offset(-10); }由于约束条件的设置比较复杂，我们先来看看初始化和安装。初始化- (id)initWithView:(MAS_VIEW *)view { self = [super init]; if (!self) return nil; self.view = view; self.constraints = NSMutableArray.new; return self;}初始化的代码比较简单，将传入的view放入MASConstraintMaker成员，然后创建MASConstraintMaker的约束容器（NSMutableArray）。安装- (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;}安装的代码分为三块：判断是否需要移除已有的约束。如果需要，会遍历已有约束，然后逐个uninstallcopy已有的约束，遍历，并逐一installremove掉所有约束，并将已添加的constraints返回。install的方法，还是继续封装到了Constraint中，我们继续跟进阅读：我们会发现Constraint只是一个接口，Masonry中对于Constraint接口有两个实现，分别是：MASViewConstraint和MASCompositeConstraint。这两个类，分别是单个约束和约束集合。在上面的例子中，我们只是对单个UIView进行约束，所以我们先看MASViewConstraint的代码。以下代码MASViewConstraint进行了一定程度的简化，省略了一些扩展属性，只展示我们的例子中，会执行的代码：- (void)install { if (self.hasBeenInstalled) { return; } … 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) if (!self.firstViewAttribute.isSizeAttribute && !self.secondViewAttribute) { secondLayoutItem = self.firstViewAttribute.view.superview; secondLayoutAttribute = firstLayoutAttribute; } 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; 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; } MASLayoutConstraint *existingConstraint = nil; … else { [self.installedView addConstraint:layoutConstraint]; self.layoutConstraint = layoutConstraint; [firstLayoutItem.mas_installedConstraints addObject:self]; }}自动布局是一种相对布局，所以，绝大部分情况下，需要两个UIView（约束方与参照方）。在上面的方法中：firstLayoutItem是约束方，secondLayoutItem是参照方firstLayoutAttribute是约束方的属性，secondLayoutAttribute是参照方的属性。MASLayoutConstraint就是NSLayoutConstraint的子类，只是添加了mas_key属性。到这里，我们就与系统提供的API对应上了。 NSLayoutConstraint *leftLc = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:20]; [self.view addConstraint:leftLc];再看看我们之前用系统API完成的例子，是不是格外熟悉？那么接下来，我们就是要阅读 make.left.equalTo(self).with.offset(20); make.right.equalTo(self).with.offset(-20); make.height.mas_equalTo(200); make.bottom.equalTo(self);是如何变成firstLayoutItem, secondLayoutItem, firstLayoutAttribute, secondLayoutAttribute和layoutRelation的。约束条件的设置回到前面的：- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *))block { self.translatesAutoresizingMaskIntoConstraints = NO; MASConstraintMaker *constraintMaker = [[MASConstraintMaker alloc] initWithView:self]; block(constraintMaker); return [constraintMaker install];}我们接下来，就要看block的实现：block其实是一个函数指针。此处真正调用的方法是： make.left.equalTo(self).with.offset(20); make.right.equalTo(self).with.offset(-20); make.height.mas_equalTo(200); make.bottom.equalTo(self);我们挑选其中一个，来看看源码实现：left- (MASConstraint *)addConstraintWithLayoutAttribute:(NSLayoutAttribute)layoutAttribute { return [self constraint:nil addConstraintWithLayoutAttribute:layoutAttribute];}- (MASConstraint *)left { return [self addConstraintWithLayoutAttribute:NSLayoutAttributeLeft];}- (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;}在对单个view添加约束时，constraint为nil。我们直接生成了一个新约束newConstraint。它的firstViewAttribute就是我们传入的NSLayoutAttributeLeftequalTo- (MASConstraint * (^)(id))equalTo { return ^id(id attribute) { return self.equalToWithRelation(attribute, NSLayoutRelationEqual); };}- (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; } };}此处，我们依然先看attribute不是NSArray的情况。这里在单个属性的约束中，就比较简单了，将relation和attribue传入MASConstraint对应的成员。在上面介绍install方法时，我们就曾提到过： MASLayoutConstraint *layoutConstraint = [MASLayoutConstraint constraintWithItem:firstLayoutItem attribute:firstLayoutAttribute relatedBy:self.layoutRelation toItem:secondLayoutItem attribute:secondLayoutAttribute multiplier:self.layoutMultiplier constant:self.layoutConstant];firstLayoutItem和secondLayoutItem在install方法中已收集完成，此时，经过left和equalTo我们又收集到了:firstViewAttribute、secondViewAttribute和layoutRelation胜利即在眼前。- (MASConstraint * (^)(CGFloat))offset { return ^id(CGFloat offset){ self.offset = offset; return self; };}- (void)setOffset:(CGFloat)offset { self.layoutConstant = offset;}通过OC的set语法，Masonry将offset传入layoutConstant。至此，layoutConstraint就完成了全部的元素收集，可以使用添加约束的方式，只需要解决最后一个问题，约束添加到哪里呢？我们似乎在调用时，并不需要关心这件事情，那说明框架帮我们完成了这个工作。closestCommonSuperview我们在MASViewConstraint中，可以找到这样一段： 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; }注意到，closetCommonSuperview就是Masonry为我们找到的最近公共父控件。- (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;}实现也比较简单。至此，我们完成了所有准备，就可以开始愉快的自动布局啦。以上就是Masonry对iOS自动布局封装的解读。如有问题，欢迎指正。问答iOS：如何使用自动布局约束？相关阅读走进 MasonryiOS自动布局框架之MasonryiOS学习——布局利器Masonry框架源码深度剖析 【每日课程推荐】机器学习实战！快速入门在线广告业务及CTR相应知识