AutoLayout中一些需要注意的点--结合Masonry使用

转载自：

简书博客地址：自动布局与Masonry使用注意事项

1 理解自身内容尺寸约束与抗压抗拉

自身内容尺寸约束:一般来说，要确定一个视图的精确位置，至少需要4个布局约束（以确定水平位置x、垂直位置y、宽度w和高度h）。但是，某些用来展现内容的用户控件，例如文本控件UILabel、按钮UIButton、图片视图UIImageView等，它们具有自身内容尺寸（Intrinsic Content Size），此类用户控件会根据自身内容尺寸添加布局约束。也就是说，如果开发者没有显式给出其宽度或者高度约束，则其自动添加的自身内容约束将会起作用。因此看似“缺失”约束，实际上并非如此。

关于自身内容尺寸约束，简单来说就是某些用来展现内容的用户控件，它们会根据自身内容尺寸添加布局约束。

自身内容尺寸约束的抗挤压与抗拉抻效果。弹簧会有自身固有长度，当有外力作用时，弹簧会抵抗外力作用，尽量接近固有长度。
抗拉抻：当外力拉长弹簧时，弹簧长度大于固有长度，且产生向内收的力阻止外力拉抻，且尽量维持长度接近自身固有长度。
抗挤压：当外力挤压弹簧时，弹簧长度小于固有长度，且产生向外顶的力阻止外力挤压，且尽量维持长度接近自身固有长度。

关于抗压抗拉，就是布局冲突需要牺牲某些控件的某些宽度或者高度约束时，抗压高的控件越不容易被压缩，抗拉高的控件越不容易被拉升。即自身布局对抗外界布局的能力。

样例：

一种常见的业务场景是用户修改地址，在输入新地址之前先读取用户之前的地址作为填充。UI实现是水平平行的UILabel和UITextField。
代码实现如下：

- (NSString *)aLongAddress{    return @"A long long long long long long long long long address";}- (NSString *)aShortAddress{    return @"A short address";}- (void)sampleCode{    UIView *layoutView = [UIView new];    layoutView.frame = CGRectMake(0, 200, [UIScreen mainScreen].bounds.size.width, 100);    layoutView.backgroundColor = [[UIColor alloc] initWithRed:0.5 green:0.5 blue:0.5 alpha:0.5];    [self.view addSubview:layoutView];    UILabel *address = [[UILabel alloc] init];    [layoutView addSubview:address];    address.text = @"地址:";    address.backgroundColor = [UIColor blueColor];    [address mas_makeConstraints:^(MASConstraintMaker *make) {        make.centerY.equalTo(layoutView);        make.left.equalTo(layoutView).offset(10);    }];    UITextField *addressTextField = [[UITextField alloc] init];    [layoutView addSubview:addressTextField];    addressTextField.returnKeyType = UIReturnKeyDone;    addressTextField.font = [UIFont systemFontOfSize:15];    addressTextField.clearButtonMode = UITextFieldViewModeWhileEditing;    addressTextField.layer.borderWidth = 1 / [UIScreen mainScreen].scale;    addressTextField.layer.borderColor =  [[[UIColor alloc] initWithRed:1 green:1 blue:0 alpha:1] CGColor];    addressTextField.layer.cornerRadius = 3;    addressTextField.text = [self aLongAddress];    [addressTextField mas_makeConstraints:^(MASConstraintMaker *make) {        make.height.equalTo(address);        make.centerY.equalTo(address);        make.right.equalTo(layoutView.mas_right).offset(-10);        make.left.equalTo(address.mas_right).offset(10);    }];}

此处使用了UILabel的自身内容尺寸约束，当houseNumberTextField.text = [self aShortAddress]UI表现正常。

但，当houseNumberTextField.text = [self aLongAddress]时会出现address UILabel被挤压掉的情况，如下图所示：

出错实例
原因是address Label的水平抗压缩没有设置。

在address Label创建的时候添加如下代码

[address setContentCompressionResistancePriority:UILayoutPriorityRequired forAxis:UILayoutConstraintAxisHorizontal]

则显示正常。

另，在某些情况下存在view被拉升，极有可能是没有设置抗拉升，此处不一一列举。

附，抗压抗拉相关API如下：

- (UILayoutPriority)contentHuggingPriorityForAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);- (void)setContentHuggingPriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);- (UILayoutPriority)contentCompressionResistancePriorityForAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);- (void)setContentCompressionResistancePriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis NS_AVAILABLE_IOS(6_0);

2 NSLayoutConstraint只能修改constant

NSLayoutConstraint即自动布局的约束类，它是自动布局的关键之一。该类有如下属性我们需要重点关注。

NS_CLASS_AVAILABLE_IOS(6_0)@interface NSLayoutConstraint : NSObject// other code@property UILayoutPriority priority;@property BOOL shouldBeArchived;/* accessors firstItem.firstAttribute {==,<=,>=} secondItem.secondAttribute * multiplier + constant */@property (readonly, assign) id firstItem;@property (readonly) NSLayoutAttribute firstAttribute;@property (readonly) NSLayoutRelation relation;@property (nullable, readonly, assign) id secondItem;@property (readonly) NSLayoutAttribute secondAttribute;@property (readonly) CGFloat multiplier;/* Unlike the other properties, the constant may be modified after constraint creation.  Setting the constant on an existing constraint performs much better than removing the constraint and adding a new one that's just like the old but for having a new constant. */@property CGFloat constant;/* The receiver may be activated or deactivated by manipulating this property.  Only active constraints affect the calculated layout.  Attempting to activate a constraint whose items have no common ancestor will cause an exception to be thrown.  Defaults to NO for newly created constraints. */@property (getter=isActive) BOOL active NS_AVAILABLE(10_10, 8_0);// other code@end

布局公式：

firstItem.firstAttribute {==,<=,>=} secondItem.secondAttribute * multiplier + constant

解释：firstItem与secondItem分别是界面中受约束的视图与被参照的视图。

注意：当使用代码来修改约束时，只能修改约束的常量值constant。一旦创建了约束，其他只读属性都是无法修改的，特别要注意的是比例系数multiplier也是只读的。

Masonry是基于NSLayoutConstraint等类的封装，也正是如此，我们在调用- (NSArray )mas_updateConstraints:(void(^)(MASConstraintMaker ))block的时候也只能更新NSLayoutConstraint中的@property CGFloat constant。

在MASViewConstraint找到如下代码可以佐证：

- (void)install {    // other code    MASLayoutConstraint *existingConstraint = nil;    if (self.updateExisting) { //如果是update，则去匹配对应的existingConstraint        existingConstraint = [self layoutConstraintSimilarTo:layoutConstraint];    }    if (existingConstraint) { //找到了existingConstraint，最终也只更新了existingConstraint.constant        // just update the constant        existingConstraint.constant = layoutConstraint.constant;        self.layoutConstraint = existingConstraint;    } else { //没有找到existingConstraint，添加一个新的约束        [self.installedView addConstraint:layoutConstraint];        self.layoutConstraint = layoutConstraint;        [firstLayoutItem.mas_installedConstraints addObject:self];    }}// 除了constant，其它都一样的约束是Similar约束- (MASLayoutConstraint *)layoutConstraintSimilarTo:(MASLayoutConstraint *)layoutConstraint {    // check if any constraints are the same apart from the only mutable property constant    // go through constraints in reverse as we do not want to match auto-resizing or interface builder constraints    // and they are likely to be added first.    for (NSLayoutConstraint *existingConstraint in self.installedView.constraints.reverseObjectEnumerator) {        if (![existingConstraint isKindOfClass:MASLayoutConstraint.class]) continue;        if (existingConstraint.firstItem != layoutConstraint.firstItem) continue;        if (existingConstraint.secondItem != layoutConstraint.secondItem) continue;        if (existingConstraint.firstAttribute != layoutConstraint.firstAttribute) continue;        if (existingConstraint.secondAttribute != layoutConstraint.secondAttribute) continue;        if (existingConstraint.relation != layoutConstraint.relation) continue;        if (existingConstraint.multiplier != layoutConstraint.multiplier) continue;        if (existingConstraint.priority != layoutConstraint.priority) continue;        return (id)existingConstraint;    }    return nil;}

样例：

@interface ViewController ()@property (nonatomic, strong) UILabel *lbl;@property (nonatomic, strong) UIButton *btn;@end@implementation ViewController- (void)viewDidLoad {    [super viewDidLoad];    // Do any additional setup after loading the view, typically from a nib.    self.btn = [UIButton buttonWithType:UIButtonTypeCustom];    self.btn.backgroundColor = [UIColor blueColor];    [self.btn setTitle:@"按钮" forState:UIControlStateNormal];    [self.btn addTarget:self action:@selector(onTest:) forControlEvents:UIControlEventTouchDown];    [self.view addSubview:self.btn];    [self.btn mas_updateConstraints:^(MASConstraintMaker *make) {        make.top.equalTo(self.view).offset(200);        make.centerX.equalTo(self.view);        make.size.mas_equalTo(CGSizeMake(100, 33));    }];    self.lbl = [[UILabel alloc] init];    self.lbl.text = @"一个label";    self.lbl.backgroundColor = [UIColor redColor];    self.lbl.textAlignment = NSTextAlignmentCenter;    [self.view addSubview:self.lbl];    [self.lbl mas_updateConstraints:^(MASConstraintMaker *make) {        make.top.equalTo(self.view).offset(300);        make.centerX.equalTo(self.view);        make.size.equalTo(self.btn);    }];}- (void)onTest:(id)sender{    [self.lbl mas_updateConstraints:^(MASConstraintMaker *make) {        make.size.mas_equalTo(CGSizeMake(200, 100));    }];}@end

当按钮被按下时，控制台出现如下警告

2016-08-03 18:49:13.110 layout[47924:2886276] Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want.  Try this:   (1) look at each constraint and try to figure out which you don't expect;   (2) find the code that added the unwanted constraint or constraints and fix it. (    "<MASLayoutConstraint:0x7ffecb632470 UIButton:0x7ffecb4f28e0.width == 100>",    "<MASLayoutConstraint:0x7ffecb637550 UILabel:0x7ffecb637030.width == UIButton:0x7ffecb4f28e0.width>",    "<MASLayoutConstraint:0x7ffecb71fc10 UILabel:0x7ffecb637030.width == 200>")Will attempt to recover by breaking constraint <MASLayoutConstraint:0x7ffecb71fc10 UILabel:0x7ffecb637030.width == 200>Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.2016-08-03 18:49:13.111 layout[47924:2886276] Unable to simultaneously satisfy constraints. Probably at least one of the constraints in the following list is one you don't want.  Try this:   (1) look at each constraint and try to figure out which you don't expect;   (2) find the code that added the unwanted constraint or constraints and fix it. (    "<MASLayoutConstraint:0x7ffecb612bc0 UIButton:0x7ffecb4f28e0.height == 33>",    "<MASLayoutConstraint:0x7ffecb625300 UILabel:0x7ffecb637030.height == UIButton:0x7ffecb4f28e0.height>",    "<MASLayoutConstraint:0x7ffecb486f10 UILabel:0x7ffecb637030.height == 100>")Will attempt to recover by breaking constraint <MASLayoutConstraint:0x7ffecb486f10 UILabel:0x7ffecb637030.height == 100>Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

原因是，lbl创建时其size约束是make.size.equalTo(self.btn)，但btn被点击时，企图去update size约束为make.size.mas_equalTo(CGSizeMake(200, 100))，然而无法找到existingConstraint，因此实际上是额外添加了一个约束make.size.mas_equalTo(CGSizeMake(200, 100))出现了布局冲突。

这件事可以这么看，NSLayoutConstraint只能修改constant决定了mas_updateConstraints的实现方式为：找到既有约束就去改变constant找不到既有约束就添加新约束。

3 被Masonry布局的view一定要与比较view有共同的祖先view

这句话比较拗口，其中涉及三类view，解释如下。

被Masonry布局的view：执行了

- (NSArray *)mas_makeConstraints:(void(^)(MASConstraintMaker *make))block;- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block;- (NSArray *)mas_remakeConstraints:(void(^)(MASConstraintMaker *make))block

等函数的view。

比较view：以上3函数block块里面出现的view。

共同的祖先view：【1】和【2】的共同祖先view。

样例1：

- (void)sampleCode{    UIView *v0 = [UIView new];    [self.view addSubview:v0];    UIView *v1 = [UIView new];    [v0 addSubview:v1];    [v1 mas_makeConstraints:^(MASConstraintMaker *make) {        make.size.mas_equalTo(CGSizeMake(10, 10));    }];    UIView *v2 = [UIView new];    [v0 addSubview:v2];    [v2 mas_makeConstraints:^(MASConstraintMaker *make) {        make.size.equalTo(v1);    }];}

针对如下代码块来说

UIView *v2 = [UIView new];[v0 addSubview:v2];[v2 mas_makeConstraints:^(MASConstraintMaker *make) {    make.size.equalTo(v1);}];

v2是被Masonry布局的view，v1是比较view，v0是共同的祖先view。

样例2：

@implementation AutoLayoutViewController- (void)viewDidLoad{    [super viewDidLoad];    [self useMasonryWithoutSuperView];}- (void)useMasonryWithoutSuperView{    UIView *masView = [UIView new];    [masView mas_makeConstraints:^(MASConstraintMaker *make) {        make.center.equalTo(self.view);    }];}@end

以上代码执行时会crash，crash log如下：

2016-08-04 00:52:47.542 CommonTest[1731:22953] *** Assertion failure in -[MASViewConstraint install], /Users/shuncheng/SourceCode/SampleCode/AutoLayout/Pods/Masonry/Masonry/MASViewConstraint.m:3382016-08-04 00:52:47.548 CommonTest[1731:22953] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'couldn't find a common superview for <UIView: 0x7fa59bd30dd0; frame = (0 0; 0 0); layer = <CALayer: 0x7fa59bd2f3c0>> and <UIView: 0x7fa59bd30c60; frame = (0 0; 414 736); autoresize = W+H; layer = <CALayer: 0x7fa59bd24780>>'

crash的原因显而易见，即，masView(被Masonry布局的view)与self.view(比较view)没有共同祖先view，因为masView没有父view，所以它和self.view必然没有共同祖先view。

被Masonry布局的view没有添加到superview上其实比较容易被发现，最怕的是出现如样例3一样的鬼畜情况。

样例3：

@implementation AutoLayoutViewController- (void)viewDidLoad{    [super viewDidLoad];    [self sampleCode];}- (void)sampleCode{    AutoLayoutViewController * __weak weakSelf = self;    [fooNetworkModel fetchData:^{        AutoLayoutViewController * self = weakSelf;        [AutoLayoutViewController showSampleViewAtView:self.view];    }];}+ (void)showSampleViewAtView:(UIView *)view{    UIView *v1 = [UIView new];    [view addSubview:v1];    [v1 mas_makeConstraints:^(MASConstraintMaker *make) {        make.size.mas_equalTo(CGSizeMake(10, 10));    }];    UIView *v2 = [UIView new];    [view addSubview:v2];    [v2 mas_makeConstraints:^(MASConstraintMaker *make) {        make.size.equalTo(v1);    }];}@end

以上代码通常不会出错，但是一种异常情况是：在AutoLayoutViewController析构后，网络数据返回，此时AutoLayoutViewController * self = weakSelf则self == nil。执行[AutoLayoutViewController showSampleViewAtView:nil]，则会出现【样例2】一样的crash。

原因是：v1和v2都没有添加到view上去（因为view为空）所以make.size.equalTo(v1)会出错（v1和v2没有共同的父view）。由此也引申到weakSelf的副作用，即必须要确保weakSelf是nil时，执行逻辑完全没有问题（目前已经两次被坑）。

4 不要被update迷惑

这里说的update有两层含义：

UIView的方法- (void)updateConstraints NS_AVAILABLE_IOS(6_0)
Masonry的方法- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block
这里先来讨论一下UIView的- (void)updateConstraints方法。

- (void)updateConstraints方法是用来更新view约束的，它有一个常见的使用场景——批量更新约束。比如你的多个约束是由多个不同的property决定，每次设置property都会直接更新局部约束。这样效率不高。不如直接override- (void)updateConstraints方法，在方面里面对property进行判断，每次设置property的时候调用一下- (void)setNeedsUpdateConstraints。伪代码如下：

优化前：

@implementation AutoLayoutView- (void)setFactor1:(NSInteger)factor1{    _factor1 = factor1;    if (_factor1满足条件) {        更新约束1    }}- (void)setFactor2:(NSInteger)factor2{    _factor2 = factor2;    if (_factor2满足条件) {        更新约束2    }}- (void)setFactor3:(NSInteger)factor3{    _factor3 = factor3;    if (_factor3满足条件) {        更新约束3    }}@end

优化后：

@implementation AutoLayoutView- (void)setFactor1:(NSInteger)factor1{    _factor1 = factor1;    [self setNeedsUpdateConstraints];}- (void)setFactor2:(NSInteger)factor2{    _factor2 = factor2;    [self setNeedsUpdateConstraints];}- (void)setFactor3:(NSInteger)factor3{    _factor3 = factor3;    [self setNeedsUpdateConstraints];}- (void)updateConstraints{    if (self.factor1满足) {        更新约束1    }    if (self.factor2满足) {        更新约束2    }    if (self.factor3满足) {        更新约束3    }    [super updateConstraints];}@end

注意：一种有误区的写法是在- (void)updateConstraints方法中进行初次constraint设置，这是不被推荐的。推荐的写法是在init或者viewDidLoad中进行view的初次constraint设置。

Masonry的方法- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block我们在第二节已经讨论过了。刚接触自动布局和Masonry的同学很容易跟着感觉在- (void)updateConstraints函数里面调用Masonry的- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block。实际上两者并没有必然联系。大多数情况在- (void)updateConstraints里面调用- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block很有可能产生布局冲突。

样例

// 头文件typedef NS_ENUM(NSUInteger, AutoLayoutType) {    HorizontalLayout,    VerticalLayout,};@interface AutoLayoutView : UIView@property (nonatomic, strong) UILabel *name;@property (nonatomic, strong) UILabel *address;@property (nonatomic, assign) AutoLayoutType layoutType;@end// 实现文件@implementation AutoLayoutView- (instancetype)initWithFrame:(CGRect)frame{    if (self = [super initWithFrame:frame]) {        _name = [[UILabel alloc] init];        [self addSubview:_name];        _address = [[UILabel alloc] init];        [self addSubview:_address];        [_name mas_updateConstraints:^(MASConstraintMaker *make) {            make.left.top.equalTo(self);        }];    }    return self;}- (void)updateConstraints{    if (self.layoutType == HorizontalLayout) {       // // 此处误用mas_updateConstraints        [self.address mas_updateConstraints:^(MASConstraintMaker *make) {            make.top.equalTo(self.name);            make.left.equalTo(self.name.mas_right).offset(10);        }];    } else {        // 此处误用mas_updateConstraints        [self.address mas_updateConstraints:^(MASConstraintMaker *make) {            make.left.equalTo(self.name);            make.top.equalTo(self.name.mas_bottom).offset(10);        }];    }    [super updateConstraints];}- (void)setLayoutType:(AutoLayoutType)layoutType{    _layoutType = layoutType;    [self setNeedsUpdateConstraints];}@end// 外部调用代码- (void)sampleCode{    AutoLayoutView *view = [[AutoLayoutView alloc] init];    view.name.text = @"name";    view.address.text = @"address";    [self.view addSubview:view];    [view mas_makeConstraints:^(MASConstraintMaker *make) {        make.center.equalTo(self.view);        make.size.mas_equalTo(CGSizeMake(200, 300));    }];    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{        view.layoutType = VerticalLayout; //修改布局方式后，出现布局冲突    });}

5 总结

本文梳理了一下自动布局和Masonry使用的误区。在基本概念没搞清的情况下，很容易犯错。总结起来就如下4点：

• 理解自身内容尺寸约束与抗压抗拉
• “NSLayoutConstraint只能修改constant”和 “- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block实现细节”之间的关系
• 被Masonry布局的view一定要与比较view有共同的祖先view
• 区分UIView的- (void)updateConstraints方法和- (NSArray *)mas_updateConstraints:(void(^)(MASConstraintMaker *make))block

6 参考资料

WWDC-Mysteries of Auto Layout, Part 2

作者：宿于松下
链接：http://www.jianshu.com/p/337ef580eef7
來源：简书
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