ios - 自动布局问题

Question

我正在开发一个使用自动布局的应用程序。我正在执行以下步骤:

1. 第 1 步:在 viewDidLoad 中创建一个按钮

   [super viewDidLoad];
   
   self.view.translatesAutoresizingMaskIntoConstraints = NO;
   
   _button1 = [UIButton buttonWithType:UIButtonTypeRoundedRect];
   _button1.translatesAutoresizingMaskIntoConstraints = NO;
   [_button1 setTitle:@"B" forState:UIControlStateNormal];
   [self.view addSubview:_button1];
   

2. 第 2 步:在 updateViewConstraints 方法中实现约束

   [super updateViewConstraints];
   
   [self.view removeConstraints:self.view.constraints];
   
   if (UIInterfaceOrientationIsPortrait([UIApplication sharedApplication].statusBarOrientation))
   {
       NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0f constant:100.0f];
       [self.view addConstraint:constraint];
   
       NSLayoutConstraint *constraint1 = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0f constant:-100.0f];
       [self.view addConstraint:constraint1];
   
       NSLayoutConstraint *constraint2 = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0f constant:200.0f];
       [self.view addConstraint:constraint2];
   
       NSLayoutConstraint *constraint3 = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0f constant:-100.0f];
       [self.view addConstraint:constraint3];
   
   
       _button1.backgroundColor = [UIColor redColor];
   } else{
       NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0f constant:200.0f];
       [self.view addConstraint:constraint];
   
       NSLayoutConstraint *constraint1 = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0f constant:-200];
       [self.view addConstraint:constraint1];
   
       NSLayoutConstraint *constraint2 = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0f constant:50.0f];
       [self.view addConstraint:constraint2];
   
       NSLayoutConstraint *constraint3 = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0f constant:-50.0f];
       [self.view addConstraint:constraint3];
   
       _button1.backgroundColor = [UIColor blueColor];
   }
   

但是当我切换设备方向时，控制台会打印以下内容:

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. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) ( "UIView:0x8a461c0 (Names: '|':UIWindow:0x8a42970 )>", "", "", "UIButton:0x8a45ea0 (Names: '|':UIView:0x8a461c0 )>", "" )

Will attempt to recover by breaking constraint

Break on objc_exception_throw to catch this in the debugger. The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in may also be helpful.

谁能告诉我这个布局有什么问题吗？

Answer 1

问题在于，您在 updateViewConstraints 中调用 [super updateViewConstraints]，而按钮仍有约束。因此，当您从横向过渡到纵向时，您仍然具有横向按钮约束(在纵向中无法满足)，但要求主视图更新其(纵向)约束。如果在删除所有现有按钮约束后将调用移至 [super updateViewConstraints] 任何位置，那么您应该处于良好状态。

---

一些旁白:

1. 如果使用 Storyboard/NIBS，您应该删除以下行:

   self.view.translatesAutoresizingMaskIntoConstraints = NO;
   

   但请保留以下内容:

   _button1.translatesAutoresizingMaskIntoConstraints = NO;
   

2. 我对全面取消所有限制持谨慎态度。我通常保留要删除的约束的数组，这样我就可以轻松删除那些我需要删除并将要重建的约束。在您的情况下，删除所有内容可能没问题，但是当您向 View 添加越来越多的约束时，跟踪您想要删除和重建的内容可能会更容易:

   @property (nonatomic, strong) NSArray *verticalConstraints;
   @property (nonatomic, strong) NSArray *horizontalConstraints;
   

3. 我可能建议使用 VFL，它更简洁一点:

   - (void)updateViewConstraints
   {
       if (self.horizontalConstraints)
           [self.view removeConstraints:self.horizontalConstraints];
   
       if (self.verticalConstraints)
           [self.view removeConstraints:self.verticalConstraints];
   
       [super updateViewConstraints];
   
       NSDictionary *views   = NSDictionaryOfVariableBindings(_button1);            
       NSDictionary *metrics = nil;
   
       if (UIInterfaceOrientationIsPortrait([UIApplication sharedApplication].statusBarOrientation))
       {
           metrics = @{@"left"   : @100,
                       @"right"  : @100,
                       @"top"    : @200,
                       @"bottom" : @100};
   
           _button1.backgroundColor = [UIColor redColor];
       } else{
           metrics = @{@"left"   : @200,
                       @"right"  : @200,
                       @"top"    : @50,
                       @"bottom" : @50};
   
           _button1.backgroundColor = [UIColor blueColor];
       }
   
       self.horizontalConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(left)-[_button1]-(right)-|" options:0 metrics:metrics views:views];
       self.verticalConstraints   = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-(top)-[_button1]-(bottom)-|" options:0 metrics:metrics views:views];
       [self.view addConstraints:self.horizontalConstraints];
       [self.view addConstraints:self.verticalConstraints];
   }