使用iOS自动布局在子视图控制器及其父视图之间建立约束

在我的根视图控制器中，我将子视图控制器的视图添加为子视图，如下所示：

ChildViewController *cvc = [[ChildViewController alloc] init];
[self addChildViewController:cvc];
[self.view addSubview:cvc.view];
[cvc didMoveToParentViewController:self];

现在我想使用NSLayoutConstraint在父视图（self.view）中定位cvc.view，这样cvc.view在父视图底部上方定位25个点。我的理解是，以下方面应该有效：

UIView *superview = self.view;
UIView *childview = cvc.view;
NSLayoutConstraint *cn = 
    [NSLayoutConstraint withItem:childview 
      attribute:NSLayoutAttributeBottom 
      relatedBy:NSLayoutRelationEqual 
      toItem:superview attribute:NSLayoutAttributeBottom 
      multiplier: 1.0 
      constant: -25.0];
[superview addConstraint: cn];

但约束在运行时失败。起初我认为可能是子视图中的自动调整大小掩码导致了问题（在WWDC 2012自动布局介绍视频之后），所以我设置了

[childview settranslatesAutoResizengMackintoConstraints:NO]

，但随后子视图根本没有出现

---

我做错了什么？

我不确定，但以下方法应该有效，或者类似的方法：

UIView *superview = self.view;
UIView *childview = cvc.view;
NSDictionary *constrainedViews = NSDictionaryOfVariableBindings(childview);
NSArray *constraints =
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[childview]-25-|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:constrainedViews];

如果您不能确保您实际设置了childview的大小，例如：

UIView *superview = self.view;
UIView *childview = cvc.view;
NSDictionary *constrainedViews = NSDictionaryOfVariableBindings(childview);
NSArray *constraints =
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[childview]|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:constrainedViews];

比方说，让它填满视图的宽度。或：

UIView *superview = self.view;
UIView *childview = cvc.view;
NSDictionary *constrainedViews = NSDictionaryOfVariableBindings(childview);
NSArray *constraints =
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[childview(>100,<304)]-|"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:constrainedViews];

UIView*superview=self.view；
UIView*childview=cvc.view；
NSDictionary*constrainedViews=NSDictionaryOfVariableBindings（childview）；
NSArray*约束=
[superview添加约束：[NSLayoutConstraint Constraints with VisualFormat:@“H:|-[childview（>100，要使用自动布局正确设置和调整子视图控制器的大小，请执行以下操作：

childView.translatesAutoresizingMaskIntoConstraints = NO;

NSLayoutConstraint *left = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeLeft multiplier:1 constant:0];
[self.view addConstraint:left];
NSLayoutConstraint *top = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeTop multiplier:1 constant:0];
[self.view addConstraint:top];
NSLayoutConstraint *width = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeWidth multiplier:1 constant:0];
[self.view addConstraint:width];
NSLayoutConstraint *height = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeHeight multiplier:1 constant:0];
[self.view addConstraint:height];

childView.translatesAutoresizingMaskIntoConstraints = NO;

NSDictionary *views = @{@"childview": childview};
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|-0-[childview]-0-|" options:0 metrics:nil views:views]];
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-0-[childview]-0-|" options:0 metrics:nil views:views]];

如果您喜欢视觉格式，可以执行以下操作：

childView.translatesAutoresizingMaskIntoConstraints = NO;

NSLayoutConstraint *left = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeLeft multiplier:1 constant:0];
[self.view addConstraint:left];
NSLayoutConstraint *top = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeTop multiplier:1 constant:0];
[self.view addConstraint:top];
NSLayoutConstraint *width = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeWidth multiplier:1 constant:0];
[self.view addConstraint:width];
NSLayoutConstraint *height = [NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:childview attribute:NSLayoutAttributeHeight multiplier:1 constant:0];
[self.view addConstraint:height];

childView.translatesAutoresizingMaskIntoConstraints = NO;

NSDictionary *views = @{@"childview": childview};
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"|-0-[childview]-0-|" options:0 metrics:nil views:views]];
[superview addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-0-[childview]-0-|" options:0 metrics:nil views:views]];