iphone - 为什么约束不适用于 UIView 但适用于 UILabel？

Question

我这里有一个简单(完整)的例子，看起来很奇怪，但我肯定只是漏掉了一些小东西……对吧？你能帮忙调试下面的简单代码吗？此代码使 aView 消失，但是当我将 aLabel 放在 aView 所在的约束中时，它会完美运行。为什么？感谢您的任何意见，这对我来说似乎很疯狂。

奥斯汀

UIView *aView = [[UIView alloc] initWithFrame:CGRectMake(0, 100, 100, 30)];
aView.backgroundColor = [UIColor redColor];
aView.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:aView];

UILabel *aLabel = [[UILabel alloc] initWithFrame:CGRectMake(0, 100, 100, 30)];
aLabel.backgroundColor = [UIColor redColor];
aLabel.text = @"Label";
aLabel.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:aLabel];

NSLayoutConstraint *myConstraint =[NSLayoutConstraint
                                   constraintWithItem:aView
                                   attribute:NSLayoutAttributeCenterY
                                   relatedBy:NSLayoutRelationEqual
                                   toItem:self.view
                                   attribute:NSLayoutAttributeCenterY
                                   multiplier:1.0
                                   constant:0];

[self.view addConstraint:myConstraint];

myConstraint =[NSLayoutConstraint
               constraintWithItem:aView
               attribute:NSLayoutAttributeCenterX
               relatedBy:NSLayoutRelationEqual
               toItem:self.view
               attribute:NSLayoutAttributeCenterX
               multiplier:1.0
               constant:0];

[self.view addConstraint:myConstraint];

Answer 1

好吧，行 aView.translatesAutoresizingMaskIntoConstraints = NO;正在使 View 的大小为零。所以你必须添加这些代码行:

NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:aView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:100];
[aView addConstraint:widthConstraint];


NSLayoutConstraint *heightConstraint = [NSLayoutConstraint constraintWithItem:aView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:30];
[aView addConstraint:heightConstraint];