NSLayoutConstraint提供宽度但不提供高度

Question

我创建了一个自定义UIView：

我正在使用NSLayoutConstraints来创建布局和大小。由于UIView没有内在大小＆＃39;就像UILabel或UITextField一样，我将视图的NSLayoutAttributeBottom等于加20到最低的子视图。这会强制视图向下和向右延伸，根据其子视图的位置给出大小。

[self addConstraints:[NSLayoutConstraint
                      constraintsWithVisualFormat:@"V:|[topBarView]-[nameLabel]"
                      options:0
                      metrics:nil
                      views:views]];
NSLayoutConstraint *imageViewBottomConstraint =
[NSLayoutConstraint constraintWithItem:self
                             attribute:NSLayoutAttributeBottom
                             relatedBy:NSLayoutRelationEqual
                                toItem:imageView
                             attribute:NSLayoutAttributeBottom
                            multiplier:1 constant:20];
imageViewBottomConstraint.priority = 400;
[self addConstraint:imageViewBottomConstraint];

我将bottom属性设置为imageView（带齿轮图像的蓝框）并设置优先级。

[self addConstraints:[NSLayoutConstraint
                      constraintsWithVisualFormat:@"V:[nameLabel][nameTextField]-(8@600)-[commentsLabel]-(0@600)-[commentsTextView]"
                      options:NSLayoutFormatAlignAllLeading
                      metrics:nil
                      views:views]];
NSLayoutConstraint *commentsTextViewBottomConstraint =
[NSLayoutConstraint constraintWithItem:self
                             attribute:NSLayoutAttributeBottom
                             relatedBy:NSLayoutRelationEqual
                                toItem:commentsTextView
                             attribute:NSLayoutAttributeBottom
                            multiplier:1 constant:20];
commentsTextViewBottomConstraint.priority = 300;
[self addConstraint:commentsTextViewBottomConstraint];

我还将底部属性设置为评论文本视图，其优先级较低，因此很容易破解。

[self addConstraints:[NSLayoutConstraint
                      constraintsWithVisualFormat:@"H:|-[imageView]-(50)-[nameTextField]"
                      options:NSLayoutFormatAlignAllTop
                      metrics:nil
                      views:views]];
[self addConstraint:[NSLayoutConstraint
                     constraintWithItem:commentsTextView
                     attribute:NSLayoutAttributeWidth
                     relatedBy:NSLayoutRelationEqual
                     toItem:nameTextField
                     attribute:NSLayoutAttributeWidth
                     multiplier:1 constant:0]];
[self addConstraint:[NSLayoutConstraint constraintWithItem:self
                                                 attribute:NSLayoutAttributeRight
                                                 relatedBy:NSLayoutRelationEqual
                                                    toItem:nameTextField
                                                 attribute:NSLayoutAttributeRight
                                                multiplier:1 constant:20]];

然后我设置正确的属性给它宽度。

我遇到高度为0的问题，直到我将其添加到子视图中：

PhotoDisplayPanel *photoPanel = [[PhotoDisplayPanel alloc] initWithPhotoData:nil];

[self.view layoutIfNeeded];

DLog(@"BEFORE | photoPanel.frame.size: (%f, %f)", photoPanel.frame.size.width, photoPanel.frame.size.height);

[self.view addSubview:photoPanel];

[self.view layoutIfNeeded];

DLog(@"AFTER | photoPanel.frame.size: (%f, %f)", photoPanel.frame.size.width, photoPanel.frame.size.height);

结果：

DEBUG | -[PhotoDisplayPanel initWithPhotoData:] | self.frame.size: (790.000000, 0.000000)
DEBUG | -[LoginViewController viewDidLoad] | BEFORE | photoPanel.frame.size: (790.000000, 0.000000)
DEBUG | -[LoginViewController viewDidLoad] | AFTER | photoPanel.frame.size: (790.000000, 389.000000)

在将视图添加到视图之前，我很困惑为什么视图有宽度而不是高度。这是我在尝试将视图设置为UIPopoverController时遇到的问题。

编辑：

在玩完之后，原因是我添加到2个底部属性约束的优先级。我删除了＆＃39; commentsTextViewbottomConstraint＆＃39;约束并删除了`imageViewBottomConstraint＆＃39;的优先级，它几乎可以工作：

    [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|[topBarView]-[nameLabel]" options:0 metrics:nil views:views]];
    NSLayoutConstraint *imageViewBottomConstraint = [NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:imageView attribute:NSLayoutAttributeBottom multiplier:1 constant:20];
//    imageViewBottomConstraint.priority = 400;
    [self addConstraint:imageViewBottomConstraint];

    [self addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[nameLabel][nameTextField]-(8@600)-[commentsLabel]-(0@600)-[commentsTextView]" options:NSLayoutFormatAlignAllLeading metrics:nil views:views]];
//    NSLayoutConstraint *commentsTextViewBottomConstraint = [NSLayoutConstraint constraintWithItem:self attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:commentsTextView attribute:NSLayoutAttributeBottom multiplier:1 constant:20];
//    commentsTextViewBottomConstraint.priority = 300;
//    [self addConstraint:commentsTextViewBottomConstraint];

新结果：

DEBUG | -[PhotoDisplayPanel initWithPhotoData:] | self.frame.size: (590.000000, 572.000000)
DEBUG | -[LoginViewController viewDidLoad] | BEFORE | photoPanel.frame.size: (590.000000, 572.000000)
DEBUG | -[LoginViewController viewDidLoad] | AFTER | photoPanel.frame.size: (590.000000, 589.000000)

我以这种方式开发了代码，因此如果图像视图比文本视图的底部属性更大，则会拉伸以显示完整图像。或者，如果图像小于文本视图的底部属性，则它将保持显示文本视图的最小高度，例如：

知道如何维护此功能吗？