将 UILabel 作为子视图添加到 UITextView 时的不明确约束

Question

The Problem
I have subclassed a UITextView and added a UILabel as a subview. I want to position the label in the bottom right hand corner.

Code

  fileprivate lazy var counterLabel: UILabel = {
    let label = UILabel()
    label.translatesAutoresizingMaskIntoConstraints = false

    self.addSubview(label)

    // add constraints
    self.addConstraints([
      NSLayoutConstraint(item: label, equalTo: self, attribute: .right),
      NSLayoutConstraint(item: label, equalTo: self, attribute: .bottom)]
    )

    return label
  }()

Screenshot

As seen in the above screenshot, the red label is shown to have an ambiguous layout

If I use a storyboard, and add the same two constraints (bottom, right) the label does not have an ambiguous position. What have I done wrong?

Answer 1

What you want is a superview. Add the label and the textview as subviews to this superview.

Answer 2

The problem may be because of instrict constraints. All labels has it's own constraints to expand due to text inside. That constraints has some priority. Possible problem may be that your created constraints have lower priority, which causes such problem. Try creating constraints with full function. It works fine for me. Here is mine code with text field.

self.leadingLabelConstraint = [NSLayoutConstraint constraintWithItem:_markLabel
                                                               attribute:NSLayoutAttributeLeading
                                                               relatedBy:NSLayoutRelationEqual
                                                                  toItem:self
                                                               attribute:NSLayoutAttributeLeading
                                                              multiplier:1.0 constant:0];
    self.topLabelConstraint = [NSLayoutConstraint constraintWithItem:_markLabel
                                                           attribute:NSLayoutAttributeTop
                                                           relatedBy:NSLayoutRelationEqual
                                                              toItem:self
                                                           attribute:NSLayoutAttributeTop
                                                          multiplier:1.0 constant:0];
    [self.superview addConstraints:@[self.topLabelConstraint, self.leadingLabelConstraint]];
    self.heightLabelConstraint = [NSLayoutConstraint constraintWithItem:_markLabel attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:20];
    [self.markLabel addConstraint:self.heightLabelConstraint];

self is - UITextField

and markLabel is - UILabel.