Adding subview to another subview using Auto Layout

Question

I'm sure I am missing something stupid here, but I have mainView, subviewA, and subviewB. I am trying to add subviewB to subviewA and anchor it inside of subviewA, however it is not anchoring (just remains in top left corner. However, if I add subviewB to mainView and then anchor it, it works fine.

Example (using a custom anchoring function that I believe is self-explanatory):

addSubview(questionContainerView)
questionContainerView.anchor(topAnchor, left: leftAnchor, bottom: centerYAnchor, right: rightAnchor, topConstant: 0, leftConstant: 0, bottomConstant: 0, rightConstant: 0, widthConstant: 0, heightConstant: 0)


// does not work
questionContainerView.addSubview(questionTextLabel)
questionTextLabel.anchor(questionContainerView.topAnchor, left: questionContainerView.leftAnchor, bottom: questionContainerView.bottomAnchor, right: questionContainerView.rightAnchor, topConstant: 25, leftConstant: 10, bottomConstant: 25, rightConstant: 10, widthConstant: 0, heightConstant: 0)

// does work
addSubview(questionTextLabel)
questionTextLabel.anchor(questionContainerView.topAnchor, left: questionContainerView.leftAnchor, bottom: questionContainerView.bottomAnchor, right: questionContainerView.rightAnchor, topConstant: 25, leftConstant: 10, bottomConstant: 25, rightConstant: 10, widthConstant: 0, heightConstant: 0)

Answer 1

You cannot add the same view to two separate parent views. Once you add it to a different view, it will be removed from the previous view. If you want the label view on both views, simply create two instances of it.

questionContainerView.addSubview(questionTextLabel) // first time
addSubview(questionTextLabel) // second time the questionTextLabel is removed from questionContainerView

Answer 2

I tried the same code with different background color and found nothing wrong with it, maybe because of same background color, it wont be visible.

Please find the code below

        self.view.addSubview(questionContainerView)
    questionContainerView.anchor(top: self.view.topAnchor, left: self.view.leftAnchor, bottom: self.view.centerYAnchor, right: self.view.rightAnchor, topConstant: 0, leftConstant: 0, bottomConstant: 0, rightConstant: 0, widthConstant: 0, heightConstant: 0)
    questionContainerView.backgroundColor = .blue

    questionContainerView.addSubview(questionTextLabel)
    questionTextLabel.anchor(top: questionContainerView.topAnchor, left: questionContainerView.leftAnchor, bottom: questionContainerView.bottomAnchor, right: questionContainerView.rightAnchor, topConstant: 25, leftConstant: 10, bottomConstant: 25, rightConstant: 10, widthConstant: 0, heightConstant: 0)
    questionTextLabel.backgroundColor = .black