在所有子视图具有自动布局之前调用viewWillTransitionToSize

Question

I have a View Controller with a view hierarchy within two nested views that are placed with autolayout constraints. I then place in code some UIViews into one of those views but I don't want to use constraints for these as the user can drag them around.

If the device gets rotated, or the user splits the view using the slide over functionality of the iPad I want to reset the locations of these UIViews into their new place based on the automatic layout of the parent views:

I have a function in the questionView called resetAnswerPosition() that correctly positions all of the Views based on the position of the reference View after the auto-layout positioning has kicked in. QuestionView has stores a reference to the ReferenceView, to make this easier.

The documentation suggests using viewWillTransitionToSize & willTransitionToTraitCollection to run these changes which works for rotation, or for when the slide over changes the aspect ratio from landscape to portrait or back again. However on an iPad Pro when change the split from 2/3 to 1/2 the aspect ratio isn't changing that the resetAnswerTilesPositions is being called before the Reference View has been resized.

I have also tried to use the viewDidLayoutSubViews but again have the issue that this is run before the Reference view has been resized.

override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {

    super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)

    coordinator.animateAlongsideTransition({ (context) -> Void in
        self.questionView.resetAnswerTilePositions()
        },
        completion: { (context) -> Void in
    })
}

override func willTransitionToTraitCollection(newCollection: UITraitCollection, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
    self.questionView.resetAnswerTilePositions()
}

override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    self.questionAnswerView.resetAnswerTilePositions()
}

What function can I register either with the View Controller or the QuestionView which will only be called once all of the views have been laid out according to the updated constraints?

Answer 1

As always posting a question led me to the right answer.

In the Question View you can override the layoutSubviews routine which is really nice from a separation of concerns of point of view. Only the Question View then needs to know how to layout its own subviews.

override func layoutSubviews() {
    super.layoutSubviews()
    self.resetAnswerTilePositions()
}