如何解决视图居中之谜？

Question

I have a UIStackView, which holds an image view and a text view. The image view has an activity indicator view as a subview (I add this programmatically). When the view initially view loads, I center the activity indicator view within the image view and starting the animation for the AIV. But, in the process of waiting for the image to load, if I rotate the image view, the AIV needs to be re-centered. I added code to listen for the status bar orientation change notification, and have a method to invoke upon the notification (UIApplicationDidChangeStatusBarOrientationNotification). In that method, I attempt to get the bounds of the image view and use that to re-center the AIV within the image view. But, it seems that the value of the bounds of the image view is not changing when the device orientation changes. Is there a “reliable" way to get the updated bounds for a view after the device orientation has changed? Code ->

// start listening for orientation changes
NSNotificationCenter.defaultCenter().addObserver(self, selector: "orientationDidChange:",
           name: UIApplicationDidChangeStatusBarOrientationNotification, object: nil)

Then I have:

func orientationDidChange(notification: NSNotification) {

   // get image view bounds
   let viewBounds = _imageView.bounds

   // recenter the activity indicator
   _activityIndicator.center = CGPointMake(CGRectGetMidX(viewBounds), CGRectGetMidY(viewBounds))
}

Update:

So I got rid of the above and tried this:

// setup activity indicator
_activityIndicatorView.translatesAutoresizingMaskIntoConstraints = true

        _imageView.addSubview(_activityIndicatorView)

        // set constraints
        let centerXConstraint = NSLayoutConstraint(item: _activityIndicatorView, attribute: .CenterX,
            relatedBy: .Equal, toItem: _imageView,
            attribute: .CenterX, multiplier: 1.0,
            constant: 0.0)
        _activityIndicatorView.addConstraint(centerXConstraint)

        let centerYConstraint = NSLayoutConstraint(item: _activityIndicatorView, attribute: .CenterY,
            relatedBy: .Equal, toItem: _imageView,
            attribute: .CenterY, multiplier: 1.0,
            constant: 0.0)
        _activityIndicatorView.addConstraint(centerYConstraint)

which is in my view did load method. The console spits out this:

2015-10-27 19:26:09.303 Historic Sites Navigator[373:93902] The view hierarchy is not prepared for the constraint: When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled. Break on -[UIView(UIConstraintBasedLayout) _viewHierarchyUnpreparedForConstraint:] to debug. 2015-10-27 19:26:09.308 Historic Sites Navigator[373:93902] View hierarchy unprepared for constraint. Constraint: Container hierarchy: > | > View not found in container hierarchy: > That view's superview: > 2015-10-27 19:26:09.310 Historic Sites Navigator[373:93902] * Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to install constraint on view. Does the constraint reference something from outside the subtree of the view? That's illegal. constraint: view:>' * First throw call stack: (0x2643767b 0x377b2e17 0x264375c1 0x27177983 0x2a628f85 0x2a628dd5 0x2a628cef 0x2ad33971 0x2a628bd7 0xdf6c4 0xdfe34 0x2a5271c7 0x2a526dad 0x2ad8ca09 0x2a8689ad 0x2a88c095 0x2a88e6c5 0x2a88e931 0x2a61e989 0x2a891bb7 0x2ab33085 0x2ac09b39 0x2ac09967 0x2a883d95 0xccac8 0xccc10 0x2a659487 0x2a71440b 0x2a7cc0b9 0x2a7d79eb 0x2a519ecd 0x263fa4c9 0x263f87cd 0x263f8bff 0x2634c119 0x2634bf05 0x2f502ac9 0x2a58ef15 0xd6e74 0x37f21873) libc++abi.dylib: terminating with uncaught exception of type NSException

Answer 1

For iOS 8 and 9 the following function is called on size change of the view (which is eg at interface rotation):

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

    coordinator.animateAlongsideTransition({ (coordinator) -> Void in
        // here the new size is already set, so place your code here
    }, completion: nil)
}

Apart from this you could use constraints (auto layout) for positioning of the UIActivityIndicatorView.

You current autolayout errors are because you have to:

• set translatesAutoresizingMaskIntoConstraints to false
• add the constraints to the image view, not to the ai-view

Answer 2

After watching the Autolayout videos from WWDC 2015 I was able to write the correct and working code:

_activityIndicatorView.translatesAutoresizingMaskIntoConstraints = false

_imageView.addSubview(_activityIndicatorView)

NSLayoutConstraint(item: _activityIndicatorView, attribute: .CenterX,
            relatedBy: .Equal, toItem: _imageView,
            attribute: .CenterX, multiplier: 1.0,
            constant: 0.0).active = true

NSLayoutConstraint(item: _activityIndicatorView, attribute: .CenterY,
            relatedBy: .Equal, toItem: _imageView,
            attribute: .CenterY, multiplier: 1.0,
            constant: 0.0).active = true

@13:48 of this video was the key: https://developer.apple.com/videos/play/wwdc2015-219/