Adding observer for KVO without pointers using Swift

Question

In Objective-C, I would normally use something like this:

static NSString *kViewTransformChanged = @\"view transform changed\";
// or
static const void *kVie         


        

Answer 1

Swift 4 - observing contentSize change on UITableViewController popover to fix incorrect size

I had been searching for an answer to change to a block based KVO because I was getting a swiftlint warning and it took me piecing quite a few different answers together to get to the right solution. Swiftlint warning:

Block Based KVO Violation: Prefer the new block based KVO API with keypaths when using Swift 3.2 or later. (block_based_kvo).

My use case was to present a popover controller attached to a button in a Nav bar in a view controller and then resize the popover once it's showing - otherwise it would be too big and not fitting the contents of the popover. The popover itself was a UITableViewController that contained static cells, and it was displayed via a Storyboard segue with style popover.

To setup the block based observer, you need the following code inside your popover UITableViewController:

// class level variable to store the statusObserver
private var statusObserver: NSKeyValueObservation?

// Create the observer inside viewWillAppear
override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    statusObserver = tableView.observe(\UITableView.contentSize,
        changeHandler: { [ weak self ] (theTableView, _) in self?.popoverPresentationController?.presentedViewController.preferredContentSize = theTableView.contentSize
        })
}

// Don't forget to remove the observer when the popover is dismissed.
override func viewDidDisappear(_ animated: Bool) {
    if let observer = statusObserver {
        observer.invalidate()
        statusObserver = nil
    }

    super.viewDidDisappear(animated)
}

I didn't need the previous value when the observer was triggered, so left out the options: [.new, .old] when creating the observer.