observing contentSize (CGSize) with KVO in swift - xcode

Question

I'm trying to observering collectionView.contentSize like this :
func startObserveCollectionView() {
collectionView.addObserver(self, forKeyPath: "contentSize", options: NSKeyValueObservingOptions.Old.union(NSKeyValueObservingOptions.New), context: &SearchDasboardLabelContext)
}
override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer<Void>) {
if context == &SearchDasboardLabelContext {
if object === collectionView && keyPath! == "contentSize" {
print(change)
}
}
}
and in xcode terminal I got a NSSize not CGSize like this :
Optional(["old": NSSize: {320, 0}, "new": NSSize: {375, 39.5}, "kind": 1])
In objective-c I used method CGSizeValue
CGSize newContentSize = [[change objectForKey:NSKeyValueChangeNewKey] CGSizeValue];
Is there any method like CGSizeValue in swift
I have tried in swift var newContentSize = change[NSKeyValueChangeNewKey]?.CGSizeValue() but got error
could not find member 'CGSizeValue'
need help anyone? Thanks

Answer 1

With Swift 4, you can cast the result of the change dictionary for the key NSKeyValueChangeKey.newKey as being of type CGSize:
if let size = change?[NSKeyValueChangeKey.newKey] as? CGSize {
/* ... */
}
The following UIViewController implementation shows how to set a KVO stack in order to observe the changes of the contentSize property of any UIScrollView subclass (e.g UITextView):
import UIKit
private var myContext = 0
class ViewController: UIViewController {
#IBOutlet weak var textView: UITextView!
/* ... */
override func viewDidLoad() {
super.viewDidLoad()
textView.addObserver(self, forKeyPath: #keyPath(UITextView.contentSize), options: [NSKeyValueObservingOptions.new], context: &myContext)
}
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
if context == &myContext,
keyPath == #keyPath(UITextView.contentSize),
let contentSize = change?[NSKeyValueChangeKey.newKey] as? CGSize {
print("contentSize:", contentSize)
}
}
deinit {
textView.removeObserver(self, forKeyPath: #keyPath(UITextView.contentSize))
}
}
Note that with Swift 4, as an alternative to addObserver(_:, forKeyPath:, options:, context:) and observeValue(forKeyPath:, of:, change:, context:), you can use observe(_:, options:, changeHandler:) in order to track your UIScrollView subclass contentSize property changes:
import UIKit
class ViewController: UIViewController {
#IBOutlet weak var textView: UITextView!
var observer: NSKeyValueObservation?
/* ... */
override func viewDidLoad() {
super.viewDidLoad()
let handler = { (textView: UITextView, change: NSKeyValueObservedChange<CGSize>) in
if let contentSize = change.newValue {
print("contentSize:", contentSize)
}
}
observer = textView.observe(\UITextView.contentSize, options: [NSKeyValueObservingOptions.new], changeHandler: handler)
}
}

Answer 2

Are you on iOS? Because I am, I did the same thing and arrived at the same question; why NSSize? Maybe that's just the xcode terminal playing a trick on us.
Anyway, you can cast it to an NSValue then you will be able to use CGSizeValue:
if let zeChange = change as? [NSString: NSValue] {
let oldSize = zeChange[NSKeyValueChangeOldKey]?.CGSizeValue()
let newSize = zeChange[NSKeyValueChangeNewKey]?.CGSizeValue()
}

Answer 3

There's a simpler and arguably swiftier alternative.
You can subclass UICollectionViewLayout (or any of its subclasses, like UICollectionViewFlowLayout) and override a computed property collectionViewContentSize. By calling super you'll get the contentSize of your collection and be able to delegate this value back to your code.
So you'll have something like this:
protocol FlowLayoutDelegate: class {
func collectionView(_ collectionView: UICollectionView?, didChange contentSize: CGSize)
}
class FlowLayout: UICollectionViewFlowLayout {
weak var delegate: FlowLayoutDelegate?
override var collectionViewContentSize: CGSize {
let contentSize = super.collectionViewContentSize
delegate?.collectionView(collectionView, didChange: contentSize)
return contentSize
}
}

Answer 4

Check out this example code:
if context == ApprovalObservingContext{
if let theChange = change as? [NSString: Bool]{
var newContentSize = change[NSKeyValueChangeNewKey]?.CGSizeValue()
}
}
This is not giving any error.