xcode - swift 使用 KVO 观察 contentSize (CGSize)

Question

我正在尝试像这样观察 collectionView.contentSize :

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)               
        }
    }
}

在 xcode 终端中，我得到的是 NSSize 而不是 CGSize ，如下所示:

Optional(["old": NSSize: {320, 0}, "new": NSSize: {375, 39.5}, "kind": 1])

在 objective-c 中我使用了方法 CGSizeValue

CGSize newContentSize = [[change objectForKey:NSKeyValueChangeNewKey] CGSizeValue];

swift中有没有类似CGSizeValue的方法

我在 swift 中尝试过 var newContentSize = change[NSKeyValueChangeNewKey]?.CGSizeValue() 但出现错误

could not find member 'CGSizeValue'

需要帮助吗？谢谢

Answer 1

使用 Swift 4，您可以将键 NSKeyValueChangeKey.newKey 的 change 字典的结果转换为 CGSize 类型:

if let size = change?[NSKeyValueChangeKey.newKey] as? CGSize {
    /* ... */
}

---

下面的 UIViewController 实现展示了如何设置一个 KVO 堆栈，以便观察任何 UIScrollView 子类的 contentSize 属性的变化(例如 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))
    }

}

---

请注意，在 Swift 4 中，作为 addObserver(_:, forKeyPath:, options:, context:) 和 observeValue(forKeyPath:, of:, change:, context :)，您可以使用 observe(_:, options:, changeHandler:) 来跟踪您的 UIScrollView 子类 contentSize属性变化:

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)
    }

}