为什么剪贴板UIPasteboardChanged通知被触发两次？

Question

我订阅了剪贴板文字更改通知。每次在我的应用程序中将某些内容复制到剪贴板中时，它都会触发该事件两次。换句话说，onCopy（）被调用两次。 （xCode 8.1; iOS 9,10）

import Foundation

protocol Clipboard {
    func onCopy()
}

class SecureClipboard : NSObject, Clipboard {
    static let sharedReader = SecureClipboard()
    private var clipboardContent: String?
    private var clipboardBeingCleared: Bool = false

    /// Lifycycle

    private override init() {
        super.init()
        subscribForClipboardChanges()
    }

    private func subscribForClipboardChanges() {
        NotificationCenter.default.addObserver(self,
                                               selector: #selector(onCopy),
                                               name:NSNotification.Name.UIPasteboardChanged,
                                               object: nil)
    }

    /// Clipboard

    func onCopy() {
        // Called twice
    }
}