iOS: return value from NSNotificationCenter in an extension (swift)

Question

In my extension I put the control for the keyboard notification in this way

protocol KeyboardSpaceProtocol {
    func addObservers()
    func removeObservers()
    func keyboardWillShow(notification:NSNotification) -> CGFloat
}

extension UIView: KeyboardSpaceProtocol {

    func addObservers() {
        NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(keyboardWillShow), name: UIKeyboardWillShowNotification, object: nil)
    }

    func removeObservers() {
        NSNotificationCenter.defaultCenter().removeObserver(self)
    }

    func keyboardWillShow(notification:NSNotification) -> CGFloat {
        let userInfo:NSDictionary = notification.userInfo!
        let keyboardFrame:NSValue = userInfo.valueForKey(UIKeyboardFrameEndUserInfoKey) as! NSValue
        let keyboardRectangle = keyboardFrame.CGRectValue()
        return keyboardRectangle.height
    }
}

now I want update a CGFloat value (keyboardheight) in my UIView class... how can I in my view controller? I don't know when the keyboard go out and how to update my value. Any suggestions?

Answer 1

You can use objc_associatedObject to add properties at run time.

My example uses UIViewController instead of UIView just because it seems more logical to me.

import Foundation
import UIKit
import ObjectiveC

protocol KeyboardSpaceProtocol {
    func addObservers()
    func removeObservers()
    func keyboardWillShow(notification:NSNotification) -> CGFloat
    var keyBoardHeight: CGFloat {get set}
}

private var keyBoardKey: UInt8 = 0
extension  UIViewController: KeyboardSpaceProtocol {

    var keyBoardHeight: CGFloat {
        get {
            return objc_getAssociatedObject(self, &keyBoardKey) as! CGFloat
        }
        set { objc_setAssociatedObject(self, &keyBoardKey, newValue,  objc_AssociationPolicy(rawValue: 0)!) }
    }

    func addObservers() {
        NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(UIViewController.keyboardWillShow(_:)), name: UIKeyboardWillShowNotification, object: nil)
    }

    func removeObservers() {
        NSNotificationCenter.defaultCenter().removeObserver(self)
    }

    func keyboardWillShow(notification:NSNotification) -> CGFloat {
        let userInfo:NSDictionary = notification.userInfo!
        let keyboardFrame:NSValue = userInfo.valueForKey(UIKeyboardFrameEndUserInfoKey) as! NSValue
        let keyboardRectangle = keyboardFrame.CGRectValue()

        var selfAsProtocol = self as KeyboardSpaceProtocol
        selfAsProtocol.keyBoardHeight = keyboardRectangle.height

        return keyboardRectangle.height
    }
}