Associated object

gistfile1.txt

import ObjectiveC
import UIKit

var associatedObjectHandle: UInt8 = 0

class TextDidChangeHandler: NSObject {
    unowned let textField: UITextField
    let handler: (textField: UITextField) -> Void
    init(textField: UITextField, handler: (textField: UITextField) -> Void) {
        self.handler = handler
        self.textField = textField
        
        super.init()
        
        NSNotificationCenter.defaultCenter().addObserver(self, selector: "textDidChange:", name: UITextFieldTextDidChangeNotification, object: self.textField)
    }
    
    func textDidChange(notification: NSNotification) {
        handler(textField: self.textField)
    }
    
    deinit {
        NSNotificationCenter.defaultCenter().removeObserver(self)
    }
}

extension UITextField {
    public var textDidChange: ((textField: UITextField) -> Void)? {
        get {
            let object = objc_getAssociatedObject(self, &associatedObjectHandle) as? TextDidChangeHandler
            return object?.handler
        }
        set {
            if let handler = newValue {
                let object = TextDidChangeHandler(textField: self, handler: handler)
                objc_setAssociatedObject(self, &associatedObjectHandle, object, objc_AssociationPolicy(OBJC_ASSOCIATION_RETAIN_NONATOMIC))
            } else {
                objc_removeAssociatedObjects(self)
            }
        }
    }
}