Implement KVO pattern in UIButton-SWIFT

Question

I am trying to observe the change in UIButton title using KVO pattern.Added observer in viewDidLoad.

 @IBOutlet weak var KVOBTn: UIButton!

  override func viewDidLoad() {

    super.viewDidLoad()
    KVOBTn.titleLabel!.addObserver(self, forKeyPath: "btntest", options: NSKeyValueObservingOptions.New | NSKeyValueObservingOptions.Old , context: nil)

  }

This is the method that listens if there is any change in the title

 override func observeValueForKeyPath(keyPath: String, ofObject object: AnyObject, change: [NSObject : AnyObject], context: UnsafeMutablePointer<Void>) {
     if keyPath == "btntest"{

      KVOBTn.backgroundColor = UIColor.greenColor()

  }

 }

I have changed the button title through another button action

  @IBAction func changeTitle(sender: AnyObject) {

     KVOBTn.setTitle("testAgain", forState: UIControlState.Normal)

  }

The thing is the observeValueForKeyPath method is never being called.What am i doing wrong?

It would be better if you explain more about `KVOBTn`. – Wonjung Kim Oct 30 '15 at 05:24 — Wonjung Kim, Oct 30 '15 at 05:24

Johnykutty · Accepted Answer · 2015-10-30T08:45:50.300

0

Your add observer code is not proper, it should be like

 KVOBTn.titleLabel!.addObserver(self, forKeyPath: "text", options: [.New, .Old] , context: nil)

Note that the key path is "text" not "btntest". UILabel dent have a key path "btntest"

Also don't forget to chage the check in observeValueForKeyPath method

UPDATE

This forKeyPath: "text" really mater. This means you are observing the change for property text of the buttons title label. If you want to observe change in the text color of the label, the key path should be textColor

UPDATE 2

I am not recommending to use KVO with UI kit elements. KVO usually use for observing changes in model objects. Please don't misinterrupt.

edited Oct 30 '15 at 08:45

answered Oct 30 '15 at 06:01

Johnykutty

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yes that was the error but why the keypath text does that really matter?? – Oct 30 '15 at 06:17
UIButton is not KVO compliant for the key "title". – Nikolai Ruhe Oct 30 '15 at 06:57
in the question he asked about a problem and he is adding observer for UIButton.titleLabel not for UIButton to key path title – Johnykutty Oct 30 '15 at 07:02
@Johnykutty Yes, but your answer only superficially fixes the problem. It's still bad advice to use KVO on a property which is not KVO compliant. – Nikolai Ruhe Oct 30 '15 at 08:13
I agree, but I am not advising or recommending to use KVO for UI elements. Just pointing out the issue with his code. – Johnykutty Oct 30 '15 at 08:46

score 0 · Answer 2 · edited May 23 '17 at 12:32

0

UIButton is not KVO compliant for the key "title".

None of the Apple provided framework objects is supposed to be used with KVO, unless documented otherwise (see here, here or here). It might work in some cases, but it's fragile and bad code.

You have to use other means to react on label changes.

edited May 23 '17 at 12:32

Community

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answered Oct 30 '15 at 07:00

Nikolai Ruhe

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that was for test purpose and it worked fine after if changed the keypath to "text" as i was observing the button title... – Oct 30 '15 at 08:17
@swiftBUTCHER No harm done if KVO is used like this for debugging. Just make sure the code is removed before going to production. – Nikolai Ruhe Oct 30 '15 at 08:21
ok..that was just for self learning never ever used KVO :)..but can you please tell me when to use these NSKeyValueObservingOptions.New or Old – Oct 30 '15 at 08:24

Implement KVO pattern in UIButton-SWIFT

2 Answers2