Usage of NSTabView.DidSelect event

johannes_schmitt · July 2018

Im using NSTabViewController to create a preferences window. In order to execute some code when the active tab changes, I would like to use NSTabView's DidChange event (as also suggested in Xamarin Forums discussion/comment/118342/#Comment_118342 - unfortunately I can't post links yet).

This is what I have:

var tabViewController = new NSTabViewController();
tabViewController.TabView.DidSelect += (sender, e) => // exception thrown here
   {
      // do stuff here
   };

Unfortunately, an InvalidOperationException is thrown when attaching the event handler:

Event registration is overwriting existing delegate. Either just use events or your own delegate: AppKit.NSTabViewController AppKit.NSTabView+_NSTabViewDelegate

I am actually only using events and not using my own delegate (contrary what the exception message is saying). Or do I misinterpret this?

Anyway, I checked the (generated) source code for NSTabView (/Library/Frameworks/Xamarin.Mac.framework/Versions/4.4.1.193/src/Xamarin.Mac/AppKit/NSTabView.g.cs). This is what I understand (and I might be wrong...): In order to use the DidSelect event, the Delegate property should be of (internal) type _NSTabViewDelegate. I also saw that if I set Delegate to null before using DidSelect, an instance of _NSTabViewDelegate is created and everything should be okay, so I came up with this:

var tabViewController = new NSTabViewController();
tabViewController.TabView = null; // silently crashing here
tabViewController.TabView.DidSelect += (sender, e) =>
   {
      // do stuff here
   };

Unfortunately, the app silently crashes without any exception when assigning null.

Can someone explain how DidSelect event should/can actually be used?
Thanks in advance!

YuriKuznetsov · July 2018

you can create your own delegate something like that

public class MyDelegate : NSTabViewDelegate {

    public event Action DidSelected;  

    public override void DidSelect(NSTabView tabView, NSTabViewItem item)
    {
        DidSelected?.Invoke();
    }
}

and you need to initialize TabView

var dlg = new MyDelegate();
dlg.DidSelected += () => {
        //you code here        
    };

var tabViewController = new NSTabViewController();
tabViewController.TabView.Delegate = dlg;

johannes_schmitt · July 2018

Hi @YuriKuznetsov , thanks for sharing your the idea.
Unfortunately, the app is crashing when I assign the delegate:
tabViewController.TabView.Delegate = dlg;

The unhandled exception is an NSInternalInconsistencyException:

A TabView managed by a TabViewController cannot have its delegate modified

JGoldberger · July 2018

@johannes_schmitt

Did you try just using a NSTabView instead of the NSTabViewController?

If you want to use the NSTabViewController, I would recommend subclassing it and then overriding the DidSelect method.

A simple sample is attached.

Xamarin

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Usage of NSTabView.DidSelect event

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