Xamarin, iOS Get Instance of viewcontroller from tabbarcontroller

Question

I'm in need of some help. I'm using Xamarin IOS in Visual Studio 2017 on Windows 10. I'm using storyboard and I have a tab bar controller with 2 view controllers. My view controllers are A_ViewController and B_ViewController. What I would like to do is click a button in A_ViewController that calls an API function in a background thread. The function periodically return some string that I would like to have displayed in a TextView in B_ViewController.

I have the API callbacks working and can recieve the return strings but I'm not sure how I can get the instance of B_ViewController to update the TextView and display the API return strings.

I know that the tabbarcontroller has a property that contains the instances of all its view controllers because I can change view controllers by calling "this.TabBarController.SelectedIndex = 1;" but from A_ViewController how do I use tabbarcontroller (or any other way) to get B_ViewController's instance?

I've tried the following from A_ViewController:

    B_ViewController vc = this.TabBarController.ViewControllers[1] as B_ViewController;
    vc.Output_Update("Hello World");  <--- This code creates an exception.

My 2 ViewControllers look something like this:

public partial class A_ViewController : UIViewController
{
    public override void ViewDidLoad(){ 
        base.ViewDidLoad();
        mbtnCallAPI.TouchUpInside += BtnCallAPI_TouchUpInside;
    }

    public void BtnCallAPI_TouchUpInside(object sender, EventArgs e) {
        // Call API here.  API Callback will return strings
        // Send return string to B_ViewController's TextView.
    }
}



public partial class B_ViewController : UIViewController
{   
    public void Output_Update(string msg) {
        mTextView.Text += msg;  
    }
}

Answer 1

I assume that mTextView wasn't created yet. If you now try to set the text it will throw a NullReferenceException.

In my opinion it is not a good way to make the controller depending on each other. For example A_ViewController shouldn't know about B_ViewController. Instead persist the response-strings somewhere and update the text view in B_Controller when it will be visible.