Getting phone state in Xamarin

Question

I have following code:

public class StateListener : PhoneStateListener
    {
        StateListener phoneStateListener = new StateListener();
        TelephonyManager telephonyManager = (TelephonyManager)GetSystemService(Context.TelephonyService);
        telephonyManager.Listen(phoneStateListender, PhoneStateListenerFlags.CallState);

        public void onCallStateChanged(CallState state, String incomingNumber)
        {
            base.OnCallStateChanged(state, incomingNumber);
            switch (state)
            {
                case CallState.Ringing:
                    break;
                case CallState.Offhook:
                    break;
                case CallState.Idle:
                    break;
            }
        }
    }

This is an inner class because my base class is "MainActivity : Activity". The above code was from Java, so I tried to convert it to C#. However, I get erros in this case:

 (TelephonyManager)GetSystemService(Context.TelephonyService); 

throws an error that field initializers must be static and

 telephonyManager.Listen(phoneStateListender, PhoneStateListenerFlags.CallState);

is not available in this context. I don't even know if the code above would even work. How to achieve in Xamarin that I can get the state of an outgoing call?

Answer 1

You have to move the creation code into the OnCreate method of your Activity.

protected override void OnCreate(Bundle bundle)
{
    base.OnCreate(bundle);

    StateListener phoneStateListener = new StateListener();
    TelephonyManager telephonyManager = (TelephonyManager)GetSystemService(Context.TelephonyService);
    telephonyManager.Listen(phoneStateListener, PhoneStateListenerFlags.CallState);
}

And then you can create the class:

public class StateListener : PhoneStateListener
{
    public override void OnCallStateChanged(CallState state, string incomingNumber)
    {
        base.OnCallStateChanged(state, incomingNumber);
        switch (state)
        {
            case CallState.Ringing:
                break;
            case CallState.Offhook:
                break;
            case CallState.Idle:
                break;
        }
    }
}

If you want to do something on you activity after OnCallStateChanged you have to pass the activity (e.g. in the constructor):

public class StateListener : PhoneStateListener
{
    private readonly MainActivity _activity;

    public StateListener(MainActivity activity)
    {
        _activity = activity;
    }

    public override void OnCallStateChanged(CallState state, string incomingNumber)
    {
        base.OnCallStateChanged(state, incomingNumber);
        _activity.UpdateCallState(state, incomingNumber);
    }
}

public class MainActivity : Activity
{
    protected override void OnCreate(Bundle bundle)
    {
        base.OnCreate(bundle);

        StateListener phoneStateListener = new StateListener(this);
        TelephonyManager telephonyManager = (TelephonyManager)GetSystemService(Context.TelephonyService);
        telephonyManager.Listen(phoneStateListener, PhoneStateListenerFlags.CallState);
    }

    public void UpdateCallState(CallState state, string incomingNumber)
    {
        // numberLabel.Text = ...
    }
}