gsm - Answer incoming call using android.telecom and InCallService

Question

Since API 21, Google has been adding features to android.telecom in general, especially by implementing more members of TelecomManager and the addition of InCallService. This last one is is supposed to allow non-system, 3rd-party apps to provide and replace the functionality of the system Calls app in-call screen - the Window that pops up and allows action on EXTRA_STATE_OFFHOOK or EXTRA_STATE_RINGING broadcasts (i.e. incoming and outgoing phone calls).

Currently, only this screen has full control of ringing and active calls and associated system callbacks with fine-grained information, by means of the root-restricted MODIFY_PHONE_STATE permission and a lot of secured AOSP code not even accessible by reflection. It's notably one of the most changed pieces of code in different manufacturers' ROM flavours, together with the launcher, contacts and camera.

This is all very pretty but...

How do you actually develop a 3rd-party InCallService?

Namely:

1. How do you get notified about, and acquire instances of GSM Calls
2. How does one answer these calls
3. What is the life-cycle of the callbacks on this class
4. Does Google provide any actual tutorial for this that I haven't found

I won't ask answers for all of these at once, but any one answer probably associates to the other questions. This is broad but intrinsically it needs to be: there's no example on the web I've stumbled upon other than AOSP-code, and that code is based on the assumption of root-privileges, which makes it unusable for 3rd-party app development purposes.

Answer 1

How do you get notified about, and acquire instances of GSM Calls

First, the user will need to select your app as the default Phone app. Refer to Replacing default Phone app on Android 6 and 7 with InCallService for a way to do that.

You also need to define an InCallService implementation the system will bind to and notify you about the call:

<service
    android:name=".CallService"
    android:permission="android.permission.BIND_INCALL_SERVICE">
    <meta-data
        android:name="android.telecom.IN_CALL_SERVICE_UI"
        android:value="true" />
    <intent-filter>
        <action android:name="android.telecom.InCallService" />
    </intent-filter>
</service>

There you should handle at least onCallAdded (set up listeners on Call, start your UI - activity - for the call) and onCallRemoved (remove listeners).

How does one answer these calls

If the user wants to answer the call, you need to invoke the method Call#answer(int) with VideoProfile.STATE_AUDIO_ONLY for example.

What is the life-cycle of the callbacks on this class

Check out Call.Callback for events that can happen with a single call.

Does Google provide any actual tutorial for this that I haven't found

I don't know about Google, but you can check out my simplified example https://github.com/arekolek/simple-phone