MainActivity.java

package org.saintandreas.audiotest;

import android.Manifest;
import android.app.Activity;
import android.content.pm.PackageManager;
import android.media.MediaRecorder;
import android.opengl.GLES32;
import android.opengl.GLSurfaceView;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.util.Log;

import java.io.IOException;
import java.util.Timer;
import java.util.TimerTask;

import javax.microedition.khronos.egl.EGLConfig;
import javax.microedition.khronos.opengles.GL10;

public class MainActivity extends Activity implements GLSurfaceView.Renderer {
    private final String TAG = MainActivity.class.getName();
    private boolean permissionToRecordAccepted = false;
    private String [] permissions = {Manifest.permission.RECORD_AUDIO};
    private static final int REQUEST_RECORD_AUDIO_PERMISSION = 200;
    private float maxAudio = 1.0f;
    private float lastAudio = 0.0f;
    private GLSurfaceView mGLView;
    private MediaRecorder mRecorder = null;

    //private native void nativeOnCreate();

    @Override
    public void onSurfaceCreated(GL10 unused, EGLConfig config) {
    }

    @Override
    public void onDrawFrame(GL10 unused) {
        float audioLevel = lastAudio / maxAudio;
        GLES32.glClearColor(audioLevel, audioLevel > 0.2f ? 1.0f :0.0f, 0.0f, 1.0f);
        GLES32.glClear(GLES32.GL_COLOR_BUFFER_BIT);
    }

    @Override
    public void onSurfaceChanged(GL10 unused, int width, int height) {
        GLES32.glViewport(0, 0, width, height);
    }


    @Override
    public void onCreate(Bundle savedInstanceState) {
        System.loadLibrary("audioTest");
        super.onCreate(savedInstanceState);
        requestPermissions(permissions, REQUEST_RECORD_AUDIO_PERMISSION);
        mGLView = new GLSurfaceView(this);
        mGLView.setEGLContextClientVersion(3);
        mGLView.setRenderer(MainActivity.this);
        setContentView(mGLView);
        //nativeOnCreate();
    }

    public void monitorAudio() {
        if (mRecorder != null) {
            lastAudio = mRecorder.getMaxAmplitude();
            maxAudio = Math.max(lastAudio, maxAudio);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
        switch (requestCode){
            case REQUEST_RECORD_AUDIO_PERMISSION:
                permissionToRecordAccepted  = grantResults[0] == PackageManager.PERMISSION_GRANTED;
                break;
        }
        if (!permissionToRecordAccepted ) {
            finish();
        } else {
            new Timer().schedule(new TimerTask() { @Override public void run() { startRecording(); }}, 1000);
        }
    }

    Timer mMonitorTimer = null;

    private void startRecording() {
        mRecorder = new MediaRecorder();
        mRecorder.setAudioSource(MediaRecorder.AudioSource.VOICE_COMMUNICATION);
        mRecorder.setOutputFormat(MediaRecorder.OutputFormat.THREE_GPP);
        mRecorder.setAudioEncoder(MediaRecorder.AudioEncoder.AMR_NB);
        mRecorder.setOutputFile("/dev/null");
        try {
            mRecorder.prepare();
        } catch (IOException e) {
            Log.e(TAG, "prepare() failed");
        }

        mRecorder.start();
        mMonitorTimer = new Timer();
        mMonitorTimer.scheduleAtFixedRate(new TimerTask() { @Override public void run(){ monitorAudio(); } }, 100, 100);
    }

    @Override
    public void onStop() {
        super.onStop();
        if (mRecorder != null) {
            mMonitorTimer.cancel();
            mMonitorTimer = null;
            mRecorder.release();
            mRecorder = null;
        }
    }
}