PianoView for a tutorial on the SSaurel's Channel

PianoView.java

package com.ssaurel.piano;

import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RectF;
import android.os.Handler;
import android.os.Message;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.View;

import java.util.ArrayList;

/**
 * Created by ssaurel on 15/03/2018.
 */
public class PianoView extends View {

  public static final int NB = 14;
  private Paint black, yellow, white;
  private ArrayList<Key> whites = new ArrayList<>();
  private ArrayList<Key> blacks = new ArrayList<>();
  private int keyWidth, height;
  private AudioSoundPlayer soundPlayer;

  public PianoView(Context context, AttributeSet attrs) {
    super(context, attrs);
    black = new Paint();
    black.setColor(Color.BLACK);
    white = new Paint();
    white.setColor(Color.WHITE);
    white.setStyle(Paint.Style.FILL);
    yellow = new Paint();
    yellow.setColor(Color.YELLOW);
    yellow.setStyle(Paint.Style.FILL);
    soundPlayer = new AudioSoundPlayer(context);
  }

  @Override
  protected void onSizeChanged(int w, int h, int oldw, int oldh) {
    super.onSizeChanged(w, h, oldw, oldh);
    keyWidth = w / NB;
    height = h;
    int count = 15;

    for (int i = 0; i < NB; i++) {
      int left = i * keyWidth;
      int right = left + keyWidth;
 
      if (i == NB - 1) {
        right = w;
      }

      RectF rect = new RectF(left, 0, right, h);
      whites.add(new Key(rect, i + 1));

      if (i != 0  &&   i != 3  &&  i != 7  &&  i != 10) {
        rect = new RectF((float) (i - 1) * keyWidth + 0.5f * keyWidth + 0.25f * keyWidth, 0, 
	                   (float) i * keyWidth + 0.25f * keyWidth, 0.67f * height);
        blacks.add(new Key(rect, count));
        count++;
      }
    }
  }

  @Override
  protected void onDraw(Canvas canvas) {
    for (Key k : whites) {
      canvas.drawRect(k.rect, k.down ? yellow : white);
    }

    for (int i = 1; i < NB; i++) {
      canvas.drawLine(i * keyWidth, 0, i * keyWidth, height, black);
    }

    for (Key k : blacks) {
      canvas.drawRect(k.rect, k.down ? yellow : black);
    }
  }

  @Override
  public boolean onTouchEvent(MotionEvent event) {
    int action = event.getAction();
    boolean isDownAction = action == MotionEvent.ACTION_DOWN || action == MotionEvent.ACTION_MOVE;

    for (int touchIndex = 0; touchIndex < event.getPointerCount(); touchIndex++) {
      float x = event.getX(touchIndex);
      float y = event.getY(touchIndex);

      Key k = keyForCoords(x,y);

      if (k != null) {
        k.down = isDownAction;
      }
    }

    ArrayList<Key> tmp = new ArrayList<>(whites);
    tmp.addAll(blacks);

    for (Key k : tmp) {
      if (k.down) {
        if (!soundPlayer.isNotePlaying(k.sound)) {
          soundPlayer.playNote(k.sound);
          invalidate();
        } else {
          releaseKey(k);
        }
      } else {
        soundPlayer.stopNote(k.sound);
        releaseKey(k);
      }
    }

    return true;
  }

  private Key keyForCoords(float x, float y) {
    for (Key k : blacks) {
      if (k.rect.contains(x,y)) {
        return k;
      }
    }

    for (Key k : whites) {
      if (k.rect.contains(x,y)) {
        return k;
      }
    }

    return null;
  }

  private void releaseKey(final Key k) {
    handler.postDelayed(new Runnable() {
      @Override
      public void run() {
        k.down = false;
        handler.sendEmptyMessage(0);
      }
    }, 100);
  }

  private Handler handler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
      invalidate();
    }
  };
}