La funcionalidad de deshacer y rehacer NO funciona con el siguiente código. A continuación se muestra la implementación de mi código de dibujo.

¿Hay algún problema en el siguiente código?

public class DrawingArea extends View {

    private Path drawPath;
    private Paint drawPaint, canvasPaint;
    private int paintColor = 0xFF660000;
    private Canvas drawCanvas;
    private Bitmap canvasBitmap;
    private ArrayList paths = new ArrayList();
    private ArrayList undonePaths = new ArrayList();
    private float mX, mY;
    private static final float TOUCH_TOLERANCE = 4;

    // for Undo, Redo
    private int historyPointer = 0;

    private boolean erase = false;

    public DrawingArea(Context context) {
        super(context);
        setupDrawing();
    }

    public DrawingArea(Context context, AttributeSet attrs) {
        super(context, attrs);
        setupDrawing();
    }

    public void setupDrawing() {
        drawPath = new Path();
        drawPaint = new Paint();
        drawPaint.setAntiAlias(true);
        drawPaint.setStrokeWidth(20);
        drawPaint.setStyle(Paint.Style.STROKE);
        drawPaint.setStrokeJoin(Paint.Join.ROUND);
        drawPaint.setStrokeCap(Paint.Cap.ROUND);
        canvasPaint = new Paint(Paint.DITHER_FLAG);

        //paths.add(drawPath);
    }

    public void setColor(String newColor) {
        invalidate();
        paintColor = Color.parseColor(newColor);
        drawPaint.setColor(paintColor);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawBitmap(canvasBitmap, 0, 0, canvasPaint);
        canvas.drawPath(drawPath, drawPaint);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        canvasBitmap = Bitmap.createBitmap(w, h, Bitmap.Config.ARGB_8888);
        drawCanvas = new Canvas(canvasBitmap);
    }

    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                touch_start(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_MOVE:
                touch_move(x, y);
                invalidate();
                break;
            case MotionEvent.ACTION_UP:
                touch_up();
                invalidate();
                break;
        }
        return true;
    }

    private void touch_start(float x, float y) {
        drawPath.reset();
        drawPath.moveTo(x, y);
        mX = x;
        mY = y;
    }

    private void touch_move(float x, float y) {
        float dx = Math.abs(x - mX);
        float dy = Math.abs(y - mY);
        if (dx >= TOUCH_TOLERANCE || dy >= TOUCH_TOLERANCE) {
            drawPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
            mX = x;
            mY = y;
        }
    }

    private void touch_up() {
        drawPath.lineTo(mX, mY);
        // commit the path to our offscreen
        drawCanvas.drawPath(drawPath, drawPaint);
        // kill this so we don't double draw
        drawPath = new Path();
        paths.add(drawPath);
    }

    public void setErase() {
        canvasBitmap.eraseColor(Color.TRANSPARENT);
        drawPath.reset();
        invalidate();
    }

    public void startNew() {
        drawCanvas.drawColor(0, PorterDuff.Mode.CLEAR);
        invalidate();
    }

    public void undo() {
        if (paths.size() > 0) {
            undonePaths.add(paths.remove(paths.size() - 1));
            invalidate();
        }
    }

    public void redo() {
        if (undonePaths.size() > 0) {
            paths.add(undonePaths.remove(undonePaths.size() - 1));
            invalidate();
        }
    }
}

respuesta

Cambia tu OnDraw() así

protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);
    canvas.drawBitmap(mBitmap, 0, 0, canvasPaint);
    canvas.drawPath(mPath,drawPaint);

    for (Path p : paths) {
        canvas.drawPath(p, drawPaint);
    }



}

y no olvide agregar el tipo de retención de datos por rutas lista de matriz

ArrayList<Path> paths = new ArrayList<Path>();