Undo/Redo not working in android Canvas

Question

Undo and redo functionality NOT working with below code. Below is the my drawing code implementation.

Is there any problem in below code?

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();
 }
 }
}

Answer 1

Change your OnDraw() like this

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);
 }
}

and don't forget to add type of data hold by paths arraylist

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

• Pnemonic: But without drawing the bitmap. The paths are being drawn onto the bitmap, so any "undo"s do no affect the underlying bitmap.