如何在Android中的绘画应用程序中实现撤消功能？

我想在我的应用程序中执行撤消功能。为此，我进行了搜索，发现采用 x,y 点的数组列表，我在下面的代码中完成了这些操作，但我无法撤消绘图？

public Drawview(Context context, float x, float y, int r) {
            super(context);


            mBitmap = Bitmap.createBitmap(800, 1200, Bitmap.Config.ARGB_8888);
           // mainbitmap.add(mBitmap);
            //System.out.println("**mBitmapPaint***"+mBitmapPaint);
            mCanvas = new Canvas(mBitmap);
            mPath = new Path();
            mBitmapPaint = new Paint(Paint.DITHER_FLAG);



        }

       @Override
       protected void onSizeChanged(int w, int h, int oldw, int oldh) {
           super.onSizeChanged(w, h, oldw, oldh);
       }

        @Override 
        protected void onDraw(Canvas canvas) {

            System.out.println("***Drawinglines****"+Drawinglines);

                canvas.drawBitmap(mBitmap, 0, 0, mBitmapPaint);
                canvas.drawPath(mPath, mPaint);



        }


        private float mX, mY;
        private static final float TOUCH_TOLERANCE = 4;

        private void touch_start(float x, float y) {
            mPath.reset();
            mPath.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) {
                mPath.quadTo(mX, mY, (x + mX)/2, (y + mY)/2);
                mX = x;
                mY = y;
            }
        }

        private void touch_up() {


            mPath.lineTo(mX, mY);
            mCanvas.drawPath(mPath, mPaint);
        ///    path.
            //graphics.add(mPath);
            mPath.reset();
        }

        // events when touching the screen
        public boolean onTouchEvent(MotionEvent event) {

             event.getAction();
            int eventaction = event.getAction(); 

            int X = (int)event.getX(); 
            int Y = (int)event.getY(); 

            switch (eventaction ) { 

            case MotionEvent.ACTION_DOWN: 

            {
                touch_start(X, Y);
                }
                 invalidate(); 
                 break; 


            case MotionEvent.ACTION_MOVE:  

                System.out.println("***ACTION_MOVE**************"+X+"************"+Y);
                if((Drawinglines.equals("Pen"))||(Drawinglines.equals("Erase")))
                {
                  touch_move(X, Y);
                  temp_point.add(new Point(X,Y));

                }
                invalidate(); 
                break; 

            case MotionEvent.ACTION_UP: 

                main_Points.add(temp_point);
                temp_point = null;

                 touch_up();  

                 invalidate(); 
                 break; 
            } 

            return true; 

        }




        public void undopath()
        {   
              main_Points.remove(main_Points.size()-1);
             invalidate();
        }

    }
    @Override
    public void onClick(View v) {


        if(v ==  Undo)
        {    
            Drawview abc=new Drawview();
             abc.undopath();
        }

但撤消撤消功能不起作用。

private Slate mSlate;
private TiledBitmapCanvas mTiledCanvas;

 public void clickUndo(View unused) {

        mSlate.undo();
    }


public void undo() {
        if (mTiledCanvas == null) {
            Log.v(TAG, "undo before mTiledCanvas inited");
        }
        mTiledCanvas.step(-1);

        invalidate();
    }