android：画图板的撤销和恢复功能无法正常运行，android正常运行

android：画图板的撤销和恢复功能无法正常运行，android正常运行

android 画图板 撤销 恢复 public class MyView extends View{

    public static Paint paint = null;
    public static Canvas cacheCanvas = null;
    private Path path;
    Bitmap cacheBitmap = null;

    public static int PEN_WIDTH = 10;
    public static int PAINT_STATUS = 0;
    private static int m_strokeColor = Color.RED;

    float preX;
    float preY;
    class DrawPath{
        Path path;
        Paint paint;
    }
    private static ArrayList<DrawPath> savePath;
    private static ArrayList<DrawPath> deletePath;
    private DrawPath dp;

    public SmartpadView(Context context,AttributeSet attrs) {
        super(context,attrs);
        initCanvas();
        initPaint();
        savePath = new ArrayList<DrawPath>();
        deletePath = new ArrayList<DrawPath>();
    }
    public void initCanvas() {
        cacheBitmap = Bitmap.createBitmap(3000,2000,Bitmap.Config.ARGB_8888);
        cacheCanvas = new Canvas(cacheBitmap);
        path = new Path();
        cacheCanvas.setBitmap(cacheBitmap);
    }
    public void initPaint(){
        paint = new Paint(Paint.DITHER_FLAG);
        paint.setColor(Color.RED);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(PEN_WIDTH);
        paint.setAntiAlias(true);
        paint.setDither(true);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeCap(Paint.Cap.ROUND);
    }
    @Override
    public void onDraw(Canvas canvas) {
        Paint bmpPaint = new Paint();
        canvas.drawBitmap(cacheBitmap, 0, 0, bmpPaint); // ②
        canvas.drawPath(path, paint);
    }
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float x = event.getX();
        float y = event.getY();
        switch (event.getAction())
        {
            case MotionEvent.ACTION_DOWN:
                path.moveTo(x, y);
                preX = x;
                preY = y;
                break;
            case MotionEvent.ACTION_MOVE:
                path.quadTo(preX, preY, x, y);
                preX = x;
                preY = y;
                break;
            case MotionEvent.ACTION_UP:
                dp = new DrawPath();
                dp.path = path;
                dp.paint = paint;
                savePath.add(dp);
                cacheCanvas.drawPath(path, paint); // ①
                path.reset();
                break;
        }
        invalidate();
        return true;
    }

    public void undo(){
        if(savePath!=null&&savePath.size()>0){
            initCanvas();

            DrawPath temp=savePath.get(savePath.size()-1);
            deletePath.add(temp);
            savePath.remove(savePath.size()-1);

            for(int i=0;i<savePath.size();i++){
                System.out.println("撤销0");
                DrawPath dp = savePath.get(i);
                cacheCanvas.drawPath(dp.path, dp.paint);
            }
            invalidate();// 刷新
        }
    }
    public void redo(){
        if(deletePath!=null&&deletePath.size()>0){
            DrawPath temp=deletePath.get(deletePath.size()-1);
            savePath.add(temp);
            deletePath.remove(deletePath.size()-1);

            cacheCanvas.drawPath(temp.path, temp.paint);
            invalidate();
        }
    }
}
無法正常運行是甚麼情況, 運行時錯誤