程序先将绘制的内容绘制到Bitmap图片缓冲区上，再将Bitmap绘制到View上，防止此前绘制内容的消失
绘制工具自定义View类

public class DrawView extends View
{
    float preX;
    float preY;
    private Path path;
    public Paint paint = null;
    final int VIEW_WIDTH = 960;
    final int VIEW_HEIGHT = 1440;
    // 定义一个内存中的图片，该图片将作为缓冲区
    Bitmap cacheBitmap = null;
    // 定义cacheBitmap上的Canvas对象
    Canvas cacheCanvas = null;
    public DrawView(Context context, AttributeSet set)
    {
        super(context, set);
        // 创建一个与该View相同大小的缓存区
        cacheBitmap = Bitmap.createBitmap(VIEW_WIDTH
            , VIEW_HEIGHT , Config.ARGB_8888);
        cacheCanvas = new Canvas();
        path = new Path();
        // 设置cacheCanvas将会绘制到内存中的cacheBitmap上
        cacheCanvas.setBitmap(cacheBitmap);
        //设置画笔的颜色
        paint = new Paint(Paint.DITHER_FLAG);
        paint.setColor(Color.RED);
        //设置画笔风格
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeWidth(1);
        //反锯齿
        paint.setAntiAlias(true);
        paint.setDither(true);  
    }
    @Override
    public boolean onTouchEvent(MotionEvent event)
    {
        //获取拖动事件的发生位置
        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:
                //将路径绘制到画布上
                cacheCanvas.drawPath(path, paint);     
                path.reset();
                break;
        }
        //通知View组件重绘
        invalidate();
        // 返回true表明处理方法已经处理该事件
        return true;
    }   
    @Override
    public void onDraw(Canvas canvas)
    {
        Paint bmpPaint = new Paint();
        // 将cacheBitmap绘制到该View组件上
        canvas.drawBitmap(cacheBitmap , 0 , 0 , bmpPaint);   
        // 沿着path绘制
        canvas.drawPath(path, paint);
    }
}

在xml文件中布置改类后，在activity中打开该xml文件即可实现画图板效果