问我想知道如何用GestureDetector来检测手指数

EN

 GestureDetector(
  onScaleStart: (details) {
    Tool tool = context.read<DrawProvider>().tool;
    double seletedPenWidth = context.read<DrawProvider>().seletedPenWidth;
    Color seletedPenColor = context.read<DrawProvider>().seletedPenColor;

    RenderBox box = context.findRenderObject() as RenderBox;

    // zoom test
    context.read<DrawProvider>().onScaleStart(details);

    // Use Pen
    if (tool == Tool.pen) {
      Offset point = box.globalToLocal(details.focalPoint);
      point = Offset(point.dx, point.dy);

      currentLine = DrawingModel(
        pointList: [point],
        color: seletedPenColor,
        width: seletedPenWidth,
      );

    } else {
      // TODO Other Tool
    }
  },

  onScaleUpdate: (details) {
    Tool tool = context.read<DrawProvider>().tool;
    double seletedPenWidth = context.read<DrawProvider>().seletedPenWidth;
    Color seletedPenColor = context.read<DrawProvider>().seletedPenColor;

    RenderBox box = context.findRenderObject() as RenderBox;

    // zoom test
    context.read<DrawProvider>().onScaleUpdate(details);

    if (tool == Tool.pen) {
      Offset point =
          box.globalToLocal(details.focalPoint);
      point = Offset(point.dx, point.dy);

      List<Offset> path = List.from(currentLine!.pointList!)..add(point);

      currentLine = DrawingModel(
        pointList: path,
        color: seletedPenColor,
        width: seletedPenWidth,
      );

      currentLineStreamController.add(currentLine!);
    }
  },

  onScaleEnd: (details) {
    Tool tool = context.read<DrawProvider>().tool;

    // zoom test
    context.read<DrawProvider>().onScaleEnd(details);

    if (tool == Tool.pen) {
      allLines = List.from(allLines)..add(currentLine!);
      linesStreamController.add(allLines);
    }
}

  Offset _offset = Offset.zero;
  Offset _initialFocalPoint = Offset.zero;
  Offset _sessionOffset = Offset.zero; 
  double _scale = 1.0;
  double _initialScale = 1.0;

 void onScaleStart(ScaleStartDetails details) {
    // TODO if use move tool
    // _initialFocalPoint = details.focalPoint;
    _initialScale = _scale;
    notifyListeners();
  }

  void onScaleUpdate(ScaleUpdateDetails details) {
    // TODO if use move tool
    // _sessionOffset = details.focalPoint - _initialFocalPoint;
    _scale = _initialScale * details.scale;
    notifyListeners();
  }

  void onScaleEnd(ScaleEndDetails details) {
    // TODO if use move tool
    // _offset += _sessionOffset;
    _sessionOffset = Offset.zero;

    notifyListeners();
  }

          Transform.translate(
            offset: _offset + _sessionOffset,
            child: Transform.scale(
              scale: _scale, 
              child: buildAllPaths(allLines: allLines), // drawing screen
            ),
          ),