going to next page after solving the puzzle about flutter_puzzle HOT 1 CLOSED

dependencies:
  flutter:
    sdk: flutter

  # The following adds the Cupertino Icons font to your application.
  # Use with the CupertinoIcons class for iOS style icons.
  cupertino_icons: ^0.1.2
  image_picker: ^0.4.10
  shared_preferences: ^0.4.3

import 'package:shared_preferences/shared_preferences.dart';
...
_incrementCounter(BuildContext context) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    int counter = 0;
    setState(() {
      counter = prefs.getInt("counter");
    });
    
    await prefs.setInt("counter", (counter == null) ? 1 : counter + 1);

    if (prefs.getInt("counter") == 9) {
      _gotoNextPage(context);
    }
    else {
    }
  }

_gotoNextPage(BuildContext context) {
    Navigator.push(context, MaterialPageRoute(builder: (BuildContext context) {
      return Next();
    }));
  }

@override
  Widget build(BuildContext context) {
    final imageWidth = MediaQuery.of(context).size.width;
    final imageHeight = MediaQuery.of(context).size.height *
        MediaQuery.of(context).size.width /
        widget.imageSize.width;
    final pieceWidth = imageWidth / widget.maxCol;
    final pieceHeight = imageHeight / widget.maxRow;

    if (top == null) {
      top = Random().nextInt((imageHeight - pieceHeight).ceil()).toDouble();
      top -= widget.row * pieceHeight;
    }
    if (left == null) {
      left = Random().nextInt((imageWidth - pieceWidth).ceil()).toDouble();
      left -= widget.col * pieceWidth;
    }

    return Positioned(
      top: top,
      left: left,
      width: imageWidth,
      child: GestureDetector(
        onTap: () {
          if (isMovable) {
            widget.bringToTop(widget);
            print("$_Tag inside GestureDetector 1");
          }
        },
        onPanStart: (_) {
          if (isMovable) {
            widget.bringToTop(widget);
            print("$_Tag inside GestureDetector 11");
          }
        },
        onPanUpdate: (dragUpdateDetails) {
          if (isMovable) {
            setState(() {
              top += dragUpdateDetails.delta.dy;
              left += dragUpdateDetails.delta.dx;

              if (-10 < top && top < 10 && -10 < left && left < 10) {
                top = 0;
                left = 0;
                isMovable = false;
                widget.sendToBack(widget);

                print("$_Tag inside GestureDetector 111");
              }
            });
          }
        },
        child: GestureDetector(
          onTap: () {
            if (isMovable) {
              widget.bringToTop(widget);
              print("$_Tag inside GestureDetector 2");
            }
          },
          onPanStart: (_) {
            if (isMovable) {
              widget.bringToTop(widget);
              print("$_Tag inside GestureDetector 22");
            }
          },
          onPanUpdate: (dragUpdateDetails) {
            if (isMovable) {
              setState(() {
                top += dragUpdateDetails.delta.dy;
                left += dragUpdateDetails.delta.dx;

                if (-10 < top && top < 10 && -10 < left && left < 10) {
                  top = 0;
                  left = 0;
                  isMovable = false;
                  widget.sendToBack(widget);
                  _incrementCounter(context);   // increment the counter here
                  print("$_Tag inside GestureDetector 222");
                }
              });
            }
          },
          child: ClipPath(
            child: widget.image,
            clipper: PuzzlePieceClipper(
                widget.row, widget.col, widget.maxRow, widget.maxCol),
          ),
        ),
      ),
    );
  }