Switch images with GestureDetector in Flutter

Question

I have an image Carousel with images i get from an API. Nonetheless I want to switch the images with swiping left and right. Current code works, but having an issue when I want to swipe left (back) it takes two swipes to get back on a previous image... So I'm guessing I have to change the distance in onPanUpdate?

  return GestureDetector(
                onPanEnd: (DragEndDetails details) {
                  initial = 0.0;
                  if (initial > distance) {
                    _nextImage();
                    print("next");
                  }
                },
                onPanUpdate: (DragUpdateDetails details) {
                  distance = details.globalPosition.dx - initial;
                },
                onPanStart: (DragStartDetails details) {
                  initial = details.globalPosition.dx;

                  if (initial <= distance) {
                    _previousImage();
                    print("previous");
                  }
                },

Answer 1

You can reference package https://pub.dev/packages/swipedetector or use it directly.

it use onHorizontalDrag to detect and allow Velocity and Displacement
code snippet of swipedetector

      onHorizontalDragStart: (dragDetails) {
        startHorizontalDragDetails = dragDetails;
      },
      onHorizontalDragUpdate: (dragDetails) {
        updateHorizontalDragDetails = dragDetails;
      },
      onHorizontalDragEnd: (endDetails) {
        double dx = updateHorizontalDragDetails.globalPosition.dx -
            startHorizontalDragDetails.globalPosition.dx;
        double dy = updateHorizontalDragDetails.globalPosition.dy -
            startHorizontalDragDetails.globalPosition.dy;
        double velocity = endDetails.primaryVelocity;

        if (dx < 0) dx = -dx;
        if (dy < 0) dy = -dy;
        double positiveVelocity = velocity < 0 ? -velocity : velocity;

full swipe detector code

library swipedetector;

import 'package:flutter/material.dart';

class SwipeConfiguration {
  //Vertical swipe configuration options
  double verticalSwipeMaxWidthThreshold = 50.0;
  double verticalSwipeMinDisplacement = 100.0;
  double verticalSwipeMinVelocity = 300.0;

  //Horizontal swipe configuration options
  double horizontalSwipeMaxHeightThreshold = 50.0;
  double horizontalSwipeMinDisplacement = 100.0;
  double horizontalSwipeMinVelocity = 300.0;

  SwipeConfiguration({
    double verticalSwipeMaxWidthThreshold,
    double verticalSwipeMinDisplacement,
    double verticalSwipeMinVelocity,
    double horizontalSwipeMaxHeightThreshold,
    double horizontalSwipeMinDisplacement,
    double horizontalSwipeMinVelocity,
  }) {
    if (verticalSwipeMaxWidthThreshold != null) {
      this.verticalSwipeMaxWidthThreshold = verticalSwipeMaxWidthThreshold;
    }

    if (verticalSwipeMinDisplacement != null) {
      this.verticalSwipeMinDisplacement = verticalSwipeMinDisplacement;
    }

    if (verticalSwipeMinVelocity != null) {
      this.verticalSwipeMinVelocity = verticalSwipeMinVelocity;
    }

    if (horizontalSwipeMaxHeightThreshold != null) {
      this.horizontalSwipeMaxHeightThreshold = horizontalSwipeMaxHeightThreshold;
    }

    if (horizontalSwipeMinDisplacement != null) {
      this.horizontalSwipeMinDisplacement = horizontalSwipeMinDisplacement;
    }

    if (horizontalSwipeMinVelocity != null) {
      this.horizontalSwipeMinVelocity = horizontalSwipeMinVelocity;
    }
  }
}

class SwipeDetector extends StatelessWidget {
  final Widget child;
  final Function() onSwipeUp;
  final Function() onSwipeDown;
  final Function() onSwipeLeft;
  final Function() onSwipeRight;
  final SwipeConfiguration swipeConfiguration;

  SwipeDetector(
      {@required this.child,
      this.onSwipeUp,
      this.onSwipeDown,
      this.onSwipeLeft,
      this.onSwipeRight,
      SwipeConfiguration swipeConfiguration})
      : this.swipeConfiguration = swipeConfiguration == null
            ? SwipeConfiguration()
            : swipeConfiguration;

  @override
  Widget build(BuildContext context) {
    //Vertical drag details
    DragStartDetails startVerticalDragDetails;
    DragUpdateDetails updateVerticalDragDetails;

    //Horizontal drag details
    DragStartDetails startHorizontalDragDetails;
    DragUpdateDetails updateHorizontalDragDetails;

    return GestureDetector(
      child: child,
      onVerticalDragStart: (dragDetails) {
        startVerticalDragDetails = dragDetails;
      },
      onVerticalDragUpdate: (dragDetails) {
        updateVerticalDragDetails = dragDetails;
      },
      onVerticalDragEnd: (endDetails) {
        double dx = updateVerticalDragDetails.globalPosition.dx -
            startVerticalDragDetails.globalPosition.dx;
        double dy = updateVerticalDragDetails.globalPosition.dy -
            startVerticalDragDetails.globalPosition.dy;
        double velocity = endDetails.primaryVelocity;

        //Convert values to be positive
        if (dx < 0) dx = -dx;
        if (dy < 0) dy = -dy;
        double positiveVelocity = velocity < 0 ? -velocity : velocity;

        if (dx > swipeConfiguration.verticalSwipeMaxWidthThreshold) return;
        if (dy < swipeConfiguration.verticalSwipeMinDisplacement) return;
        if (positiveVelocity < swipeConfiguration.verticalSwipeMinVelocity)
          return;

        if (velocity < 0) {
          //Swipe Up
          if (onSwipeUp != null) {
            onSwipeUp();
          }
        } else {
          //Swipe Down
          if (onSwipeDown != null) {
            onSwipeDown();
          }
        }
      },
      onHorizontalDragStart: (dragDetails) {
        startHorizontalDragDetails = dragDetails;
      },
      onHorizontalDragUpdate: (dragDetails) {
        updateHorizontalDragDetails = dragDetails;
      },
      onHorizontalDragEnd: (endDetails) {
        double dx = updateHorizontalDragDetails.globalPosition.dx -
            startHorizontalDragDetails.globalPosition.dx;
        double dy = updateHorizontalDragDetails.globalPosition.dy -
            startHorizontalDragDetails.globalPosition.dy;
        double velocity = endDetails.primaryVelocity;

        if (dx < 0) dx = -dx;
        if (dy < 0) dy = -dy;
        double positiveVelocity = velocity < 0 ? -velocity : velocity;

        print("$dx $dy $velocity $positiveVelocity");

        if (dx < swipeConfiguration.horizontalSwipeMinDisplacement) return;
        if (dy > swipeConfiguration.horizontalSwipeMaxHeightThreshold) return;
        if (positiveVelocity < swipeConfiguration.horizontalSwipeMinVelocity)
          return;

        if (velocity < 0) {
          //Swipe Up
          if (onSwipeLeft != null) {
            onSwipeLeft();
          }
        } else {
          //Swipe Down
          if (onSwipeRight != null) {
            onSwipeRight();
          }
        }
      },
    );
  }
}

full example code

import 'package:flutter/material.dart';
import 'package:swipedetector/swipedetector.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(),
      debugShowCheckedModeBanner: false,
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  String _swipeDirection = "";

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(),
      body: Center(
        child: Container(
          child: Row(
            children: <Widget>[
              Expanded(
                child: SwipeDetector(
                  child: Card(
                    child: Container(
                      padding: EdgeInsets.only(
                        top: 80.0,
                        bottom: 80.0,
                        left: 16.0,
                        right: 16.0,
                      ),
                      child: Column(
                        mainAxisSize: MainAxisSize.min,
                        children: <Widget>[
                          Text(
                            'Swipe Me!',
                            style: TextStyle(
                              fontSize: 40.0,
                            ),
                          ),
                          Text(
                            '$_swipeDirection',
                            style: TextStyle(),
                          ),
                        ],
                      ),
                    ),
                  ),
                  onSwipeUp: () {
                    setState(() {
                      _swipeDirection = "Swipe Up";
                    });
                  },
                  onSwipeDown: () {
                    setState(() {
                      _swipeDirection = "Swipe Down";
                    });
                  },
                  onSwipeLeft: () {
                    setState(() {
                      _swipeDirection = "Swipe Left";
                    });
                  },
                  onSwipeRight: () {
                    setState(() {
                      _swipeDirection = "Swipe Right";
                    });
                  },
                  swipeConfiguration: SwipeConfiguration(
                      verticalSwipeMinVelocity: 100.0,
                      verticalSwipeMinDisplacement: 50.0,
                      verticalSwipeMaxWidthThreshold:100.0,
                      horizontalSwipeMaxHeightThreshold: 50.0,
                      horizontalSwipeMinDisplacement:50.0,
                      horizontalSwipeMinVelocity: 100.0),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }
}