Is there a better way to check Left/Right Drag in #flutter?

using the GestureDetector widget and its panUpdate method, calculate the distance moved.

    GestureDetector(
     onPanStart: (DragStartDetails details) {
      initial = details.globalPosition.dx;
     },
     onPanUpdate: (DragUpdateDetails details) {
      distance= details.globalPosition.dx - initial;  
     },
     onPanEnd: (DragEndDetails details) {
      initial = 0.0; 
     print(distance);
     //+ve distance signifies a drag from left to right(start to end)
     //-ve distance signifies a drag from right to left(end to start)
});

You can use onHorizontalDragUpdate:

onHorizontalDragUpdate: (details){
    print(details.primaryDelta);
},

if details.primaryDelta is positive ,the drag is left to right. if details.primaryDelta is negative ,the drag is right to left

I would just use a Dismissible widget for this. It's pretty configurable.

Note: If you don't want to provide visual feedback on the swipe, you could use a Stack to put a transparent Dismissible on top of another widget.

import 'package:flutter/material.dart';

void main() {
  runApp(new MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  State createState() => new MyHomePageState();
}

class MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      body: new Dismissible(
        resizeDuration: null,
        onDismissed: (DismissDirection direction) {
          setState(() {
            _counter += direction == DismissDirection.endToStart ? 1 : -1;
          });
        },
        key: new ValueKey(_counter),
        child: new Center(
          child: new Text(
            '$_counter',
            style: Theme.of(context).textTheme.display4,
          ),
        ),
      ),
    );
  }
}