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

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,
          ),
        ),
      ),
    );
  }
}

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

primaryVelocity on onHorizontalDragEnd works fine, just check for greater or lesser than zero.

GestureDetector(
  onHorizontalDragEnd: (DragEndDetails drag) {
    if(drag.primaryVelocity == null) return;
    if(drag.primaryVelocity! < 0) {
      // drag from right to left
    }else{
      // drag from left to right
    }
  }
);

Since details which is a DragUdate class instance returns double , you can do.( And also Paageroute provides this by default)

GestureDetector(
       onHorizontalDragUpdate: (details) {
        if(details.primaryDelta! > 1.0) {
           Navigator.pop(context);
                      }
          },
      child:Scaffold(..)

Use TabBarView() Create a TabController specify the length of the widgets in TabBarView children .