main.dart

/// shiv
/// om namah shivay✊🏻omNS omNS
/// omNS maro BaBUDO maro mahakal

import 'package:flutter/material.dart';
import 'dart:ui';
import 'dart:math';

void main() => runApp(MaterialApp(home: OMNS()));

class OMNS extends StatefulWidget {
  @override
  _OMNSState createState() => _OMNSState();
}

class _OMNSState extends State<OMNS> with SingleTickerProviderStateMixin {
  AnimationController controller;
  double sum = 0;
  final double minh = 120;
  double get maxh => MediaQuery.of(context).size.height;

  @override
  void initState() {
    controller =
        AnimationController(vsync: this, duration: Duration(seconds: 1));
    controller.addListener(() {
      setState(() {
      });
    });
    super.initState();
  }

  void dragup(DragUpdateDetails details) {
    sum += details.primaryDelta;

    /*
      Amazing knowledge by my omNS shiva bholiyo bhandari babo

      when we set value of controller.value more than it's range then
      controller.value set to maximum range (set to end value  > Tween(begin:...,end: value))

//    controller.value = 10000;
//    print("omNS controller.value = ${controller.value}");

     */

    /*
    1. if we want to understand the how below line work then just
    restart app, drag from bottom to top without stopping and
    see the value of sum variable = it see same as maxh
    so logic is ::: when drag become maxh then make controller.value = 1
      [
        now, why we want controller.value  = 1 because when it is 1 then
        lerp() will gives maxh, (see lerpDouble() on docs. )
      ]
    means
        when  total drag(sum) =  "maxh"   then  controller.value = "1"
     so when  total drag      =  "k"      then  controller.value = "?"
    maxh =  1
     k   =  ?
    after solving question we got
    controller.value should be = k*1/maxh;
    */

    controller.value -= details.primaryDelta / maxh;
    print(details.primaryDelta / maxh);
  }

  void dragend(DragEndDetails details) {
    double velocityY = details.velocity.pixelsPerSecond.dy;

    // here velocityY is normally around 2000 so we are dividing it by 1000 (a Random value)
    // to get small value which should be between
    if (velocityY < 0) {
      controller.fling(
          velocity: max(2, -velocityY / 1000),
          animationBehavior: AnimationBehavior.preserve);
    } else if (velocityY > 0) {
      controller.fling(velocity: min(-2, -velocityY / 1000));
      // NOTE: here also we are putting  ^ (minus) because as velocity will >0 (as per condition)
      // se to decrease the controller.value we need to give negative value
    } else {
      // now if we are just dragging little bit and then take finger up
      // then velocity will be 0.0
      // so in this case we should increase/decrease value of controller
      // to the nearest edge(Top/Bottom) so we can get information about nearest edge(up/down)
      // using controller.value
      // if it is < 0.5 then we are nearest to BOTTOM else ...

      controller.fling(velocity: controller.value < 0.5 ? -1.368 : 1.368);
    }
    print(".velocity : ${details.velocity}");
    print(
        ".velocity.pixelPerSecond.dy : ${details.velocity.pixelsPerSecond.dy}");
  }

  void get() {
    // by default animation value range between 0 to 1 so we can use controller.status
    // without using tween
    bool isfull = controller.status == AnimationStatus.completed;

    // this is a rate through which animation.value should increases/decrease
    // twist here is value of animation increase / decrease from current value of animation
    // if isfull = false then it increase value of animation by 1.3689 unit of maximum value to maximum value
    // here we have not specify (Tween())maximum value so it will go up to 1
    // similarly if iffull = true then .filng() will decrease value by -3 unit of maximum value
    // until minimum value is not comes
    // if Tween()/minimum value is not given then it will take the 0 as minimum
    controller.fling(velocity: isfull ? -3 : 1.3689);
    /*

    here try this to understand and also see the value in console
               controller.fling(velocity: isfull ? -3 :50);
    and also unable the slow animation option at Flutter Inspector

      controller.fling =  animation.forward() + rate of increasing value is equal to velocity

    */
  }

  @override
  Widget build(BuildContext context) {
    double height = MediaQuery.of(context).size.height;
    double width = MediaQuery.of(context).size.width;
    return Scaffold(
      body: Container(
        width: width,
        height: height,
        child: Stack(
          children: [
            Positioned(
            bottom: height / 2,
              left: width / 2.9,
              child: Text("Fun With Flutter", style: TextStyle(fontSize: 36, color: Color(0xff000000), fontWeight: FontWeight.bold)),
            ),
            Positioned(
              bottom: 0,
              left: 0,
              right: 0,
              child: GestureDetector(
                onTap: get,
                onVerticalDragUpdate: dragup,
                onVerticalDragEnd: dragend,
                child: Container(
                  alignment: Alignment.center,
                  height:  lerpDouble(minh, maxh, controller.value),
                  child: Text("Drag Up or Down", style: TextStyle(fontSize: 36, color: Color(0xff000000), fontWeight: FontWeight.bold)),
                  color: Colors.yellow,
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}