see your widget

see_your_demo.dart

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

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

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => new MyAppState();
}

const double itemHeight = 100.0;

class MyAppState extends State<MyApp> {
  GlobalKey<State> key = new GlobalKey();

  bool widgetIn = false;

  @override
  Widget build(BuildContext context) {

    return new MaterialApp(
      home: new Scaffold(
        appBar: new AppBar(
          title: new Text("Check whether I'm in"),
        ),
        body: NotificationListener<ScrollNotification>(
          child: new ListView(
            itemExtent: itemHeight,
            children: [
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              MyWidget(key: key),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget(),
              YourWidget()
            ],
          ),
          onNotification: (ScrollNotification scroll) {
            var currentContext = key.currentContext;
            if (currentContext == null) return false;
            //final double screenHeight = MediaQuery.of(currentContext).size.height;
            var renderObject = currentContext.findRenderObject();
            RenderAbstractViewport viewport =
                RenderAbstractViewport.of(renderObject);

            /// The `alignment` argument describes where the target should be positioned
            /// after applying the returned offset. If `alignment` is 0.0, the child must
            /// be positioned as close to the leading edge of the viewport as possible. If
            /// `alignment` is 1.0, the child must be positioned as close to the trailing
            /// edge of the viewport as possible. If `alignment` is 0.5, the child must be
            /// positioned as close to the center of the viewport as possible.

            /// Distance between top edge of screen and MyWidget bottom edge
            var offsetToRevealLeading =
                viewport.getOffsetToReveal(renderObject, 0.0);

            /// Distance between bottom edge of screen and MyWidget top edge
            var offsetToRevealTrailingEdge =
                viewport.getOffsetToReveal(renderObject, 1.0);

            print(
                " ${scroll.metrics.pixels}  ${offsetToRevealLeading.offset} ${offsetToRevealTrailingEdge.offset}");

            var offset = scroll.metrics.pixels;
            //in
            //
            if (offsetToRevealTrailingEdge.offset <= offset + itemHeight &&
                offset <= offsetToRevealLeading.offset + itemHeight) {
              if (!widgetIn) {
                setState(() {
                  widgetIn = true;
                });
              }
            }
            //out
            else {
              if (widgetIn) {
                setState(() {
                  //out
                  widgetIn = false;
                });
              }
            }

            return false;
          },
        ),
        floatingActionButton: new FloatingActionButton(
          child: Container(
            child: Text(widgetIn ? "I see you " : "bye bye"),
            height: 200.0,
            width: 200.0,
            alignment: Alignment.center,
          ),
          onPressed: () {},
        ),
      ),
    );
  }
}

class MyWidget extends StatefulWidget {
  const MyWidget({Key key}) : super(key: key);

  @override
  State<StatefulWidget> createState() => new MyWidgetState();
}

class MyWidgetState extends State<MyWidget> {
  @override
  Widget build(BuildContext context) {
    return new Container(height: itemHeight, color: Colors.red);
  }
}

class YourWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Container(
      decoration:
          new BoxDecoration(border: new Border.all(), color: Colors.grey),
    );
  }
}