How to make copyable Text Widget in Flutter?

You can use a SnackBar to notify the user about the copy.

Here is a relevant code:

String _copy = "Copy Me";

  @override
  Widget build(BuildContext context) {
    final key = new GlobalKey<ScaffoldState>();
    return new Scaffold(
      key: key,
      appBar: new AppBar(
        title: new Text("Copy"),
        centerTitle: true,
      ),
      body:
      new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new GestureDetector(
              child: new Text(_copy),
              onLongPress: () {
                Clipboard.setData(new ClipboardData(text: _copy));
                key.currentState.showSnackBar(
                    new SnackBar(content: new Text("Copied to Clipboard"),));
              },
            ),
            new TextField(
                decoration: new InputDecoration(hintText: "Paste Here")),
          ]),


    );
  }

EDIT

I was working on something and I did the followin, so I thought of revisiting this answer:

import "package:flutter/material.dart";
import 'package:flutter/services.dart';

void main() {
  runApp(new MaterialApp(home: new MyApp(),
  ));
}

class MyApp extends StatefulWidget {
  @override
  _MyAppState createState() => new _MyAppState();
}

class _MyAppState extends State<MyApp> with TickerProviderStateMixin {
  String _copy = "Copy Me";

  @override
  Widget build(BuildContext context) {
    final key = new GlobalKey<ScaffoldState>();
    return new Scaffold(
      key: key,
      appBar: new AppBar(
        title: new Text("Copy"),
        centerTitle: true,
      ),
      body:
      new Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            new GestureDetector(
              child: new CustomToolTip(text: "My Copyable Text"),
              onTap: () {

              },
            ),
            new TextField(
                decoration: new InputDecoration(hintText: "Paste Here")),
          ]),


    );
  }
}

class CustomToolTip extends StatelessWidget {

  String text;

  CustomToolTip({this.text});

  @override
  Widget build(BuildContext context) {
    return new GestureDetector(
      child: new Tooltip(preferBelow: false,
          message: "Copy", child: new Text(text)),
      onTap: () {
        Clipboard.setData(new ClipboardData(text: text));
      },
    );
  }
}

Since Flutter 1.9 you can use

SelectableText("Lorem ipsum...")

When text is selected the "Copy" context button will appear.