在不单击菜单项的情况下打开SelectableText的上下文菜单

import 'package:flutter/material.dart';

class SelectableTextPage extends StatefulWidget {
  @override
  _SelectableTextPageState createState() => _SelectableTextPageState();
}

class _SelectableTextPageState extends State<SelectableTextPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('SelectableText'),
      ),
      body: GestureDetector(
        onLongPress: () {
          final RenderBox box = context.findRenderObject();
          final Offset topLeft = box.localToGlobal(Offset.zero);
          final Offset bottomRight =
              box.localToGlobal(box.size.bottomRight(Offset.zero));
          final RelativeRect position = RelativeRect.fromLTRB(
            topLeft.dx,
            topLeft.dy,
            bottomRight.dx,
            bottomRight.dy,
          );

          showMenu(
            context: context,
            position: position,
            items: <PopupMenuEntry>[
              PopupMenuItem(
                child: Text('复制'),
                value: 'copy',
              ),
              PopupMenuItem(
                child: Text('粘贴'),
                value: 'paste',
              ),
            ],
          );
        },
        child: Center(
          child: SelectableText(
            '这是可选择的文本',
            style: TextStyle(fontSize: 18.0),
          ),
        ),
      ),
    );
  }
}