my_own_popup_menu.dart

import 'package:flutter/material.dart';

class MyOwnPopupMenuButton<T> extends StatefulWidget {
  /// Creates a button that shows a popup menu.
  ///
  /// The [itemBuilder] argument must not be null.
  const MyOwnPopupMenuButton({
    Key key,
    @required this.itemBuilder,
    this.initialValue,
    this.onSelected,
    this.onCanceled,
    this.tooltip,
    this.elevation = 8.0,
    this.enabled = true,
    this.padding = const EdgeInsets.all(8.0),
    this.child,
    this.icon,
    this.offset = Offset.zero,
  })  : assert(itemBuilder != null),
        assert(offset != null),
        assert(!(child != null &&
            icon != null)), // fails if passed both parameters
        super(key: key);

  /// Called when the button is pressed to create the items to show in the menu.
  final PopupMenuItemBuilder<T> itemBuilder;

  /// The value of the menu item, if any, that should be highlighted when the menu opens.
  final T initialValue;

  /// Called when the user selects a value from the popup menu created by this button.
  ///
  /// If the popup menu is dismissed without selecting a value, [onCanceled] is
  /// called instead.
  final PopupMenuItemSelected<T> onSelected;

  /// Called when the user dismisses the popup menu without selecting an item.
  ///
  /// If the user selects a value, [onSelected] is called instead.
  final PopupMenuCanceled onCanceled;

  /// Text that describes the action that will occur when the button is pressed.
  ///
  /// This text is displayed when the user long-presses on the button and is
  /// used for accessibility.
  final String tooltip;

  /// The z-coordinate at which to place the menu when open. This controls the
  /// size of the shadow below the menu.
  ///
  /// Defaults to 8, the appropriate elevation for popup menus.
  final double elevation;

  /// Matches IconButton's 8 dps padding by default. In some cases, notably where
  /// this button appears as the trailing element of a list item, it's useful to be able
  /// to set the padding to zero.
  final EdgeInsetsGeometry padding;

  /// If provided, the widget used for this button.
  final Widget child;

  /// If provided, the icon used for this button.
  final Icon icon;

  /// The offset applied to the Popup Menu Button.
  ///
  /// When not set, the Popup Menu Button will be positioned directly next to
  /// the button that was used to create it.
  final Offset offset;

  final bool enabled;

  @override
  _PopupMenuButtonState<T> createState() => _PopupMenuButtonState<T>();
}

class _PopupMenuButtonState<T> extends State<MyOwnPopupMenuButton<T>> {
  void showButtonMenu() {
    final RenderBox button = context.findRenderObject();
    final RenderBox overlay = Overlay.of(context).context.findRenderObject();
    final RelativeRect position = RelativeRect.fromRect(
      Rect.fromPoints(
        button.localToGlobal(widget.offset, ancestor: overlay),
        button.localToGlobal(button.size.bottomRight(Offset.zero),
            ancestor: overlay),
      ),
      Offset.zero & overlay.size,
    );
    showMenu<T>(
      context: context,
      elevation: widget.elevation,
      items: widget.itemBuilder(context),
      initialValue: widget.initialValue,
      position: position,
    ).then<void>((T newValue) {
      if (!mounted) return null;
      if (newValue == null) {
        if (widget.onCanceled != null) widget.onCanceled();
        return null;
      }
      if (widget.onSelected != null) widget.onSelected(newValue);
    });
  }

  Icon _getIcon(TargetPlatform platform) {
    assert(platform != null);
    switch (platform) {
      case TargetPlatform.android:
      case TargetPlatform.fuchsia:
        return const Icon(Icons.more_vert);
      case TargetPlatform.iOS:
        return const Icon(Icons.more_horiz);
    }
    return null;
  }

  @override
  Widget build(BuildContext context) {
    assert(debugCheckHasMaterialLocalizations(context));
    return widget.child != null
        ? InkWell(
            onTap: widget.enabled ? showButtonMenu : null,
            child: widget.child,
          )
        : IconButton(
            icon: widget.icon ?? _getIcon(Theme.of(context).platform),
            padding: widget.padding,
            tooltip: widget.tooltip ??
                MaterialLocalizations.of(context).showMenuTooltip,
            onPressed: widget.enabled ? showButtonMenu : null,
          );
  }
}