MasterDetail でもメニューバーを表示するためのテンプレート試作

App.xaml

<?xml version="1.0" encoding="utf-8" ?>
<prism:PrismApplication xmlns="http://xamarin.com/schemas/2014/forms"
                        xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
                        xmlns:prism="clr-namespace:Prism.Unity;assembly=Prism.Unity.Forms"
                        x:Class="Smallgeek.App">
  <Application.Resources>
    <ResourceDictionary>
      <ControlTemplate x:Key="NavigationMenuPageTemplate">
        <Grid RowSpacing="0">
          <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition />
          </Grid.RowDefinitions>

          <c:NavigationBar Grid.Row="0" Grid.Column="0"
                           Title="{TemplateBinding Title}"
                           ToolbarItems="{TemplateBinding ToolbarItems}"
                           BackCommand="GoBack"
                           HeightRequest="56">
            <c:NavigationBar.IsVisible>
              <OnIdiom x:TypeArguments="x:Boolean">
                <OnIdiom.Phone>False</OnIdiom.Phone>
                <OnIdiom.Tablet>True</OnIdiom.Tablet>
              </OnIdiom>
            </c:NavigationBar.IsVisible>
          </c:NavigationBar>
          <ContentPresenter Grid.Row="1" Grid.Column="0" />
        </Grid>
      </ControlTemplate>

    </ResourceDictionary>
  </Application.Resources>
</prism:PrismApplication>

NavigationBar.cs

using System.Collections.Generic;
using System.Collections.ObjectModel;
using Xamarin.Forms;

namespace Smallgeek.Controls
{
    public class NavigationBar : ContentView
    {
        private readonly Label title;
        private readonly Button back;
        private readonly StackLayout menuItemView;

        public NavigationBar()
        {
            var main = new Grid();
            main.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });
            main.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Star });
            main.ColumnDefinitions.Add(new ColumnDefinition { Width = GridLength.Auto });

            main.Padding = new Thickness(10, 18, 18, 0);

            back = new Button
            {
                Text = "<",
                VerticalOptions = LayoutOptions.Center,
                HorizontalOptions = LayoutOptions.Center,
            };
            main.Children.Add(back, 0, 0);

            title = new Label
            {
                VerticalOptions = LayoutOptions.Center,
                HorizontalOptions = LayoutOptions.Center,
                FontAttributes = FontAttributes.Bold
            };
            main.Children.Add(title, 0, 0);
            Grid.SetColumnSpan(title, 3);

            menuItemView = new StackLayout
            {
                Orientation = StackOrientation.Horizontal,
                VerticalOptions = LayoutOptions.Center,
                HorizontalOptions = LayoutOptions.End
            };
            main.Children.Add(menuItemView, 2, 0);

            Content = main;
        }

        public static readonly BindableProperty HasBackButtonProperty =
            BindableProperty.Create(
                propertyName: nameof(HasBackButton),
                returnType: typeof(bool),
                declaringType: typeof(NavigationBar),
                defaultValue: false,
                propertyChanged: (bindable, oldValue, newValue) =>
                {
                    var bar = (NavigationBar)bindable;
                    bar.back.IsVisible = (bool)newValue;
                });

        public bool HasBackButton
        {
            get { return (bool)GetValue(HasBackButtonProperty); }
            set { SetValue(HasBackButtonProperty, value); }
        }

        public static readonly BindableProperty BackButtonTitleProperty =
            BindableProperty.Create(
                propertyName: nameof(BackButtonTitle),
                returnType: typeof(string),
                declaringType: typeof(NavigationBar),
                defaultValue: "<",
                propertyChanged: (bindable, oldValue, newValue) =>
                {
                    var bar = (NavigationBar)bindable;
                    bar.back.Text = newValue?.ToString();
                });

        public string BackButtonTitle
        {
            get { return (string)GetValue(BackButtonTitleProperty); }
            set { SetValue(BackButtonTitleProperty, value); }
        }

        public static readonly BindableProperty TitleProperty =
            BindableProperty.Create(
                propertyName: nameof(Title),
                returnType: typeof(string),
                declaringType: typeof(NavigationBar),
                defaultValue: "",
                propertyChanged: (bindable, oldValue, newValue) =>
                {
                    var bar = (NavigationBar)bindable;
                    bar.title.Text = newValue?.ToString();
                });

        public string Title
        {
            get { return (string)GetValue(TitleProperty); }
            set { SetValue(TitleProperty, value); }
        }

        public static readonly BindableProperty ToolbarItemsProperty =
            BindableProperty.Create(
                propertyName: nameof(ToolbarItems),
                returnType: typeof(IList<ToolbarItem>),
                declaringType: typeof(NavigationBar),
                propertyChanged: (bindable, oldValue, newValue) =>
                {
                    var bar = (NavigationBar)bindable;
                    bar.menuItemView.Children.Clear();

                    var newItems = newValue as ObservableCollection<ToolbarItem>;
                    if (newItems != null)
                    {
                        newItems.CollectionChanged += bar.OnToolbarItemsCollectionChanged;
                    }

                    var oldItems = oldValue as ObservableCollection<ToolbarItem>;
                    if (oldItems != null)
                    {
                        oldItems.CollectionChanged -= bar.OnToolbarItemsCollectionChanged;
                    }
                });

        public IList<ToolbarItem> ToolbarItems
        {
            get { return (IList<ToolbarItem>)GetValue(ToolbarItemsProperty); }
            set { SetValue(ToolbarItemsProperty, value); }
        }

        private void OnToolbarItemsCollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
        {
            foreach (ToolbarItem item in e.NewItems)
            {
                var button = new Button();
                button.SetBinding(Button.ImageProperty, new Binding(ToolbarItem.IconProperty.PropertyName, BindingMode.OneWay, source: item));
                button.SetBinding(Button.TextProperty, new Binding(ToolbarItem.TextProperty.PropertyName, BindingMode.OneWay, source: item));
                button.SetBinding(Button.CommandProperty, new Binding(ToolbarItem.CommandProperty.PropertyName, BindingMode.OneWay, source: item));

                menuItemView.Children.Add(button);
            }
        }
    }
}