Xamarin.Forms 在 OnBindingContextChanged 中将子级添加到 TabbedPage

Question

我正在尝试使用 MvvmCross 实现 TabbedPage 进行导航。问题是 MvvmCross 使用 ViewModel 第一个导航，这似乎与将子项添加到 TabbedPage 的一般方法不兼容；因为在页面构建期间我无法访问非空 ViewModel，但我可以在 OnBindingContextChanged 中访问它。

这是我目前所拥有的......

DashboardPage.xaml：

<?xml version="1.0" encoding="UTF-8"?>
<TabbedPage xmlns="http://xamarin.com/schemas/2014/forms" 
        xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
        x:Class="DashboardPage"
        xmlns:local="clr-namespace:CoreUI;assembly=CoreUI"
        SelectedItem="{Binding CurrentSection, Mode=TwoWay}">
</TabbedPage>

DashboardPage.xaml.cs：

public partial class DashboardPage : TabbedPage
{
    public DashboardPage()
    {
        InitializeComponent();
    }

    protected override void OnBindingContextChanged()
    {
        var vm = (BindingContext as DashboardViewModel);
        if (vm == null)
        {
            return;
        }

        ObservableCollection<MainMenuSection> sections = vm.MenuSections;
        foreach (var section in sections)
        {
            MainMenuViewModel main_menu_vm = new MainMenuViewModel
            {
                Section = section
            };

            // Question 2:
            // Going against the MvvmCross grain here by referring to other pages from within a page, as opposed to doing everything from a ViewModel. How do I get around this?
            Children.Add(new MainMenuPage(main_menu_vm));
        }
    }
}

MainMenuPage.xaml（注意这里的cmets）：

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
        xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
        x:Class="MainMenuPage"
        xmlns:local="clr-namespace:CoreUI;assembly=CoreUI"
        Title="{Binding Title}" > <!-- The tabs that are displayed on Dashboard have the correct labels, so Binding appears to be working here. -->

    <ContentPage.Content>
        <StackLayout x:Name="Body" IsVisible="false">
            <Label Text="{Binding Title}"/> <!-- Label doesn't get displayed, but does get displayed if Text is bound to something static, so Binding not quite working here. -->
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

MainMenuPage.xaml.cs：

public partial class MainMenuPage : ContentPage
{
    public MainMenuPage(MainMenuViewModel vm)
    {
        InitializeComponent();
        BindingContext = vm;
    }

    protected override void OnBindingContextChanged()
    {
        Body.IsVisible = true;
    }
}

上面的 MainMenuPage 是我必须说明我的观点的简化版本，即我为 DashboardPage 中的每个选项卡获得一个空白页。

问题一：为什么标签页是空白的？

问题 2：参考 DashboardPage.xaml.cs 中的注释。

Answer 1

你为什么要自己做这件事？正如您在示例 (https://github.com/MvvmCross/MvvmCross/tree/develop/TestProjects/Playground/Playground.Forms.UI/Pages) 中所见，您只需使用 MvxTabbePagePresentation 属性装饰您的视图，MvvmCross 将为您处理其余的事情！

我还建议使用 Mvx 类型的页面来利用许多功能。