Xamarin.Forms在OnBindingContextChanged

Question

我正在尝试使用MvvmCross为我的导航实现TabbedPage。问题是MvvmCross使用ViewModel第一个导航，这似乎不能很好地用于将子项添加到TabbedPage的一般方法;因为我在页面构建期间无法访问非null的ViewModel，但我可以在OnBindingContextChanged内访问它。

这是我到目前为止所拥有的......

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（注意此处的评论）：

<?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是我要说明的观点的简化版本，即DashboardPage中的每个标签都有一个空白页面。

问题1 ：为什么标签页空白？

问题2 ：请参阅DashboardPage.xaml.cs中的评论。

Answer 1

你为什么要自己做这整件事？正如您在样本（https://github.com/MvvmCross/MvvmCross/tree/develop/TestProjects/Playground/Playground.Forms.UI/Pages）中看到的那样，您可以使用MvxTabbePagePresentation属性装饰您的视图，MvvmCross将为您处理其余的视图！

我还建议使用Mvx类型页面来利用许多功能。