xamarin.forms如何初始化数据并使数据绑定正常工作？

Question

我从这个 tutorial 中学到了东西，并尝试使用主详细信息页面构建我自己的时间跟踪器应用程序。但是我遇到了一个问题。这是针对该问题的类似快速演示：

对于MasterDetailPageDetail.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"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             xmlns:local="clr-namespace:QuickDemo"          
             x:Class="QuickDemo.MasterDetailPageDetail"
             Title="Detail">
    <TabbedPage.Children>
        <NavigationPage Title="Tab One">
            <x:Arguments>
                <local:FirstPage BindingContext="{Binding FirstPageModel}" />
            </x:Arguments>
        </NavigationPage>
        <NavigationPage Title="Tab Two">

        </NavigationPage>
    </TabbedPage.Children>
  
</TabbedPage>

对于FirstPage.xaml:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:d="http://xamarin.com/schemas/2014/forms/design"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             mc:Ignorable="d"
             NavigationPage.HasNavigationBar="False"
             xmlns:buttons="clr-namespace:QuickDemo.Views.Buttons"
             x:Class="QuickDemo.FirstPage">
    <ContentPage.Content>
        <StackLayout>
            <StackLayout Padding="20,20,20,5">
                <Label Text="{Binding CurrentStartDate, StringFormat='{0:h:mm tt}'}"
                       FontSize="20"
                       TextColor="Black"/>
                <Label Text="{Binding RunningTotal, StringFormat='{}{0:h\:mm\:ss}'}"
                       FontSize="50" HorizontalTextAlignment="Center"
                       TextColor="Black"/>
            </StackLayout>
        </StackLayout>
    </ContentPage.Content>
</ContentPage>

对于DetailPageModel.cs:

public class DetailPageModel : PageModelBase
    {
        private FirstPageModel _firstPageModel;
        public FirstPageModel FirstPageModel
        {
            get => _firstPageModel;
            set => SetProperty(ref _firstPageModel, value);
        }

        public DetailPageModel(
            FirstPageModel firstPageModel)
        {
            FirstPageModel = firstPageModel;
        }

        public override Task InitializeAsync(object navigationData = null)
        {
            return Task.WhenAny(base.InitializeAsync(navigationData),
                FirstPageModel.InitializeAsync(null));
        }
    }

对于FirstPageModel.cs:

public class FirstPageModel : PageModelBase
    {
        private DateTime _currentStartDate;
        public DateTime CurrentStartDate
        {
            get => _currentStartDate;
            set => SetProperty(ref _currentStartDate, value);
        }

        private TimeSpan _runningTotal;
        public TimeSpan RunningTotal
        {
            get => _runningTotal;
            set => SetProperty(ref _runningTotal, value);
        }

        private Timer _timer;

        public FirstPageModel()
        {
           
            this.InitializeTimer();
        }

        private void InitializeTimer()
        {
            _timer = new Timer();
            _timer.Interval = 1000;
            _timer.Enabled = false;
            _timer.Elapsed += TimerElapsed;
        }

        private void TimerElapsed(object sender, ElapsedEventArgs e)
        {
            RunningTotal += TimeSpan.FromSeconds(1);
        }

        public override async Task InitializeAsync(object navigationData)
        {
            CurrentStartDate = DateTime.Now;
            RunningTotal = new TimeSpan();
            await base.InitializeAsync(navigationData);
        }
    }

并且我在PageModelLocator.cs中注册了页面和页面模型：

public class PageModelLocator
    {
        static TinyIoCContainer _container;
        static Dictionary<Type, Type> _lookupTable;

        static PageModelLocator()
        {
            _container = new TinyIoCContainer();
            _lookupTable = new Dictionary<Type, Type>();

            // Register pages and page models
            Register<DetailPageModel, MasterDetailPageDetail>();
            Register<FirstPageModel, FirstPage>();


            // Register services (services are registered as singletons default)

        }

        public static T Resolve<T>() where T : class
        {
            return _container.Resolve<T>();
        }

        public static Page CreatePageFor(Type pageModelType)
        {
            var pageType = _lookupTable[pageModelType];
            var page = (Page)Activator.CreateInstance(pageType);
            var pageModel = _container.Resolve(pageModelType);
            page.BindingContext = pageModel;
            return page;
        }

        static void Register<TPageModel, TPage>() where TPageModel : PageModelBase where TPage : Page
        {
            _lookupTable.Add(typeof(TPageModel), typeof(TPage));
            _container.Register<TPageModel>();
        }
    }

PageModelBase.cs 和 ExtendedBindableObject.cs 与教程相同。当我运行模拟器时，我得到了这个结果：

我以为会有一个 DateTime 字符串和一个零时间跨度。我觉得 FirstPageModel 中的数据根本没有初始化。我还尝试在构造函数中设置 CurrentStartTime。得到了相同的结果。

我是否错过了在首页上显示 CurrentStartDate 和 RunningTotal 的内容？任何帮助或提示将不胜感激。提前致谢。

Answer 1

我相信您的 MasterDetailPageDetail 没有被初始化（或者 BindingContext 没有被初始化）。本教程中的体系结构在导航事件期间设置 BindingContext。由于您在 MasterDetailPage 的 XAML 中显式设置了 Master 和 Detail，因此未显式设置绑定上下文并且未调用 InitializeAsync 方法。

在导航服务中添加初始化

使用以下方法更新 NavigationService 的 NavigateToAsync 方法，就在 if (setRoot) 之后和 if (page is TabbedPage tabbedPage) 之前：

if (page is MasterDetailPage mdp)
{
     App.Current.MainPage = mdp;
     // We need to initialize both Master's BindingContext as
     // well as Detail's BindingContext if they are PageModelBases
     if (mdp.Master.BindingContext is PageModelBase masterPM)
     {
         await masterPM.InitializeAsync(null);
     }
     if (mdp.Detail.BindingContext is PageModelBase detailPM)
     {
         await detailPM.InitializeAsync(null);
     }
}
else if (page is TabbedPage tabbedPage) 
// .... existing code here

确保您正在为页面（主页和详细信息）设置绑定上下文。您可以在 XAML 中执行此操作，或者像在 Tabbed Page

中那样解析 MasterDetailPage 和 PageModel

如果需要，我可以制作一个视频来介绍这个，并将其添加到系列中。希望这对您有所帮助！

干杯，帕特里克

xamarin.forms如何初始化数据并使数据绑定正常工作？

How to initialize data and make data binding work properly in xamarin.forms?

data-binding

xamarin.ios

xamarin.android

xamarin

xamarin.forms