导航和DI

Question

我正在尝试制作一个标准代码，以便在我的 xamarin.forms 应用程序中实现。我想要做的是有一种方法可以在视图模型和一个 Wey 之间导航，以正确实现依赖注入。我目前正在做的导航：

await Navigation.PushAsync(new SecondPageView());

对于 DI：

 var test = DependencyService.Get<ITestService>();
 WelcomeMessage = test.GetSystemWelcome();

我知道实现 Di 的正确方法是创建一个界面并从该步骤开始，但问题是当我尝试时，我无法尝试拥有一个好的导航系统（例如在文件中注册视图和视图模型） .

有没有人有我可以看看的示例？或者也许有一些迹象可以继续？

PD：我试图避免像 MvvMcross 这样的框架。

提前致谢！

Answer 1

（我会尽量简化所有的代码示例）。

1. 首先，我们需要一个地方，我们可以在其中注册我们所有的对象并可以选择定义它们的生命周期。这件事我们可以使用一个IOC容器，你可以自己选择一个。在这个例子中，我将使用Autofac（它是最快的可用之一）。我们可以保留对它的引用，以便在App全局范围内可用（不是一个好主意，但需要简化）：

public class DependencyResolver
{
    static IContainer container;

    public DependencyResolver(params Module[] modules)
    {
        var builder = new ContainerBuilder();

        if (modules != null)
            foreach (var module in modules)
                builder.RegisterModule(module);

        container = builder.Build();
    }

    public T Resolve<T>() => container.Resolve<T>();
    public object Resolve(Type type) => container.Resolve(type);
}

public partial class App : Application
{
    public DependencyResolver DependencyResolver { get; }

    // Pass here platform specific dependencies
    public App(Module platformIocModule)
    {
        InitializeComponent();
        DependencyResolver = new DependencyResolver(platformIocModule, new IocModule());
        MainPage = new WelcomeView();
    }

    /* The rest of the code ... */
}

2.我们将需要一个负责为Page特定检索（视图）的对象ViewModel，反之亦然。第二种情况在设置应用程序的根/主页的情况下可能很有用。为此，我们应该同意一个简单的约定，即所有的ViewModels都应该在ViewModels目录中，Pages（视图）应该在Views目录中。换句话说，ViewModels应该存在于[MyApp].ViewModels命名空间中，而Pages(Views)应该存在于命名[MyApp].Views空间中。除此之外，我们应该同意WelcomeView(Page) 应该有一个WelcomeViewModel等等。这是一个映射器的代码示例：

public class TypeMapperService
{
    public Type MapViewModelToView(Type viewModelType)
    {
        var viewName = viewModelType.FullName.Replace("Model", string.Empty);
        var viewAssemblyName = GetTypeAssemblyName(viewModelType);
        var viewTypeName = GenerateTypeName("{0}, {1}", viewName, viewAssemblyName);
        return Type.GetType(viewTypeName);
    }

    public Type MapViewToViewModel(Type viewType)
    {
        var viewModelName = viewType.FullName.Replace(".Views.", ".ViewModels.");
        var viewModelAssemblyName = GetTypeAssemblyName(viewType);
        var viewTypeModelName = GenerateTypeName("{0}Model, {1}", viewModelName, viewModelAssemblyName);
        return Type.GetType(viewTypeModelName);
    }

    string GetTypeAssemblyName(Type type) => type.GetTypeInfo().Assembly.FullName;
    string GenerateTypeName(string format, string typeName, string assemblyName) =>
        string.Format(CultureInfo.InvariantCulture, format, typeName, assemblyName);
}

3.对于设置根页面的情况，我们需要自动ViewModelLocator设置BindingContext：

public static class ViewModelLocator
{
    public static readonly BindableProperty AutoWireViewModelProperty =
        BindableProperty.CreateAttached("AutoWireViewModel", typeof(bool), typeof(ViewModelLocator), default(bool), propertyChanged: OnAutoWireViewModelChanged);

    public static bool GetAutoWireViewModel(BindableObject bindable) =>
        (bool)bindable.GetValue(AutoWireViewModelProperty);

    public static void SetAutoWireViewModel(BindableObject bindable, bool value) =>
        bindable.SetValue(AutoWireViewModelProperty, value);

    static ITypeMapperService mapper = (Application.Current as App).DependencyResolver.Resolve<ITypeMapperService>();

    static void OnAutoWireViewModelChanged(BindableObject bindable, object oldValue, object newValue)
    {
        var view = bindable as Element;
        var viewType = view.GetType();
        var viewModelType = mapper.MapViewToViewModel(viewType);
        var viewModel =  (Application.Current as App).DependencyResolver.Resolve(viewModelType);
        view.BindingContext = viewModel;
    }
}

// Usage example
<?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:viewmodels="clr-namespace:MyApp.ViewModel"
    viewmodels:ViewModelLocator.AutoWireViewModel="true"
    x:Class="MyApp.Views.MyPage">
</ContentPage>

4.最后，我们需要一个NavigationService支持的ViewModel First Navigation方法：

public class NavigationService
{
    TypeMapperService mapperService { get; }

    public NavigationService(TypeMapperService mapperService)
    {
        this.mapperService = mapperService;
    }

    protected Page CreatePage(Type viewModelType)
    {
        Type pageType = mapperService.MapViewModelToView(viewModelType);
        if (pageType == null)
        {
            throw new Exception($"Cannot locate page type for {viewModelType}");
        }

        return Activator.CreateInstance(pageType) as Page;
    }

    protected Page GetCurrentPage()
    {
        var mainPage = Application.Current.MainPage;

        if (mainPage is MasterDetailPage)
        {
            return ((MasterDetailPage)mainPage).Detail;
        }

        // TabbedPage : MultiPage<Page>
        // CarouselPage : MultiPage<ContentPage>
        if (mainPage is TabbedPage || mainPage is CarouselPage)
        {
            return ((MultiPage<Page>)mainPage).CurrentPage;
        }

        return mainPage;
    }

    public Task PushAsync(Page page, bool animated = true)
    {
        var navigationPage = Application.Current.MainPage as NavigationPage;
        return navigationPage.PushAsync(page, animated);
    }

    public Task PopAsync(bool animated = true)
    {
        var mainPage = Application.Current.MainPage as NavigationPage;
        return mainPage.Navigation.PopAsync(animated);
    }

    public Task PushModalAsync<TViewModel>(object parameter = null, bool animated = true) where TViewModel : BaseViewModel =>
        InternalPushModalAsync(typeof(TViewModel), animated, parameter);

    public Task PopModalAsync(bool animated = true)
    {
        var mainPage = GetCurrentPage();
        if (mainPage != null)
            return mainPage.Navigation.PopModalAsync(animated);

        throw new Exception("Current page is null.");
    }

    async Task InternalPushModalAsync(Type viewModelType, bool animated, object parameter)
    {
        var page = CreatePage(viewModelType);
        var currentNavigationPage = GetCurrentPage();

        if (currentNavigationPage != null)
        {
            await currentNavigationPage.Navigation.PushModalAsync(page, animated);
        }
        else
        {
            throw new Exception("Current page is null.");
        }

        await (page.BindingContext as BaseViewModel).InitializeAsync(parameter);
    }
}

如您所见，BaseViewModel对于所有ViewModels您可以定义的方法都有一个- 抽象基类，这些方法InitializeAsync将在导航后立即执行。这是一个导航示例：

public class WelcomeViewModel : BaseViewModel
{
    public ICommand NewGameCmd { get; }
    public ICommand TopScoreCmd { get; }
    public ICommand AboutCmd { get; }

    public WelcomeViewModel(INavigationService navigation) : base(navigation)
    {
        NewGameCmd = new Command(async () => await Navigation.PushModalAsync<GameViewModel>());
        TopScoreCmd = new Command(async () => await navigation.PushModalAsync<TopScoreViewModel>());
        AboutCmd = new Command(async () => await navigation.PushModalAsync<AboutViewModel>());
    }
}

正如您所了解的那样，这种方法更复杂、更难调试并且可能会令人困惑。然而，它有很多优点，而且您实际上不必自己实现它，因为大多数 MVVM 框架都开箱即用地支持它。此处演示的代码示例可在github 上找到。

有很多关于ViewModel First Navigation方法的好文章，还有一个免费的Enterprise Application Patterns using Xamarin.Forms电子书，它详细解释了这一点和许多其他有趣的主题。