Navigation and DI

Question

我试图制作一个标准代码，以便在我的xamarin.forms应用程序中实现。我想做的是有一种在视图模型和 wey 之间导航的方法，以正确实现依赖注入。 我目前正在为导航做什么：

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接口并从该步骤继续，但问题是当我尝试时，我无法尝试拥有一个好的导航系统(例如将视图和视图模型注册到一个文件中分开(。

有没有人有一个我可以看看的示例示例？或者也许一些迹象以便继续？

PD：我试图避免像MvvMcross这样的框架。

提前感谢！

Answer 1

(我将尝试尽可能简化所有代码示例(。

1. 首先，我们需要一个可以注册所有对象并选择性地定义其生命周期的地方。为此，我们可以使用IOC容器，您可以自己选择一个。在这个例子中，我将使用Autofac(它是最快的可用之一(。我们可以在App中保留对它的引用，以便全局可用(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.我们需要一个对象负责检索特定ViewModel的Page(视图(，反之亦然。在设置应用程序的根/主页的情况下，第二种情况可能很有用。为此，我们应该就一个简单的约定达成一致，即所有ViewModels都应该在ViewModels目录中，Pages(视图(应该在Views目录中。换句话说，ViewModels应该位于命名空间[MyApp].ViewModels，Pages(视图(应位于[MyApp].Views命名空间中。除此之外，我们应该同意WelcomeView(页面(应该有一个WelcomeViewModel等。下面是映射器的代码示例：

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，我们需要自动设置BindingContext的ViewModelLocator：

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，我们需要一个支持ViewModel First Navigation方法的NavigationService：

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，所有ViewModels都有一个BaseViewModel抽象基类，您可以在其中定义InitializeAsync等将在导航后立即执行的方法。下面是一个导航示例：

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>());
}
}

如您所知，此方法更复杂，更难调试，并且可能会令人困惑。但是，有很多优点，而且您实际上不必自己实现它，因为大多数 MVVM 框架都支持开箱即用。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，并且有一个免费的使用Xamarin.Forms的企业应用程序模式电子书，详细解释了这个和许多其他有趣的主题。