使用视图模型中的 INavigation

第一步是创建我们将在视图模型上使用的导航界面：

public interface IViewNavigationService
{
    void Initialize(INavigation navigation, SuperMapper navigationMapper);
    Task NavigateToAsync(object navigationSource, object parameter = null);
    Task GoBackAsync();
}

在 Initialize 方法中，我使用我的自定义映射器，其中我保留了具有关联键的页面类型的集合。

public class SuperMapper
{
    private readonly ConcurrentDictionary<Type, object> _typeToAssociateDictionary = new ConcurrentDictionary<Type, object>();

    private readonly ConcurrentDictionary<object, Type> _associateToType = new ConcurrentDictionary<object, Type>();

    public void AddMapping(Type type, object associatedSource)
    {
        _typeToAssociateDictionary.TryAdd(type, associatedSource);
        _associateToType.TryAdd(associatedSource, type);
    }

    public Type GetTypeSource(object associatedSource)
    {
        Type typeSource;
        _associateToType.TryGetValue(associatedSource, out typeSource);

        return typeSource;
    }

    public object GetAssociatedSource(Type typeSource)
    {
        object associatedSource;
        _typeToAssociateDictionary.TryGetValue(typeSource, out associatedSource);

        return associatedSource;
    }
}

枚举页面：

public enum NavigationPageSource
{
    Page1,
    Page2
}

App.cs 文件：

public class App : Application
{
    public App()
    {
        var startPage = new Page1();
        InitializeNavigation(startPage);
        MainPage = new NavigationPage(startPage);
    }

    #region Sample of navigation initialization
    private void InitializeNavigation(Page startPage)
    {
        var mapper = new SuperMapper();
        mapper.AddMapping(typeof(Page1), NavigationPageSource.Page1);
        mapper.AddMapping(typeof(Page2), NavigationPageSource.Page2);

        var navigationService = DependencyService.Get<IViewNavigationService>();
        navigationService.Initialize(startPage.Navigation, mapper);
    } 
    #endregion
}

在映射器中，我将某个页面的类型与枚举值相关联。

IViewNavigationService 实施：

[assembly: Dependency(typeof(ViewNavigationService))]
namespace SuperForms.Core.ViewNavigation
{
    public class ViewNavigationService : IViewNavigationService
    {
        private INavigation _navigation;
        private SuperMapper _navigationMapper;

        public void Initialize(INavigation navigation, SuperMapper navigationMapper)
        {
            _navigation = navigation;
            _navigationMapper = navigationMapper;
        }

        public async Task NavigateToAsync(object navigationSource, object parameter = null)
        {
            CheckIsInitialized();

            var type = _navigationMapper.GetTypeSource(navigationSource);

            if (type == null)
            {
                throw new InvalidOperationException(
                    "Can't find associated type for " + navigationSource.ToString());
            }

            ConstructorInfo constructor;
            object[] parameters;

            if (parameter == null)
            {
                constructor = type.GetTypeInfo()
                                  .DeclaredConstructors
                                  .FirstOrDefault(c => !c.GetParameters().Any());

                parameters = new object[] { };
            }
            else
            {
                constructor = type.GetTypeInfo()
                                  .DeclaredConstructors
                                  .FirstOrDefault(c =>
                                    {
                                        var p = c.GetParameters();
                                        return p.Count() == 1 &&
                                            p[0].ParameterType == parameter.GetType();
                                    });

                parameters = new[] { parameter };
            }

            if (constructor == null)
            {
                throw new InvalidOperationException(
                    "No suitable constructor found for page " + navigationSource.ToString());
            }

            var page = constructor.Invoke(parameters) as Page;

            await _navigation.PushAsync(page);
        }

        public async Task GoBackAsync()
        {
            CheckIsInitialized();

            await _navigation.PopAsync();
        }

        private void CheckIsInitialized()
        {
            if (_navigation == null || _navigationMapper == null)
                throw new NullReferenceException("Call Initialize method first.");
        }
    }
}

我得到用户想要导航的页面类型，并使用反射创建它的实例。

然后我可以在视图模型上使用导航服务：

var navigationService = DependencyService.Get<IViewNavigationService>();
await navigationService.NavigateToAsync(NavigationPageSource.Page2, "hello from Page1");