MvxException：无法从定位器

Question

拜访Xamarin和MvvmCross专家。我已经尝试调试MvvmCross.Exceptions.MvxException: Failed to construct and initialize ViewModel for type TestApp.Core.ViewModels.IngredientsViewModel from locator MvxDefaultViewModelLocator几个小时的错误了。我发现了实际上导致此错误的那段代码，这与使用EFCore sqlite从数据库中提取数据有关。

这是我第一次将其用于移动应用程序，因此也许您可能会发现我看不到的东西。以下是我认为足够的所有信息，如果需要更多信息，请告诉我！我希望该解决方案可以帮助其他人。 请注意，当我将GetIngredients注释掉时，我没有得到上面的错误。

我在核心/共享项目中的App文件

public class AppCore : MvxApplication
{
    public override void Initialize()
    {
        Mvx.IoCProvider.RegisterType<TestContext>();
        Mvx.IoCProvider.RegisterType<IIngredientRepository, IngredientRepository>();
        RegisterAppStart<IngredientsViewModel>();
    }
}

我的DbContext

public class TestContext : DbContext
{
    public TestContext() : base()
    {
    }
    private const string _databaseName = "Test.db";
    public DbSet<Ingredient> Ingredients { get; set; }    
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        string databasePath;
        switch (Device.RuntimePlatform)
        {
            case Device.iOS:
                SQLitePCL.Batteries_V2.Init();
                databasePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments), "..", "Library", _databaseName); ;
                break;
            case Device.Android:
                databasePath = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Personal), _databaseName);
                break;
            default:
                throw new NotImplementedException("Platform not supported");
        }
        optionsBuilder.UseSqlite($"Filename={databasePath}");
    }
}

存储库

public interface IRepository<TEntity> where TEntity : class
{
        IEnumerable<TEntity> GetAll();
}

public class Repository<TEntity> : IRepository<TEntity> where TEntity : class
{
    protected readonly TestContext Context;
    public Repository(TestContext context)
    {
        Context = context;
    }

    public IEnumerable<TEntity> GetAll()
    {
        return Context.Set<TEntity>().ToList(); //after this line error is thrown
    }
}

public interface IIngredientRepository : IRepository<Ingredient> {}
public class IngredientRepository : Repository<Ingredient>, IIngredientRepository
{
    public IngredientRepository(TestContext testContext) : base(testContext)
    { }
}

我的ViewModel

public class IngredientsViewModel : BaseViewModel
{
    private IIngredientRepository IngredientRepository { get; set; }

    public IngredientsViewModel(IIngredientRepository ingredientRepository)
    {
        IngredientRepository = ingredientRepository;
        GetIngredients(); //when commented out, the view loads fine
    }

    private void GetIngredients()
    {
        var ingredients = IngredientRepository.GetAll();
        Ingredients = new MvxObservableCollection<Ingredient>(ingredients);
    }

    private MvxObservableCollection<Ingredient> _ingredients;
    public MvxObservableCollection<Ingredient> Ingredients
    {
        get => _ingredients;
        set { SetProperty(ref _ingredients, value); }
    }
}

Answer 1

在您的TestContext.cs中，尝试在构造函数中添加以下行：

Database.EnsureCreated(); //returns boolean which could be used for some check if needed

而且，我的建议是像这样重写OnModelCreating：

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
        base.OnModelCreating(modelBuilder);

        //creating tables according to UML
        modelBuilder.Entity<Ingredient>();

        //add one for each table
}

如果这样做没有帮助，请尝试检查InnerException或逐步调试。