I am not a regular with the MVVM pattern and this is basically my first time playing with it.
What I used to do ("normal" WPF), was creating my Views with a Business layer and perhaps a datalayer (which usually contains my entities created by a service or the Entity Framework).
Now after some toying I created a standard template from MVVM Light and did this:
Locator:
public class ViewModelLocator
{
static ViewModelLocator()
{
ServiceLocator.SetLocatorProvider(() => SimpleIoc.Default);
if (ViewModelBase.IsInDesignModeStatic)
{
SimpleIoc.Default.Register<IUserService, DesignUserService>();
}
else
{
SimpleIoc.Default.Register<IUserService, IUserService>();
}
SimpleIoc.Default.Register<LoginViewModel>();
}
public LoginViewModel Login
{
get
{
return ServiceLocator.Current.GetInstance<LoginViewModel>();
}
}
}
Login ViewModel:
public class LoginViewModel : ViewModelBase
{
private readonly IUserService _userService;
public RelayCommand<Object> LoginCommand
{
get
{
return new RelayCommand<Object>(Login);
}
}
private string _userName;
public String UserName
{
get { return _userName; }
set
{
if (value == _userName)
return;
_userName = value;
RaisePropertyChanged("UserName");
}
}
/// <summary>
/// Initializes a new instance of the LoginViewModel class.
/// </summary>
public LoginViewModel(IUserService userService)
{
_userService = userService;
_closing = true;
}
private void Login(Object passwordBoxObject)
{
PasswordBox passwordBox = passwordBoxObject as PasswordBox;
if (passwordBox == null)
throw new Exception("PasswordBox is null");
_userService.Login(UserName, passwordBox.SecurePassword, result =>
{
if (!result)
{
MessageBox.Show("Wrong username or password");
}
});
}
}
Binding and commands work fine so there is no questions. Business mockup class for design and test time:
public class DesignUserService : IUserService
{
private readonly User _testUser;
private readonly IList<User> _users;
public void Login(String userName, SecureString password, Action<Boolean> callback)
{
var user = _users.FirstOrDefault(u => u.UserName.ToLower() == userName.ToLower());
if (user == null)
{
callback(false);
return;
}
String rawPassword = Security.ComputeHashString(password, user.Salt);
if (rawPassword != user.Password)
{
callback(false);
return;
}
callback(true);
}
public DesignUserService()
{
_testUser = new User
{
UserName = "testuser",
Password = "123123",
Salt = "123123"
};
_users = new List<User>
{
_testUser
};
}
}
UserData is a static class which makes calls to the database (Entity Framework).
Now I have my test:
[TestClass]
public class Login
{
[TestMethod]
public void IncorrectUsernameCorrectPassword()
{
IUserService userService = new DesignUserService();
PasswordBox passwordBox = new PasswordBox
{
Password = "password"
};
userService.Login("nonexistingusername", passwordBox.SecurePassword, b => Assert.AreEqual(b, false));
}
}
Now my test is not on the ViewModel itself but directly to the Business layer.
Basically I have 2 questions:
Am I on the right path, or is there a fundamental flaw in my pattern implementation?
How can I test my ViewModel?
Your view model has one relevant piece of code worth testing, which is Login method. Given that it's private, it should be tested it via LoginCommand.
Now, one might ask, what is the purpose of testing command when you already have test for underlying business logic? The purpose is to verify that business logic is called and with correct parameters.
How do one goes with such test? By using mock. Example with FakeItEasy:
var userServiceFake = A.Fake<IUserService>();
var testedViewModel = new LoginViewModel(userServiceFake);
// prepare data for test
var passwordBox = new PasswordBox { Password = "password" };
testedViewModel.UserName = "TestUser";
// execute test
testedViewModel.LoginCommand.Execute(passwordBox);
// verify
A.CallTo(() => userServiceFake.Login(
"TestUser",
passwordBox.SecurePassword,
A<Action<bool>>.Ignored)
).MustHaveHappened();
This way you verify that command calls business layer as expected. Note that Action<bool> is ignored when matching parameters - it's difficult to match Action<T> and Func<T> and usually not worth it.
Few notes:
You might want to reconsider having message box code in view model (this should belong to view, view model should either request or notify view to display popup). Doing so, will also make more through testing of view model possible (eg. not needing to ignore that Action argument)
Some people do test INotifyPropertyChanged properties (UserName in your case) - that event is raised when property value changes. Since this is lot of boilerplate code, using tool /library to automate this process is highly suggested.
You do want to have two sets of tests, one for view model (as in the example above) and one for underlying business logic (your original test). In MVVM, VM is that extra layer which might seem to be of little use - but that's the whole point - to have no business logic there and rather focus data rearrangement/preparation for views layer.
Related
Below is a simple but functional example of roughly how I would do Dependency Injection. This works great when my DbContext connection string is not dynamic. Even if it's passed in to the factory through a config file or whatever, it doesn't matter so long as it's the same one all the time.
What I need is to wrap my head around how to make (ideally minor) modifications to the below code to allow for the connection string to be determined dynamically at run time.
For example, say on the View the user was able to not only select the teacher to be passed into the Post method of the controller, but also the school. If, for simplicity sake, there are 2 schools that have the exact same database structure, but have different connection strings how do I get that down from the controller to the factory?
I've experimented with passing a value from method to method, but this isn't really sustainable for large projects, increases the likelihood of errors and overall is just messy (besides violations of SOLID) to be passing something from layer to layer like that. (If desired I can add the not exactly ideal attempts I've made, I've omitted them for brevity sake since this is already a fairly long question what with the code examples and all).
Controller
public class HomeController : Controller
{
private readonly IDataService _dataService;
public HomeController(IDataService dataService)
{
_dataService = dataService;
}
public ActionResult Index()
{
var results = _dataService.GetTeachers();
var model = new ViewModel
{
Teachers = results
};
return View(model);
}
[HttpPost]
public ActionResult Index(ViewModel model)
{
var results = _dataService.GetCourses(model.Teacher);
model.Courses = new List<string>(results);
return View(model);
}
}
Service
public class DataService : IDataService
{
private readonly IDataRepo _dataRepo;
public DataService(IDataRepo dataRepo)
{
_dataRepo = dataRepo;
}
public List<string> GetCourses(string teacherName)
{
return _dataRepo.GetCourses()
.Where(c => c.Teacher.FirstName == teacherName)
.Select(c => c.Name)
.ToList();
}
public List<string> GetTeachers()
{
return _dataRepo.GetCourses()
.Select(c => c.Teacher.FirstName)
.ToList();
}
}
Repository
public class DataRepo : IDataRepo
{
private readonly SchoolContext _context;
public DataRepo()
{
_context = ContextFactory.MakeContext();
}
public IEnumerable<Course> GetCourses()
{
return _context.Courses;
}
}
Context Factory
public static class ContextFactory
{
public static SchoolContext MakeContext()
{
var connString =
"connStringA";
return new SchoolContext(connString);
}
}
UnityConfig
public static void RegisterComponents()
{
var container = new UnityContainer();
container.RegisterType<IDataService, DataService>();
container.RegisterType<IDataRepo, DataRepo>();
DependencyResolver.SetResolver(new UnityDependencyResolver(container));
}
First, you have to decide how are you going to get the current connection string to use. Is it through a URL? or using the current user or whatever other way.
Then, create another database that has a mapping between the connection strings and the method you chose (user, url ...)
Lastly, implement a way to get the record from the database.
so, assuming that you will use the URL as an identifier for the current tenant, your entity class should be like this:
public class Tenant
{
public string Url {get;set;}
public string ConnectionString {get;set;}
}
An interface that represents the logic to get the current tenant:
public interface ICurrentTenantService
{
Tenant GetCurrentTenant();
}
And now you will put its implementation
public class CurrentTenantService : ICurrentTenantService
{
public Tenant GetCurrentTenant()
{
string currentUrl = HttpContext.Current.Url; //make sure to get only the base URL here
return TenantDbContext.Tenants.FirstOrDefault(t=>t.Url == url); //TenantDbContext should be a class that has the Tenant entity
}
}
Now you have to wire up the context factory to the tenant service like this
public static class ContextFactory
{
private readonly ICurrentTenantService currentTenantService;
//Inject it in the constructor
public static SchoolContext MakeContext()
{
var currentTenant= currentTenantService.GetCurrentTenant(); //Check for NULL
return new SchoolContext(currentTenant.ConnectionString);
}
}
I have created a struct on validating models on Business Layer which is based on Steven's answer.
It is working well but something confuses my mind. I inject UserService in CreateUserValidator to able to use GetUser method. This means I call validator in UserService and create a new UserService instance to check whether user exist.
UserService -> [ValidateUser -> new UserService().GetUser()]
It works but seems to be a very bad design. But I have to use that method.
Could you please let me know how I can solve this problem, or Shouldn't I worry about it?
public class CreateUser
{
public string Name { get; set; }
public string Email { get; set; }
}
public sealed class CreateUserValidator : Validator<CreateUser>
{
private IUserService _userService;
public CreateUserValidator(IUserService userService)
{
_userService = userService;
}
protected override IEnumerable<ValidationResult> Validate(
CreateUser entity)
{
var user = _userService.GetUserByEmail(entity.Email);
if (user != null)
{
yield return new ValidationResult("Email", "Email address is already exist!");
}
}
}
UserService.cs
public partial class UserService : IUserService
{
IGenericUnitofWork _uow = null;
private readonly IValidationProvider _validationProvider;
public UserService(IGenericUnitofWork uow, IValidationProvider validationProvider)
{
_uow = uow;
_validationProvider = validationProvider;
}
public User CreateUser(CreateUser createUser)
{
this._validationProvider.Validate(createUser);
var user = new User()
{
Email = createUser.Email,
Name = createUser.Name,
};
_uow.Repository<User>().Insert(User);
_uow.SaveChanges();
return user;
}
public User GetUser(string email)
{
var user = _uow.Repository<User>().Where(m => m.Email == email).FirstOrDefault();
return user;
}
}
You dependency graph is cyclic. As described in section 6.3 of Dependency Injection in .NET second edition, dependency cycles are often caused by Single Responsibility Principle violations, as is the case in your design.
The problem is that UserService has too many responsibilities: Creating a user is a different responsibility than getting a user. Creating a user can become a very complex use case, as the validation logic hints at, while getting a user is something typically quite simple. It would therefore be beneficial to split UserService into multiple smaller classes. This would allow the validator to depend on the service that allows retrieving the user by its mail address, while the 'create user' service can depend on the validator.
To take it even one step further, you might want to remove validation from the 'create user' service completely. Validation is a cross-cutting concern, and mixing it with the class that contains the business logic, makes such class harder to maintain.
A design that might benefit you is one where you place all state changing business actions behind a common abstraction, as described here.
My current implementation of passing UserID in my application is through the constructor.
i.e. SomeObject s = new SomeObject(userID)
Where in there is a code behind that does things based on the userID. The userID is further keep tracked by adding another property named "CurrentUser", however this seems to be a dirty solution as I have to implement it to all ViewModels and it seems to violate the "DRY" concept.
The second approach I have in mind is creating a public static variable on my MainWindowViewModel where all my other models can refer to it as MainWindowViewModel.CurrentUser.
Is one of the two approach the correct way to do this or is there a better approach that i don't know about?
You need to carefully analyze up front what you want to achieve with your application. Are you happy with there only ever being one selected client? Or will you need to have multiple clients being viewed or edited at a time (i.e. you have an MDI style app)?
Going with the single client approach is easy, you can implement the global property bag as already mentioned in other answers. But I will advise caution: if you build your app on the assumption there will only ever be one selected client it becomes a real PITA to try to refactor to make it multi-client capable. Using a centralized property bag or "session service" like this is indeed decoupling state from the VM, but the centralized service can still turn into a monstrosity over time and you build up too much dependence on it.
If you do want to go the multi-client route, then you are on the right track - but instead of passing a client identifier in on the constructor, pass (inject) the entire client data object. The chances are that you already have most of the client details available from the piece of UI that invokes the client oriented VM, so pass it in and save having to make another trip to your database to get the details.
Don't tie a current user to a ViewModel. I typically opt for a SessionService of some kind. If you're using Dependency Injection (DI), register a singleton of an ISessionService and concrete implementation. If your not using DI, then just have your app start create a singleton, like a SessionService.Current. Then you can put any items you need in here. Then each ViewModel can ask for the SessionService.Current.User and they have it. Your ViewModels shouldn't know about each other, but they can know about services. This keeps it DRY and loosely coupled, especially if you only access these session variables using the interface of an ISessionService and not the concrete implementation. This allows you to mock one up very easily without changing any ViewModel code.
What you have here is the problem of Communication between ViewModels. There are a number of solutions but my fave is the Mediator Pattern:
using System;
namespace UnitTestProject2
{
public class GetDataViewModel
{
IMediator mediator;
public GetDataViewModel(IMediator mediator)
{
this.mediator = mediator;
this.mediator.ListenFor("LoggedIn", LoggedIn);
}
protected string UserId;
protected void LoggedIn(Object sender, EventArgs e)
{
UserId = sender.ToString();
}
}
public class LoginViewModel
{
IMediator mediator;
public LoginViewModel(IMediator mediator)
{
this.mediator = mediator;
}
public string UserId { get; set; }
public void Login(string userid)
{
this.UserId = userid;
this.mediator.RaiseEvent("LoggedIn", this.UserId);
}
}
public interface IMediator
{
public void ListenFor(string eventName, EventHandler action );
public void RaiseEvent(string eventName, object data);
}
}
I Haven't implemented the Mediator here, because it can get quite involved and there are a number of packages available. but you can see the idea from my simple interface. Essentially the Mediator provides a Global list of EventHandlers which any Viewmodel can call or add to. You still have the problem of where to store the event names. Its nice to have these in enums, but that gives you a coupling problem. (a problem I usually ignore)
Alternatively you can have a Controller or (MasterViewModel if you love MVVM)
using System;
namespace UnitTestProject3
{
public class GetDataViewModel
{
protected string UserId;
public void LoggedIn(Object sender, EventArgs e)
{
UserId = sender.ToString();
}
}
public class LoginViewModel
{
public EventHandler OnLogin;
public string UserId { get; set; }
public void Login(string userid)
{
this.UserId = userid;
if (this.OnLogin != null)
{
this.OnLogin(this.UserId, null);
}
}
}
public class Controller // or MasterViewModel
{
public void SetUp()
{
GetDataViewModel vm1 = new GetDataViewModel();
LoginViewModel vm2 = new LoginViewModel();
vm2.OnLogin += vm1.LoggedIn;
//wire up to views and display
}
}
}
I'm studding MVVM in C#.
I want to use Inversion of Control (IoC). I use the framework Unity.
I don't understand how to handling exception that could be raised from Data Access Layer.
Here a little easy example I do for study:
-- i have omitted the manage of Model validation (IDataErrorInfo) and Services for ViewModel (ex: DialogService) --
XAML View
<TextBox ... Text="{Binding Path=Id}" />
<TextBox ... Text="{Binding Path=Name}"/>
DESIGN APPLICATION
MODEL
{
public class User : INotifyPropertyChanged
{
private int _id;
public int Id
{
get{return _id;}
set
{
_id = value;
this.OnPropertyChanged("Id");
}
}
private string _name;
public string Name
{
get{return _name;}
set
{
_name = value;
this.OnPropertyChanged("Name");
}
}
public User(int i, string n)
{
_id = i;
_name = n;
}
public event PropertyChangedEventHandler PropertyChanged;
protected void OnPropertyChanged(string name)
{
if (PropertyChanged != null)
{
PropertyChanged(this, new PropertyChangedEventArgs(name));
}
}
}
}
DATA ACCESS LAYER
Interface
public interface IDataAccessLayer
{
Model.User GetUser();
}
Concrete class
public class ConcreteDataAccessLayer : IDataAccessLayer
{
public ConcreteDataAccessLayer(){}
Model.User IDataAccessLayer.GetUser()
{
//could throw Exception connecting with data source
}
}
BUSINESS LAYER
public class BusinessLayer
{
public BusinessLayer(IDataAccessLayer dataAccessLayer)
{
if (dataAccessLayer == null)
{
throw new ArgumentNullException("dataAccessLayer");
}
this._dataAccessLayer = dataAccessLayer;
}
private IDataAccessLayer _dataAccessLayer;
private QuestionStak.Model.User _user;
internal QuestionStak.Model.User User
{
get
{
if (_user == null)
_user = _dataAccessLayer.GetUser();
return _user;
}
}
}
VIEWMODEL
public class ViewModel : INotifyPropertyChanged
{
public ViewModel(BusinessLayer bl)
{
if (bl == null)
{
throw new ArgumentNullException("BusinessLayer");
}
_businessLayer = bl;
}
private BusinessLayer _businessLayer;
public int Id
{
get
{
return _businessLayer.User.Id;
}
set
{
_businessLayer.User.Id = value;
}
}
public string Name
{
get
{
return _businessLayer.User.Name;
}
set
{
_businessLayer.User.Name = value;
}
}
public event PropertyChangedEventHandler PropertyChanged;
}
APPLICATION
public partial class App : Application
{
protected override void OnStartup(StartupEventArgs e)
{
base.OnStartup(e);
//Inversion of control
IUnityContainer container = new UnityContainer();
//Created as singleton
container.RegisterType<IDataAccessLayer, ConcreteDataAccessLayer>(new ContainerControlledLifetimeManager());
container.RegisterType<BusinessLayer, BusinessLayer>(new ContainerControlledLifetimeManager());
MainWindow win = container.Resolve<MainWindow>();
win.DataContext = container.Resolve<ViewModel>();
win.Show();
}
}
Principles follow
simple constructors (Unity catch all exception and follow http://blog.ploeh.dk/2011/03/03/InjectionConstructorsshouldbesimple/)
ViewModel must hide Model structure
So I have a problem that I don't understands how to solve:
If ConcreteDataAccessLayer can't load data (Ex: server not available) during loading of ViewModel the statement _dataAccessLayer.GetUser() throw the exception and in could not manage it (catch by Unity conteiner)
If somewhere during the loading I manage the exception, the data binding cause the throw of a null exception because _businessLayer.User is null (unable to load the view)
Please, have someone a clean solution for this problem?
Thanks!
If ConcreteDataAccessLayer can't load data (Ex: server not available) during loading of ViewModel the statement _dataAccessLayer.GetUser() throw the exception and in could not manage it (catch by Unity conteiner)
I'm not sure what you mean by 'in could not manage it (catch by Unity container)', as I would only expect a wrapped Unity exception if there was a problem constructing the types that Unity is resolving. Either way, you would still be able to handle the exception yourself. You would probably want to do this in your presentation layer, so that you could bring up a dialog etc.
If somewhere during the loading I manage the exception, the data binding cause the throw of a null exception because _businessLayer.User is null (unable to load the view)
Yes, if you've handled the error in your business layer, then you would need to guard against the User property being null.
However, I think your approach should be reconsidered. Your IDataAccessLayer and BusinessLayer types seem like big dependencies which is going to minimise the amount of code reuse, as well as making unit testing more difficult.
You should try and minimise the dependencies. If your view model is only interested in users for example, then you could use the repository pattern (well really a data access object pattern) to inject a user repository.
Alternatively if you wish to use rich business objects, then your view models would take your user business object as the dependency. You may then decide to expose the model directly to the view, depending on if you wish to violate the DRY principle (which you are currently doing), or the Law of Demeter.
I would also consider using an MVVM framework if you're using the MVVM design pattern.
I have a small webapp that uses EntityFramework to store stuff via repositories into the database.
What I've done so far (based on all the tutorials I read) is create a repository where I need it, as shown below:
In CustomMembershipProvider:
public CustomMembershipProvider()
{
_userRepository = new UserRepository(new TenantApplicationContext());
}
In my RegisterController:
public TenantRepository TenantRepository { get; set; }
public UserRepository UserRepository { get; set; }
protected override void Initialize(RequestContext requestContext)
{
if (MembershipService == null) { MembershipService = new AccountMembershipService(); }
if (TenantRepository == null) { TenantRepository = new TenantRepository(TenantApplicationContext); }
if (UserRepository == null) { UserRepository = new UserRepository(TenantApplicationContext); }
base.Initialize(requestContext);
}
The point is, that I instantiate the UserRepository twice. This becomes a problem when I create a User in one instance, and try to retrieve it in the other instance, and I did not call SaveChanges in between.
The problem lies here:
// Snippet from the register controller class
if (!UserRepository.Exists(model.AccountableEmailAddress))
{
// 1 - Create the user via a custom MembershipProvider
// Note, the CustomMembershipProvider has it's own instance of UserRepository
var createStatus = MembershipService.CreateUser(
model.AccountableUser,
model.Password,
model.AccountableEmailAddress);
if (createStatus == MembershipCreateStatus.Success)
{
// Left out irrelevant code
AdministerUserAndTenant(tenant.Name, model.AccountableEmailAddress);
}
}
private void AdministerUserAndTenant(string tenantName, string emailAddress)
{
// 2 - Try to retrieve the user from a different (!) instance of UserRepository
var user = UserRepository.GetUser(emailAddress);
var tenant = TenantRepository.GetTenantByName(tenantName);
tenant.Users.Add(user);
TenantApplicationContext.SaveChanges();
}
I hope you can still follow, tried to leave out unnecessary parts.
What is the best way to deal with issues like this?
PS: I'm not very fond of the Singleton pattern, so if possible don't go there :).
When exactly does it become a problem? Cause that's where the answer lies. Classes that should know of each other's unsaved changes should use the same repository instance. Since they are probably related, you'll manage passing a reference between them.
If there's reason why all of your application should have one single repository, use Dependency Injection.