Question

我有以下类和接口结构，我很难尝试让代码完成我需要的工作。

public interface IUserManager
{
    int Add(User user);
}

public class UserManagerA : IUserManager{}
public class UserManagerB : IUserManager{}

在这个例子中，我使用Ninject作为IoC容器，但如果其他容器解决了这个问题，我可以更改它：

这是我的NinjectWebCommon.cs：

void RegisterServices(IKernel kernel)
{
    string userRole = CurrentUser.Role;//this gets the user logged in
    //This is the part I do not how to do
    //I wish I could just type this in:
    kernel.Bind<IUserManager>().To<UserManagerA>()
        .When(userRole == "RoleA"); // this doesn't work obviously
    kernel.Bind<IUserManager>().To<UserManagerB>()
        .When(userRole == "RoleB"); // same doesn't work
}

所有这一切，以便在我的（MVC）控制器中我可以这样做：

public class UserController
{
    private readonly IUserManager _userManager;
    public UserController(IUserManager userManager)
    {
        _userManager = userManager;
    }
    public ActionResult Add(User user)
    {
        //this would call the correct manager
        //based on the userRole
        _userManager.Add(user);
    }
}

我一直在阅读有关抽象工厂的文章，但没有找到解释如何将工厂与IoC容器集成并传递在运行时获得的参数来解决实现的文章。

Answer 1

创建一个负责提供正确UserManager的类并将其注入您的控制器：

public class UserManagerProvider : IUserManagerProvider
{
    private readonly IContext _context;

    public UserManagerProvider(IContext context)
    {
        _context = context;
    }

    public IUserManager Create(User currentUser)
    {
        if (currentUser.Role == "User A")
            return _context.Kernel.Get<UserManagerA>();

        if (currentUser.Role == "User B")
            return _context.Kernel.Get<UserManagerB>();

        // Or bind and resolve by name
        // _context.Kernel.Get<IUserManager>(currentUser.Role);
    }
}

在控制器中：

private readonly IUserManager _userManager;

public UserController(IUserManagerProvider userManagerProvider)
{
    _userManager = userManagerProvider.Create(CurrentUser);
}

另外，作为旁注，您应该有CurrentUserProvider负责获取当前用户。依赖静态方法会使单元测试变得困难，并且您实际上在所有引用它的类中隐藏了依赖项：

private readonly IUserManager _userManager;
private readonly User _currentUser;

public UserController(IUserManagerProvider userManagerProvider, ICurrentUserProvider currentUserProvider)
{
    _currentUser = currentUserProvider.GetUser();
    _userManager = userManagerProvider.Create(_currentUser);
}

Answer 2

如果IUserManager实现的数量不是很多（不太可能达到100个实现），您可以使用Strategy Pattern在合成期间解析所有UserManager实例，然后选择最佳实例在运行时使用。

首先，我们需要一种方法将IUserManager实现映射到角色。

public interface IUserManager
{
    int Add(User user);
    bool AppliesTo(string userRole);
}

public class UserManagerA : IUserManager
{
    // Add method omitted

    public bool AppliesTo(string userRole)
    {
        // Note that it is entirely possible to 
        // make this work with multiple roles and/or
        // multiple conditions.
        return (userRole == "RoleA");
    }
}

public class UserManagerB : IUserManager
{
    // Add method omitted

    public bool AppliesTo(string userRole)
    {
        return (userRole == "RoleB");
    }
}

然后我们需要一个策略类，它只根据userRole选择正确的实例。组成应用程序时，DI容器会提供IUserManager个实例。

public interface IUserManagerStrategy
{
    IUserManager GetManager(string userRole);
}

public class UserManagerStrategy
    : IUserManagerStrategy
{
    private readonly IUserManager[] userManagers;

    public UserManagerStrategy(IUserManager[] userManagers)
    {
        if (userManagers == null)
            throw new ArgumentNullException("userManagers");

        this.userManagers = userManagers;
    }

    public IUserManager GetManager(string userRole)
    {
        var manager = this.userManagers.FirstOrDefault(x => x.AppliesTo(userRole));
        if (manager == null && !string.IsNullOrEmpty(userRole))
        {
            // Note that you could optionally specify a default value
            // here instead of throwing an exception.
            throw new Exception(string.Format("User Manager for {0} not found", userRole));
        }

        return manager;
    }
}

<强>用法

public class SomeService : ISomeService
{
    private readonly IUserManagerStrategy userManagerStrategy;

    public SomeService(IUserManagerStrategy userManagerStrategy)
    {
        if (userManagerStrategy == null)
            throw new ArgumentNullException("userManagerStrategy");
        this.userManagerStrategy = userManagerStrategy;
    }

    public void DoSomething()
    {
        string userRole = CurrentUser.Role;//this gets the user logged in

        // Get the correct UserManger according to the role
        IUserManager userManager = this.userManagerStrategy.GetManger(userRole);

        // Do something with userManger
    }
}

void RegisterServices(IKernel kernel)
{
    kernel.Bind<IUserManager>().To<UserManagerA>();
    kernel.Bind<IUserManager>().To<UserManagerB>();

    // Ninject will automatically supply both IUserManager instances here
    kernel.Bind<IUserManagerStrategy>().To<UserManagerStrategy>();

    kernel.Bind<ISomeService>().To<SomeService>();
}

此方法不要求您将容器注入应用程序。没有使用服务地点。

另请注意，每次向应用程序添加新UserManager时都不需要修改switch case语句。何时使用UserManager的逻辑是UserManager实现的一部分，逻辑执行的顺序由DI配置决定。

此外，无论您使用哪个DI容器，这都可以使用。

您可以将此与RagtimeWilly的回答中的CurrentUserProvider结合使用，以便将用户角色转换为使用此角色的服务。

参考：Best way to use StructureMap to implement Strategy pattern

抽象工厂和控制反转在运行时解决

2 个答案: