CODEWITHSUNDEEP
c#asp.net-mvc
pek
pek

Reputation: 18035

Get instance of a class using generics

I'm working on a game that uses MVCS and has, so far, clearly separated the business logic from the view.

However, I've been having trouble with one particular piece of the puzzle.

In the game we have command classes (IPlayerCommand) that execute a specific business logic. Each command class returns a result class (PlayerCommandResult). For each PlayerCommand we have a respected visual command class (IVisualPlayerCommand) that takes the PlayerCommandResult and updates the view accordingly.

I'd like the IVisualPlayerCommand to use specific classes that inherit PlayerCommandResult in order to get the information it needs (as opposed to using object). I'd also like to make it compile-time safe (as opposed to casting it before using it). For these two reasons I made the classes use generics.

Here are the declaration of the classes:

public class PlayerCommandResult
{}

public interface IPlayerCommand<T> where T : PlayerCommandResult
{
    T Execute(GameWorld world);
}

public interface IVisualPlayerComamnd<T> where T : PlayerCommandResult
{
    void Play(T commandResult);
}

Here is the Move Unit command as an example:

public class MoveUnitPlayerCommand : IPlayerCommand<MoveUnitPlayerCommandResult>
{
    private Unit unitToMove;

    public MoveUnitPlayerCommand(Unit unit)
    {
        this.unitToMove = unit;
    }

    public MoveUnitPlayerCommandResult Execute(GameWorld world)
    {
        MoveUnitPlayerCommand result = new MoveUnitPlayerCommand();
        // Do some changes to the world and store any information needed to the result
        return result;
    }
}

public class MoveUnitVisualPlayerCommand : IVisualPlayerCommand<MoveUnitPlayerCommandResult>
{
    void Play(MoveUnitPlayerCommandResult commandResult)
    {
        // Do something visual
    }
}

public class MoveUnitPlayerCommandResult : PlayerCommandResult
{
    public Unit TargetUnit { get; private set; }
    public Path MovePath { get; private set; }
}

So far, so good. However, I'm having a really hard time tying a IPlayerCommand to a IVisualPlayerCommand because of the use of generics:

public class CommandExecutorService
{
    public void ExecuteCommand<T>(IPlayerCommand<T> command) where T : PlayerCommandResult
    {
        T result = command.Execute(world);
        IVisualPlayerCommand<T> visualCommand = GetVisualPlayerCommand(command);
        visualCommand.Play(result);
    }

    public IVisualPlayerCommand<T> GetVisualPlayerCommand<T>(IPlayerCommand<T> command) where T : PlayerCommandResult
    {
        // ?!?!?!?!?!??!?!?!??!?!
    }
}

I have a feeling that what I'm trying to do is not even possible because of the way generics work in C# (as opposed to Java where I could say IVisualPlayerCommand<?>).

Could you help me figure out a way?

Any feedback for the design is welcome.

P.S. Sorry if the title doesn't reflect the question. I wasn't sure how to boil down the question in one line.
P.P.S. Which is why I also don't know if this question has been asked and answered before.

Upvotes: 0

Views: 104

Answers (1)

Matt
Matt

Reputation: 6050

You two command classes, are served as service. To me, for this case, I would use the service locator pattern. As how to implement this pattern, you can check this link

The drawback of using template, is that, if something changes, you have to compiled it again.

Here's link which provides an example of the service locator pattern.

So for you code, you want find the corresponding instance of IVisualPlayerCommand to IPlayerCommand, so the concrete service can inherit from both interface, which it actually implements the IVisualPlayerCommand interface, while the IPlayerCommand just severs as a tag.

so the code will like this:

class MoveUnitVisualPlayerCommand: IVisualPlayerCommand, IPlayerCommand {} 
services = new Dictionary<object, object>();
this.services.Add(typeof(IPlayerCommand ), new MoveUnitVisualPlayerCommand());

as how to get the service, you can refer the example.

Hope this helps.

Upvotes: 1

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