The mediator pattern is useful for managing request/reply responsibilities in an application.
To use the mediator, simply create a custom request reply objects. Send the request to the mediator.
// Set up the kernel.
var kernel = new StandardKernel();
kernel.Bind<IRequestHandler<SayHello, SayHelloReply>>().To<SayHelloHandler>().InThreadScope();
// Create a mediator instance.
var mediator = new NinjectMediator(kernel);
// Send a request to the mediator.
var sayHello = new SayHello("World");
var reply = mediator.Request(sayHello);
Console.WriteLine(reply.Hello);Note: It is not necessary that the objects be immutable.
These classes demonstrate a simple request and reply, to say hello to a given name.
public class SayHello : IRequest<SayHelloReply>
{
private readonly string _name;
public SayHello(string name)
{
_name = name;
}
public string Name
{
get { return _name; }
}
}public class SayHelloReply
{
private readonly string _hello;
public SayHelloReply(string hello)
{
_hello = hello;
}
public string Hello
{
get { return _hello; }
}
}The handler's responsibility is to handle the request.
public class SayHelloHandler : IRequestHandler<SayHello, SayHelloReply>
{
public void Dispose()
{
Dispose(true);
}
public SayHelloReply Handle(SayHello request)
{
var hello = string.Format("Hello, {0}!", request.Name);
return new SayHelloReply(hello);
}
protected virtual void Dispose(bool isDisposing)
{
/* Nothing to do here. */
}
}The most common use of this pattern is for some type of data access, whether a database or service. In the example below, the Product class would be some type of data entity.
Note: This example uses Dapper for database access.
public class GetProductById : IRequest<Product>
{
public GetProductById(int id)
{
Id = id;
}
public int Id { get; private set; }
}
public class GetProductByIdHandler : IRequestHandler<GetProductById, Product>
{
private readonly IDbConnection _connection;
public GetProductByIdHandler(IDbConnection connection)
{
if (connection == null)
throw new ArgumentNullException("connection");
if (connection.State != ConnectionState.Open)
throw new ArgumentException(string.Format("Connection is not open. Connection state is {0}.", connection.State);
_connection = connection;
}
public static string Sql
{
get { return @"SELECT TOP 1 * FROM [dbo].[Products] WHERE [ProductId] = @ProductId";
}
public void Dispose()
{
Dispose(true);
}
public Product Handle(GetProductById request)
{
var p = new
{
ProductId = request.Id
};
var product = _connection.Query<Product>(Sql, p).First();
return product;
}
protected virtual void Dispose(bool isDisposing)
{
if (isDisposing)
{
_connection.Dispose();
}
}
}You have the option to handle a request asynchronously, as well, by using the RequestAsync extension method. Instead of returning TReply, the result of the request is a Task<TReply>.
var sayHello = new SayHello("World");
var answer = mediator.RequestAsync(sayHello);
answer.Wait();
Assert.AreEqual("Hello, World!", answer.Result.Hello);The mediator pattern is right at home with the ActionResult or objects returned by ASP.NET MVC or WebAPI.
public class LogonController
{
private readonly IMediator _mediator;
public LogonController(IMediator mediator)
{
_mediator = mediator;
}
[HttpDelete]
public ActionResult Logoff()
{
var logoff = new Logoff(this);
var reply = _mediator.Request(logoff);
return reply;
}
[HttpGet]
public ActionResult Logon()
{
// The only thing this action does is return a view.
var viewModel = new LogonViewModel();
return View(viewModel);
}
[HttpPost]
public ActionResult Logon(LogonViewModel model)
{
var logon = new Logon(model, this);
var reply = _mediator.Request(logon);
return reply;
}
}Instructions coming "soon".
- Jarrett Meyer (Author)
- Jamie Kurtz
The MIT License (MIT)
Copyright (c) 2014 Fusion Alliance
Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.