Custom Handlers

Handlers

Handlers are responsible for analyzing HTTP requests and to serve responses to the connected client. All features available in the SDK are either provided by handlers or concerns.

If you would like to add an additional feature that cannot be achieved using the existing functionality to the SDK or your own web application, you may implement a custom handler instance. The following example will generate a simple text response:

public class CustomHandler : IHandler
{ 

    public IHandler Parent { get; }

    public CustomHandler(IHandler parent)
    {
        Parent = parent;
    }

    public ValueTask<IResponse?> HandleAsync(IRequest request)
    {
        var response = request.Respond()
                              .Content(Resource.FromString("Hello World"))
                              .Type(new FlexibleContentType(ContentType.TextPlain))
                              .Build();

        return new ValueTask<IResponse?>(response);
    }

    public ValueTask PrepareAsync() 
    {
        // perform CPU or I/O heavy work to initialize this
        // handler and it's children
        return new ValueTask();
    }

    public IEnumerable<ContentElement> GetContent(IRequest request) => Enumerable.Empty<ContentElement>();

}

public class CustomHandlerBuilder : IHandlerBuilder<CustomHandlerBuilder>
{
    private readonly List<IConcernBuilder> _Concerns = new List<IConcernBuilder>();

    public CustomHandlerBuilder Add(IConcernBuilder concern)
    {
        _Concerns.Add(concern);
        return this;
    }

    public IHandler Build(IHandler parent)
    {
        return Concerns.Chain(parent, _Concerns, (p) => new CustomHandler(p));
    }

}

Host.Create()
    .Handler(new CustomHandlerBuilder())
    .Run();

As handlers are invoked for every request handled by the server, it is usually worth it to optimize them for performance. For example, as the content served by our CustomHandler does not change depending on the request, the string resource instance could be cached by a field or property set in the constructor.