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Shuttle.Core.Pipelines 

PM> Install-Package Shuttle.Core.Pipelines

Observable event-based pipelines based broadly on pipes and filters.

The Pipeline class is defined in the Shuttle.Core.Pipelines package.

A Pipeline is a variation of the pipes and filters pattern and consists of 1 or more stages that each contain one or more events. When the pipeline is executed each event in each stage is raised in the order that they were registered. One or more observers should be registered to handle the relevant event(s).

Each Pipeline always has its own state that is simply a name/value pair with some convenience methods to get and set/replace values. The State class will use the full type name of the object as a key should none be specified:

var state = new State();
var list = new List<string> {"item-1"};

state.Add(list); // key = System.Collections.Generic.List`1[[System.String...]]
state.Add("my-key", "my-key-value");

Console.WriteLine(state.Get<List<string>>()[0]);
Console.Write(state.Get<string>("my-key"));

Example

Events have to derive from PipelineEvent.

We will use the following events:

public class OnAddCharacterA : PipelineEvent
{
}

public class OnAddCharacter : PipelineEvent
{
    public char Character { get; private set; }

    public OnAddCharacter(char character)
    {
        Character = character;
    }
}

The OnAddCharacterA event represents a very explicit event whereas with the OnAddCharacter event one can specify some data. In this case the character to add.

In order for the pipeline to process the events we will have to define one or more observers to handle the events. We will define only one for this sample but we could very easily add another that will handle one or more of the same, or other, events:

public class CharacterPipelineObserver : 
    IPipelineObserver<OnAddCharacterA>,
    IPipelineObserver<OnAddCharacter>
{
    public void Execute(OnAddCharacterA pipelineEvent)
    {
        var state = pipelineEvent.Pipeline.State;
        var value = state.Get<string>("value");

        value = string.Format("{0}-A", value);

        state.Replace("value", value);
    }

    public void Execute(OnAddCharacter pipelineEvent)
    {
        var state = pipelineEvent.Pipeline.State;
        var value = state.Get<string>("value");

        value = string.Format("{0}-{1}", value, pipelineEvent.Character);

        state.Replace("value", value);
    }
}

Next we will define the pipeline itself:

var pipeline = new Pipeline();

pipeline.RegisterStage("process")
    .WithEvent<OnAddCharacterA>()
    .WithEvent(new OnAddCharacter('Z'));

pipeline.RegisterObserver(new CharacterPipelineObserver());

pipeline.State.Add("value", "start");
pipeline.Execute();

Console.WriteLine(pipeline.State.Get<string>("value")); // outputs start-A-Z

We can now execute this pipeline with predictable results.

Pipelines afford us the ability to better decouple functionality. This means that we could re-use the same observer in multiple pipelines enabling us to compose functionality at a more granular level.

Extending Pipelines

Pipelines are usually instantiated using an implementation of the IPipelineFactory interface which would also provide you with the following event handlers:

event EventHandler<PipelineEventArgs> PipelineCreated;
event EventHandler<PipelineEventArgs> PipelineObtained;
event EventHandler<PipelineEventArgs> PipelineReleased;

We would make any alterations to a pipeline, such as adding an observer, when a PipelineCreated event is raised. An example of how one may go about doing is is available in the Shuttle.Esb.Module.ActiveTimeRange repository:

using System;
using Shuttle.Core.Contract;
using Shuttle.Core.Pipelines;

namespace Shuttle.Esb.Module.ActiveTimeRange
{
    public class ActiveTimeRangeModule
    {
        private readonly ActiveTimeRange _activeTimeRange;
        private readonly string _shutdownPipelineName = typeof(ShutdownPipeline).FullName;
        private readonly string _startupPipelineName = typeof(StartupPipeline).FullName;

        public ActiveTimeRangeModule(IPipelineFactory pipelineFactory,
            IActiveTimeRangeConfiguration activeTimeRangeConfiguration)
        {
            Guard.AgainstNull(pipelineFactory, nameof(pipelineFactory));
            Guard.AgainstNull(activeTimeRangeConfiguration, nameof(activeTimeRangeConfiguration));

            _activeTimeRange = activeTimeRangeConfiguration.CreateActiveTimeRange();

            pipelineFactory.PipelineCreated += PipelineCreated;
        }

        private void PipelineCreated(object sender, PipelineEventArgs e)
        {
            var pipelineName = e.Pipeline.GetType().FullName ?? string.Empty;

            if (pipelineName.Equals(_startupPipelineName, StringComparison.InvariantCultureIgnoreCase)
                ||
                pipelineName.Equals(_shutdownPipelineName, StringComparison.InvariantCultureIgnoreCase))
            {
                return;
            }

            e.Pipeline.RegisterObserver(new ActiveTimeRangeObserver(_activeTimeRange));
        }
    }
}

Transactions

The following provides a pipeline observer to handle transaction scopes.

PM> Install-Package Shuttle.Core.Transactions