Add event-driven projection support to event sourcing

src/Neuroglia.Core/Extensions/ICollectionExtensions.cs

@@ -13,7 +13,7 @@
 
 using System.Collections;
 
-namespace Neuroglia;
+namespace Neuroglia.Collections;
 
 /// <summary>
 /// Defines extensions for <see cref="ICollection"/>s

src/Neuroglia.Core/Extensions/IDictionaryExtensions.cs

@@ -0,0 +1,32 @@
+// Copyright © 2021-Present Neuroglia SRL. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"),
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+namespace Neuroglia;
+
+/// <summary>
+/// Defines extensions for <see cref="IDictionary{TKey, TValue}"/> instances
+/// </summary>
+public static class IDictionaryExtensions
+{
+
+    /// <summary>
+    /// Gets the value at the specified key
+    /// </summary>
+    /// <typeparam name="TKey">The type of the <see cref="IDictionary{TKey, TValue}"/> keys</typeparam>
+    /// <typeparam name="TValue">The type of the <see cref="IDictionary{TKey, TValue}"/> values</typeparam>
+    /// <param name="dictionary">The extended <see cref="IDictionary{TKey, TValue}"/></param>
+    /// <param name="key">The key of the value to get</param>
+    /// <returns>The value with the specified key</returns>
+    public static TValue Get<TKey, TValue>(this IDictionary<TKey, TValue> dictionary, TKey key) => dictionary[key];
+
+}

src/Neuroglia.Core/Extensions/ObjectExtensions.cs

@@ -47,4 +47,29 @@ public static class ObjectExtensions
         return expando;
     }
 
+    /// <summary>
+    /// Gets the value returned by the specified property
+    /// </summary>
+    /// <param name="source">The extended object</param>
+    /// <param name="name">The name of the property to get</param>
+    /// <returns>The value of the specified property</returns>
+    /// <remarks>This method is used to dynamically get the property of an object, specifically when building an expression, which does not allow dynamic operations</remarks>
+    public static object GetProperty(this object source, string name)
+    {
+        ArgumentNullException.ThrowIfNull(source);
+        ArgumentException.ThrowIfNullOrWhiteSpace(name);
+        var property = source.GetType().GetProperty(name) ?? throw new MissingMemberException($"Failed to find a property with the specified name '{name}'", name);
+        return property.GetValue(source)!;
+    }
+
+    /// <summary>
+    /// Gets the value returned by the specified property
+    /// </summary>
+    /// <typeparam name="T">The type of the property to get</typeparam>
+    /// <param name="source">The extended object</param>
+    /// <param name="name">The name of the property to get</param>
+    /// <returns>The value of the specified property</returns>
+    /// <remarks>This method is used to dynamically get the property of an object, specifically when building an expression, which does not allow dynamic operations</remarks>
+    public static T GetProperty<T>(this object source, string name) => (T)source.GetProperty(name);
+
 }

src/Neuroglia.Core/PropertyPath.cs

@@ -72,7 +72,7 @@ public MemberExpression ToExpression(Expression target)
     /// </summary>
     /// <param name="input">The input to parse</param>
     /// <returns>A new <see cref="PropertyPath"/> based on the specified input</returns>
-    public static PropertyPath Parse(string input) => new PropertyPath(input);
+    public static PropertyPath Parse(string input) => new(input);
 
     /// <summary>
     /// Attempts to parse the specified input into a new <see cref="PropertyPath"/>

src/Neuroglia.Data.Infrastructure.EventSourcing.Abstractions/Projection.cs

@@ -0,0 +1,35 @@
+// Copyright © 2021-Present Neuroglia SRL. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"),
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using Neuroglia.Data.Infrastructure.EventSourcing.Services;
+
+namespace Neuroglia.Data.Infrastructure.EventSourcing;
+
+/// <summary>
+/// Defines constants and statics used to help expressing event-driven projections
+/// </summary>
+public static class Projection
+{
+
+    /// <summary>
+    /// Placeholder method used by <see cref="IProjectionBuilder{TState}"/> implementations to link a processed event to a specific stream
+    /// </summary>
+    /// <param name="stream">The stream to link the processed event to</param>
+    /// <param name="e">The processed event</param>
+    public static void LinkEventTo(string stream, IEventRecord e)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(stream);
+        ArgumentNullException.ThrowIfNull(e);
+    }
+
+}

src/Neuroglia.Data.Infrastructure.EventSourcing.Abstractions/Services/Interfaces/IProjectionBuilder.cs

@@ -0,0 +1,46 @@
+// Copyright © 2021-Present Neuroglia SRL. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"),
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+using System.Linq.Expressions;
+
+namespace Neuroglia.Data.Infrastructure.EventSourcing.Services;
+
+/// <summary>
+/// Defines the fundamentals of a service used to build event-driven projections
+/// </summary>
+/// <typeparam name="TState">The type of the state of the event-driven projection to create</typeparam>
+public interface IProjectionBuilder<TState>
+{
+
+    /// <summary>
+    /// Configures the <see cref="Func{TResult}"/> used to create the projection's initial state
+    /// </summary>
+    /// <param name="factory">The <see cref="Func{TResult}"/> used to create the projection's initial state</param>
+    /// <returns>The configured <see cref="IProjectionBuilder{TState}"/></returns>
+    IProjectionBuilder<TState> Given(Expression<Func<object>> factory);
+
+    /// <summary>
+    /// Configures the predicate used to to filter incoming event records based on the current projection state
+    /// </summary>
+    /// <param name="predicate">A <see cref="Func{T1, T2, TResult}"/> used to to filter incoming event records based on the current projection state</param>
+    /// <returns>The configured <see cref="IProjectionBuilder{TState}"/></returns>
+    IProjectionBuilder<TState> When(Expression<Func<TState, IEventRecord, bool>> predicate);
+
+    /// <summary>
+    /// Configures an <see cref="Action{T1, T2}"/> to be performed on the projection state when a matching event record is processed
+    /// </summary>
+    /// <param name="action">An <see cref="Action{T1, T2}"/> to be performed on the projection state when a matching event record is processed</param>
+    /// <returns>The configured <see cref="IProjectionBuilder{TState}"/></returns>
+    IProjectionBuilder<TState> Then(Expression<Action<TState, IEventRecord>> action);
+
+}

src/Neuroglia.Data.Infrastructure.EventSourcing.Abstractions/Services/Interfaces/IProjectionManager.cs

@@ -0,0 +1,41 @@
+// Copyright © 2021-Present Neuroglia SRL. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"),
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+namespace Neuroglia.Data.Infrastructure.EventSourcing.Services;
+
+/// <summary>
+/// Defines the fundamentals of a service used to create and retrieve event-driven projections
+/// </summary>
+public interface IProjectionManager
+{
+
+    /// <summary>
+    /// Creates a new event-driven projection
+    /// </summary>
+    /// <typeparam name="TState">The type of the state of the projection to create</typeparam>
+    /// <param name="name">The name of the projection to create</param>
+    /// <param name="setup">An <see cref="Action{T}"/> used to build and configure the event-driven projection to create</param>
+    /// <param name="cancellationToken">A <see cref="CancellationToken"/></param>
+    /// <returns>A new awaitable <see cref="Task"/></returns>
+    Task CreateAsync<TState>(string name, Action<IProjectionSourceBuilder<TState>> setup, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Retrieves the current state of the event-driven projection with the specified name
+    /// </summary>
+    /// <typeparam name="TState">The type of the projection's state</typeparam>
+    /// <param name="name">The name of the event-driven projection to get the current state of</param>
+    /// <param name="cancellationToken">A <see cref="CancellationToken"/></param>
+    /// <returns>The current state of the specified event-driven projection</returns>
+    Task<TState> GetStateAsync<TState>(string name, CancellationToken cancellationToken = default);
+
+}

src/Neuroglia.Data.Infrastructure.EventSourcing.Abstractions/Services/Interfaces/IProjectionSourceBuilder.cs

@@ -0,0 +1,30 @@
+// Copyright © 2021-Present Neuroglia SRL. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License"),
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+namespace Neuroglia.Data.Infrastructure.EventSourcing.Services;
+
+/// <summary>
+/// Defines the fundamentals of a service used to build a new event-driven projection source
+/// </summary>
+/// <typeparam name="TState">The type of the state of the projection to build the source for</typeparam>
+public interface IProjectionSourceBuilder<TState>
+{
+
+    /// <summary>
+    /// Builds a new event-driven projection that processes events from the specified stream
+    /// </summary>
+    /// <param name="name">The name of the stream from which events will be processed by the projection</param>
+    /// <returns>A new <see cref="IProjectionBuilder{TState}"/> implementation used to configure the projection to build</returns>
+    IProjectionBuilder<TState> FromStream(string name);
+
+}