1 files changed, 61 insertions, 0 deletions
diff --git a/src/Core/Def/Features/IFeatureService.cs b/src/Core/Def/Features/IFeatureService.cs
new file mode 100644
index 0000000..88a8c9f
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+++ b/src/Core/Def/Features/IFeatureService.cs
@@ -0,0 +1,61 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Microsoft.VisualStudio.Utilities
+{
+    /// <summary>
+    /// Service that keeps track of <see cref="IFeatureController"/>'s requests to disable a feature in given scope.
+    /// When multiple <see cref="IFeatureController"/>s disable a feature and one <see cref="IFeatureController"/>
+    /// enables it back, it will not interfere with other disable requests, and feature will ultimately remain disabled.
+    ///
+    /// While this service does have a thread affinity, its implementation does not guarantee thread safety.
+    /// It is advised to change feature state from UI thread, otherwise simultaneous changes may result in race conditions.
+    /// </summary>
+    /// <example>
+    /// // In an exported MEF part:
+    /// [Import]
+    /// IFeatureServiceFactory FeatureServiceFactory;
+    ///
+    /// IFeatureService globalService = FeatureServiceFactory.GlobalFeatureService;
+    /// IFeatureService localService = FeatureServiceFactory.GetOrCreate(scope); // scope is an IPropertyOwner
+    ///
+    /// // Also have a reference to <see cref="IFeatureController"/>:
+    /// IFeatureController MyFeatureController;
+    /// // Interact with the <see cref="IFeatureService"/>:
+    /// globalService.Disable(PredefinedEditorFeatureNames.Popup, MyFeatureController);
+    /// localService.IsEnabled(PredefinedEditorFeatureNames.Completion); // returns false, because Popup is a base definition of Completion and because global scope is a superset of local scope.
+    /// </example>
+    public interface IFeatureService
+    {
+        /// <summary>
+        /// Checks if feature is enabled. By default, every feature is enabled.
+        /// </summary>
+        /// <param name="featureName">Name of the feature</param>
+        /// <returns>False if there are any disable requests. True otherwise.</returns>
+        bool IsEnabled(string featureName);
+
+        /// <summary>
+        /// Disables a feature.
+        /// </summary>
+        /// <param name="featureName">Name of the feature to disable</param>
+        /// <param name="controller">Object that uniquely identifies the caller.</param>
+        IFeatureDisableToken Disable(string featureName, IFeatureController controller);
+
+        /// <summary>
+        /// Provides a notification when this feature or its base feature was updated.
+        /// We use FeatureUpdatedEventArgs and not FeatureChangedEventArgs
+        /// because there are base features and disable requests from parent scopes that affect the factual state of given feature.
+        /// We use this event to let the interested parties (<see cref="IFeatureCookie"/>) recalculate the actual state of the feature.
+        /// </summary>
+        event EventHandler<FeatureUpdatedEventArgs> StateUpdated;
+
+        /// <summary>
+        /// Creates a new <see cref="IFeatureCookie"/> that provides O(1) access to the feature's value, in this service's scope.
+        /// The <see cref="IFeatureCookie" /> is updated when the feature or its base is updated in this scope or in the global scope.
+        /// </summary>
+        /// <param name="featureName">Name of the feature</param>
+        /// <returns>New instance of <see cref="IFeatureCookie"/></returns>
+        IFeatureCookie GetCookie(string featureName);
+    }
+}