Add ObserveHasAnyDatoms

Resolves #91

src/NexusMods.MnemonicDB.Abstractions/Query/ObservableDatoms.cs

@@ -1,21 +1,20 @@
 using System;
-using System.Collections.Generic;
-using System.Diagnostics.CodeAnalysis;
-using System.Linq;
 using System.Reactive.Linq;
 using System.Runtime.CompilerServices;
 using DynamicData;
+using JetBrains.Annotations;
 using NexusMods.MnemonicDB.Abstractions.Attributes;
-using NexusMods.MnemonicDB.Abstractions.DatomComparators;
 using NexusMods.MnemonicDB.Abstractions.DatomIterators;
 using NexusMods.MnemonicDB.Abstractions.IndexSegments;
-using NexusMods.MnemonicDB.Abstractions.Internals;
 
 namespace NexusMods.MnemonicDB.Abstractions.Query;
 
+/// <summary>
+/// Extension methods to observe changes in the DB.
+/// </summary>
+[PublicAPI]
 public static class ObservableDatoms
 {
-
     /// <summary>
     /// Delays the creation of the IObservable until after the first value from the src is received. Once the first
     /// value arrives the ctor function will be called to create the observable. This is useful for situations where
@@ -89,7 +88,15 @@ public static IObservable<IChangeSet<Datom, DatomKey>> ObserveDatoms(this IConne
     {
         return conn.Revisions.DelayUntilFirstValue(() => conn.ObserveDatoms(SliceDescriptor.Create(attribute, conn.AttributeCache)));
     }
-
+
+    /// <summary>
+    /// Observe whether datoms for the descriptor exist at all.
+    /// </summary>
+    public static IObservable<bool> ObserveHasAnyDatoms(this IConnection connection, SliceDescriptor descriptor)
+    {
+        return connection.Revisions.Select(revision => revision.Datoms(descriptor).Count != 0);
+    }
+
     /// <summary>
     /// Converts a set of observed datoms to a set of observed entity ids, assumes that there will be no datoms with
     /// duplicate entity ids.
@@ -112,8 +119,6 @@ public static IObservable<IChangeSet<Datom, EntityId>> AsEntityIds(this IObserva
     private static IChangeSet<Datom, DatomKey> Diff(AttributeCache cache, IndexSegment updates, SliceDescriptor descriptor)
     {
         var changes = new ChangeSet<Datom, DatomKey>();
-
-        AttributeId previousAid = default;
 
         var index = descriptor.Index;
 

tests/NexusMods.MnemonicDB.Tests/DbTests.cs

@@ -1172,4 +1172,38 @@ public async Task CanExciseEntities()
             .NotBeEmpty();
 
     }
+
+    [Fact]
+    public async Task Test_ObserveHasAnyDatoms()
+    {
+        var hasAnyDatoms = false;
+
+        using var disposable = Connection
+            .ObserveHasAnyDatoms(SliceDescriptor.Create(Loadout.Name, Connection.AttributeCache))
+            .Subscribe(value => hasAnyDatoms = value);
+
+        hasAnyDatoms.Should().BeFalse();
+
+        EntityId entityId;
+        using (var tx = Connection.BeginTransaction())
+        {
+            var loadout = new Loadout.New(tx)
+            {
+                Name = "Test Loadout"
+            };
+
+            var result = await tx.Commit();
+            entityId = result.Remap(loadout).Id;
+        }
+
+        hasAnyDatoms.Should().BeTrue();
+
+        using (var tx = Connection.BeginTransaction())
+        {
+            tx.Delete(entityId, recursive: false);
+            await tx.Commit();
+        }
+
+        hasAnyDatoms.Should().BeFalse();
+    }
 }