feat: Add overload of GetFromDeactivatableObservable that uses a Func

src/DynamicMvvm.Tests/Integration/DeactivationIntegrationTests.cs

@@ -2,6 +2,7 @@
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;
+using System.Reactive.Linq;
 using System.Reactive.Subjects;
 using System.Text;
 using System.Threading.Tasks;
@@ -61,11 +62,41 @@ bool ReadCountChanged()
 			}
 		}
 
+		[Fact]
+		public void GetFromDeactivatableObservable_WithFuncOverload_doesnt_build_observable_more_than_once()
+		{
+			var sut = new TestVM2();
+
+			// InvocationCount should be 0 because the observable is not built until the first time it is accessed.
+			sut.InvocationCount.Should().Be(0);
+			sut.Count.Should().Be(0);
+
+			// InvocationCount should be 1 because the observable is built the first time it is accessed.
+			sut.InvocationCount.Should().Be(1);
+			sut.Count.Should().Be(0);
+
+			// InvocationCount should still be 1 because the property is cached.
+			sut.InvocationCount.Should().Be(1);
+		}
+
 		public class TestVM : DeactivatableViewModelBase
 		{
 			public ReplaySubject<int> CountSubject = new ReplaySubject<int>(bufferSize: 1);
 
 			public int Count => this.GetFromDeactivatableObservable<int>(CountSubject, initialValue: 0);
 		}
+
+		public class TestVM2 : DeactivatableViewModelBase
+		{
+			public int InvocationCount { get; private set; }
+
+			public int Count => this.GetFromDeactivatableObservable<int>(GetObservable, initialValue: 0);
+
+			private IObservable<int> GetObservable()
+			{
+				++InvocationCount;
+				return Observable.Never<int>();
+			}
+		}
 	}
 }

src/DynamicMvvm/Deactivation/IDeactivatableViewModel.Extensions.cs

@@ -25,5 +25,21 @@ public static T GetFromDeactivatableObservable<T>(this IDeactivatableViewModel v
 		{
 			return viewModel.Get<T>(viewModel.GetOrCreateDynamicProperty(name, n => new DeactivatableDynamicPropertyFromObservable<T>(name, source, viewModel, initialValue)));
 		}
+
+		/// <summary>
+		/// Gets or creates a <see cref="IDynamicProperty"/> attached to this <see cref="IViewModel"/>.<br/>
+		/// The underlying <see cref="IDynamicProperty"/> implements <see cref="IDeactivatable"/> so the observation can be deactivated.
+		/// This overload uses a <see cref="Func{TResult}"/> to avoid evaluating the observable sequence more than once (which can avoid memory allocations).
+		/// </summary>
+		/// <typeparam name="T">The property type.</typeparam>
+		/// <param name="viewModel">The <see cref="IViewModel"/> owning the property.</param>
+		/// <param name="sourceProvider">The provider of the observable of values that feeds the property.</param>
+		/// <param name="initialValue">The property's initial value.</param>
+		/// <param name="name">The property's name.</param>
+		/// <returns>The property's value.</returns>
+		public static T GetFromDeactivatableObservable<T>(this IDeactivatableViewModel viewModel, Func<IObservable<T>> sourceProvider, T initialValue = default, [CallerMemberName] string name = null)
+		{
+			return viewModel.Get<T>(viewModel.GetOrCreateDynamicProperty(name, n => new DeactivatableDynamicPropertyFromObservable<T>(name, sourceProvider(), viewModel, initialValue)));
+		}
 	}
 }