Merge pull request #100 from nventive/dev/jpl/func-get-from-observable

feat: Add an overload of GetFromObservable that uses a func

README.md

@@ -171,6 +171,8 @@ public long Counter => this.GetFromObservable(Observable.Timer(
 ```
 > 💡 If you use [Chinook Snippets](https://marketplace.visualstudio.com/items?itemName=nventivecorp.ChinookSnippets), you can quickly generate a property from observable using the snippets `"ckpropo"` (**c**hinoo**k** **prop**erty from **o**bservable) or `"ckpropog"` (**c**hinoo**k** **prop**erty from **o**bservable **g**et-only).
 
+> 💡 Consider using the overload that takes a `Func<IObservable<T>>` to avoid evaluating the observable every time the property is read and potentially save some memory allocations.
+
 ### Create properties from `Task<T>`
 Using `IViewModel.GetFromTask`, you can create a property that updates itself based on a `Task<T>` result.
 ```csharp

src/DynamicMvvm.Abstractions/ViewModel/IViewModel.Extensions.Properties.cs

@@ -243,6 +243,41 @@ void OnDynamicPropertyChanged(IDynamicProperty dynamicProperty)
 			}
 		}
 
+		/// <summary>
+		/// Gets or creates a <see cref="IDynamicProperty"/> attached to this <see cref="IViewModel"/>.
+		/// 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 function to provide 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 GetFromObservable<T>(
+			this IViewModel viewModel,
+			Func<IObservable<T>> sourceProvider,
+			T initialValue = default,
+			[CallerMemberName] string name = null
+		)
+		{
+			// We don't use GetOrCreateDynamicProperty internally to avoid the performance costs of the lambda and closure.
+			if (viewModel.IsDisposed)
+			{
+				return default(T);
+			}
+
+			if (viewModel.TryGetDisposable<IDynamicProperty>(name, out var property))
+			{
+				return viewModel.Get<T>(property);
+			}
+			else
+			{
+				property = AddDynamicPropertyFromObservable(viewModel, sourceProvider(), initialValue, name);
+
+				return (T)property.Value;
+			}
+		}
+
 		/// <summary>
 		/// Sets the value of a property.
 		/// </summary>

src/DynamicMvvm.Tests/Integration/IntegrationTests.cs

@@ -1,10 +1,10 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reactive.Linq;
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
-using Chinook.DynamicMvvm;
 using Chinook.DynamicMvvm.Tests.Helpers;
 using FluentAssertions;
 using Microsoft.Extensions.DependencyInjection;
@@ -137,6 +137,23 @@ public void Disposing_a_child_removes_it_from_the_parent()
 			viewModel.TryGetDisposable(nameof(viewModel.Child), out var _).Should().BeFalse();
 		}
 
+		[Fact]
+		public void GetFromObservable_WithFuncOverload_doesnt_build_observable_more_than_once()
+		{
+			var sut = new TestVM2(_serviceProvider);
+
+			// 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);
+		}
+
 		private class MyViewModel : ViewModelBase
 		{
 			public MyViewModel(IServiceProvider serviceProvider)
@@ -226,5 +243,23 @@ public int MyNumber
 				set => this.Set(value);
 			}
 		}
+
+		public class TestVM2 : ViewModelBase
+		{
+			public TestVM2(IServiceProvider serviceProvider)
+				: base(serviceProvider: serviceProvider)
+			{
+			}
+
+			public int InvocationCount { get; private set; }
+
+			public int Count => this.GetFromObservable<int>(GetObservable, initialValue: 0);
+
+			private IObservable<int> GetObservable()
+			{
+				++InvocationCount;
+				return Observable.Never<int>();
+			}
+		}
 	}
 }