From 3f16e316b0e93e22c52840cc2eea9e9e3ffd9c57 Mon Sep 17 00:00:00 2001
From: erri120 <erri120@fossmailer.de>
Date: Sat, 10 Aug 2024 19:16:33 +0200
Subject: [PATCH] Add `WhereNotNull` operator

Using optimized `WhereSelect`, the `WhereNotNull` operator is very
common and many libraries and applications had to DIY implement
this for `System.IObservable<T>`.
---
 src/R3/Operators/WhereNotNull.cs              | 13 ++++++++++
 .../OperatorTests/WhereNotNullTest.cs         | 24 +++++++++++++++++++
 2 files changed, 37 insertions(+)
 create mode 100644 src/R3/Operators/WhereNotNull.cs
 create mode 100644 tests/R3.Tests/OperatorTests/WhereNotNullTest.cs

diff --git a/src/R3/Operators/WhereNotNull.cs b/src/R3/Operators/WhereNotNull.cs
new file mode 100644
index 00000000..70b632c1
--- /dev/null
+++ b/src/R3/Operators/WhereNotNull.cs
@@ -0,0 +1,13 @@
+namespace R3;
+
+public static partial class ObservableExtensions
+{
+    public static Observable<TResult> WhereNotNull<TResult>(this Observable<TResult?> source) where TResult : class
+    {
+        return new WhereSelect<TResult?, TResult>(
+            source: source,
+            selector: static item => item!,
+            predicate: item => item is not null
+        );
+    }
+}
diff --git a/tests/R3.Tests/OperatorTests/WhereNotNullTest.cs b/tests/R3.Tests/OperatorTests/WhereNotNullTest.cs
new file mode 100644
index 00000000..5f8057d0
--- /dev/null
+++ b/tests/R3.Tests/OperatorTests/WhereNotNullTest.cs
@@ -0,0 +1,24 @@
+namespace R3.Tests.OperatorTests;
+
+public class WhereNotNullTest(ITestOutputHelper output)
+{
+    [Fact]
+    public void WhereNotNull()
+    {
+        var p = new Subject<string?>();
+
+        using var list = p.WhereNotNull().ToLiveList();
+
+        p.OnNext(null);
+        list.AssertEqual([]);
+
+        p.OnNext("foo");
+        list.AssertEqual(["foo"]);
+
+        p.OnNext(null);
+        list.AssertEqual(["foo"]);
+
+        p.OnNext("bar");
+        list.AssertEqual(["foo", "bar"]);
+    }
+}