Utility fn for stream

app/lib/stream_extension.dart

@@ -1,3 +1,28 @@
 extension StreamExtension<T> on Stream<T> {
   Stream<U> whereType<U>() => where((event) => event is U).cast<U>();
+
+  /// Creates a new stream from this stream that emits 1 element per [count]
+  /// elements
+  ///
+  /// For a stream containing [A, B, C, D, E, F, G], return a new stream
+  /// containing [A, C, E, G] when [count] == 2
+  ///
+  /// If [count] == 1, the returned stream is practically identical to the
+  /// original stream
+  Stream<T> per(int count) async* {
+    assert(count > 0);
+    if (count <= 1) {
+      yield* this;
+    } else {
+      var i = 0;
+      await for (final e in this) {
+        if (i == 0) {
+          yield e;
+        }
+        if (++i >= count) {
+          i = 0;
+        }
+      }
+    }
+  }
 }

app/test/stream_extension_test.dart

@@ -0,0 +1,29 @@
+import 'package:nc_photos/stream_extension.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group("StreamExtension", () {
+    group("per", () {
+      test("count = 1", _perCount1);
+      test("count = 2", _perCount2);
+    });
+  });
+}
+
+Future<void> _perCount1() async {
+  final stream = () async* {
+    for (var i = 0; i < 10; ++i) {
+      yield i;
+    }
+  }();
+  expect(await stream.per(1).toList(), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+}
+
+Future<void> _perCount2() async {
+  final stream = () async* {
+    for (var i = 0; i < 10; ++i) {
+      yield i;
+    }
+  }();
+  expect(await stream.per(2).toList(), [0, 2, 4, 6, 8]);
+}