From 79abb910d461f296c043bde71ca89faf2a68cb55 Mon Sep 17 00:00:00 2001
From: Chris Bracken <chris@bracken.jp>
Date: Tue, 25 Aug 2020 23:22:38 -0700
Subject: [PATCH] Add tests for lerpDouble

The behaviour of lerpDouble with respect to null inputs isn't entirely
obvious. In the case where both inputs are null, it returns null.
Otherwise, it defaults the null parameter to 0.0 and carries on.

Post non-null by default, it might be nice to strengthen the parameter
contract to require them to be non-null. While this would be a breaking
change, it seems likely that the framework either meets this guarantee
or can provide it without a framework breaking change.

https://github.com/flutter/flutter/issues/64617 tracks the above.

In the meantime, adding a test to lock in the current behaviour.
---
 lib/ui/lerp.dart            |  2 ++
 testing/dart/lerp_test.dart | 46 +++++++++++++++++++++++++++++++++++++
 2 files changed, 48 insertions(+)
 create mode 100644 testing/dart/lerp_test.dart

diff --git a/lib/ui/lerp.dart b/lib/ui/lerp.dart
index 0bb0a08b7242d..91a04c2cbf94a 100644
--- a/lib/ui/lerp.dart
+++ b/lib/ui/lerp.dart
@@ -7,6 +7,8 @@
 part of dart.ui;
 
 /// Linearly interpolate between two numbers.
+// TODO(cbracken): Consider making a and b non-nullable.
+//                 https://github.com/flutter/flutter/issues/64617
 double? lerpDouble(num? a, num? b, double t) {
   if (a == null && b == null)
     return null;
diff --git a/testing/dart/lerp_test.dart b/testing/dart/lerp_test.dart
new file mode 100644
index 0000000000000..a2220ad4ade67
--- /dev/null
+++ b/testing/dart/lerp_test.dart
@@ -0,0 +1,46 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.10
+import 'dart:ui';
+
+import 'package:test/test.dart';
+
+void main() {
+  test('lerpDouble should return null if and only if both inputs are null', () {
+    expect(lerpDouble(null, null, 1.0), isNull);
+    expect(lerpDouble(5.0, null, 0.25), isNotNull);
+    expect(lerpDouble(null, 5.0, 0.25), isNotNull);
+  });
+
+  test('lerpDouble should treat a null input as 0 if the other input is non-null', () {
+    expect(lerpDouble(null, 10.0, 0.25), 2.5);
+    expect(lerpDouble(10.0, null, 0.25), 7.5);
+  });
+
+  test('lerpDouble should handle interpolation values < 0.0', () {
+    expect(lerpDouble(0.0, 10.0, -5.0), -50.0);
+    expect(lerpDouble(10.0, 0.0, -5.0), 60.0);
+  });
+
+  test('lerpDouble should return the start value at 0.0', () {
+    expect(lerpDouble(2.0, 10.0, 0.0), 2.0);
+    expect(lerpDouble(10.0, 2.0, 0.0), 10.0);
+  });
+
+  test('lerpDouble should interpolate between two values', () {
+    expect(lerpDouble(0.0, 10.0, 0.25), 2.5);
+    expect(lerpDouble(10.0, 0.0, 0.25), 7.5);
+  });
+
+  test('lerpDouble should return the end value at 1.0', () {
+    expect(lerpDouble(2.0, 10.0, 1.0), 10.0);
+    expect(lerpDouble(10.0, 2.0, 1.0), 2.0);
+  });
+
+  test('lerpDouble should handle interpolation values > 1.0', () {
+    expect(lerpDouble(0.0, 10.0, 5.0), 50.0);
+    expect(lerpDouble(10.0, 0.0, 5.0), -40.0);
+  });
+}