Add a new test for OsuScreenStack, fix parallax

osu.Game.Tests/Visual/TestCaseOsuScreenStack.cs

@@ -0,0 +1,45 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+using NUnit.Framework;
+using osu.Framework.Graphics;
+using osu.Framework.Screens;
+using osu.Framework.Testing;
+using osu.Game.Graphics.Containers;
+using osu.Game.Screens;
+using osu.Game.Screens.Play;
+
+namespace osu.Game.Tests.Visual
+{
+    [TestFixture]
+    public class TestCaseOsuScreenStack : OsuTestCase
+    {
+        private TestScreen baseScreen;
+        private TestOsuScreenStack stack;
+
+        [SetUpSteps]
+        public void Setup()
+        {
+            AddStep("Create new screen stack", () => { Child = stack = new TestOsuScreenStack { RelativeSizeAxes = Axes.Both }; });
+            AddStep("Push new base screen", () => stack.Push(baseScreen = new TestScreen()));
+        }
+
+        [Test]
+        public void ParallaxAssignmentTest()
+        {
+            AddStep("Push new screen to base screen", () => baseScreen.Push(new TestScreen()));
+            AddAssert("Parallax is correct", () => stack.IsParallaxSet);
+            AddStep("Exit from new screen", () => { baseScreen.MakeCurrent(); });
+            AddAssert("Parallax is correct", () => stack.IsParallaxSet);
+        }
+
+        private class TestScreen : ScreenWithBeatmapBackground
+        {
+        }
+
+        private class TestOsuScreenStack : OsuScreenStack
+        {
+            public bool IsParallaxSet => ParallaxAmount == ((TestScreen)CurrentScreen).BackgroundParallaxAmount * ParallaxContainer.DEFAULT_PARALLAX_AMOUNT;
+        }
+    }
+}