Wrote some tests

PixiEditor/Models/DataHolders/Document/Document.Layers.cs

@@ -118,10 +118,17 @@ namespace PixiEditor.Models.DataHolders
 
         public void ToggleLayer(int index)
         {
-            Layer layer = Layers[index];
-            layer.IsActive = !layer.IsActive;
+            if (index < Layers.Count && index >= 0)
+            {
+                Layer layer = Layers[index];
+                layer.IsActive = !layer.IsActive;
+            }
         }
 
+        /// <summary>
+        /// Selects all layers between active layer and layer at given index.
+        /// </summary>
+        /// <param name="index">End of range index.</param>
         public void SelectLayersRange(int index)
         {
             DeselectAllExcept(ActiveLayer);

PixiEditorTests/ModelsTests/DataHoldersTests/DocumentLayersTests.cs

@@ -0,0 +1,83 @@
+using PixiEditor.Models.DataHolders;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using Xunit;
+
+namespace PixiEditorTests.ModelsTests.DataHoldersTests
+{
+    [Collection("Application collection")]
+    public class DocumentLayersTests
+    {
+        [Fact]
+        public void TestThatToggleLayerTogglesLayer()
+        {
+            Document doc = new Document(5, 5);
+            doc.AddNewLayer("layer");
+            bool isActive = doc.Layers[^1].IsActive;
+            doc.ToggleLayer(0);
+            Assert.True(doc.Layers[^1].IsActive != isActive);
+        }
+
+        [Fact]
+        public void TestThatToggleLayerDoesNothingOnNonExistingIndex()
+        {
+            Document document = new Document(5, 5);
+            document.AddNewLayer("test");
+            document.ToggleLayer(1);
+            document.ToggleLayer(-1);
+            Assert.True(true);
+        }
+
+        [Theory]
+        [InlineData(0, 2)]
+        [InlineData(2, 0)]
+        [InlineData(1, 1)]
+        public void TestThatSelectLayersRangeSelectsRange(int startIndex, int endIndex)
+        {
+            Document document = new Document(5, 5);
+
+            document.AddNewLayer("1");
+            document.AddNewLayer("2");
+            document.AddNewLayer("3");
+
+            document.SetActiveLayer(startIndex);
+
+            document.SelectLayersRange(endIndex);
+
+            for (int i = 0; i < document.Layers.Count; i++)
+            {
+                Assert.Equal(
+                    i >= Math.Min(startIndex, endIndex)
+                    && i <= Math.Max(startIndex, endIndex), 
+                    document.Layers[i].IsActive);
+            }
+        }
+
+        [Theory]
+        [InlineData(0)]
+        [InlineData(1)]
+        [InlineData(2)]
+        public void TestThatDeselectAllExceptDeselectsAllExceptLayer(int index)
+        {
+            Document document = new Document(5, 5);
+
+            document.AddNewLayer("1");
+            document.AddNewLayer("2");
+            document.AddNewLayer("3");
+
+            document.SetActiveLayer(0);
+            document.Layers[1].IsActive = true;
+            document.Layers[2].IsActive = true;
+
+            document.DeselectAllExcept(document.Layers[index]);
+
+            foreach (var layer in document.Layers)
+            {
+                Assert.Equal(layer == document.Layers[index], layer.IsActive);
+            }
+        }
+    }
+}