Add basic tests for GetChildElementById

src/app/tests/DevToys.UnitTests/Tool/GUI/Components/UIElementWithChildrenTests.cs

@@ -0,0 +1,75 @@
+namespace DevToys.UnitTests.Tool.GUI.Components;
+
+public class UIElementWithChildrenTests
+{
+    private class TestUIElement : UIElementWithChildren
+    {
+        private readonly IEnumerable<IUIElement> _children;
+
+        public TestUIElement(string id, IEnumerable<IUIElement> children)
+            : base(id)
+        {
+            _children = children;
+        }
+
+        protected override IEnumerable<IUIElement> GetChildren() => _children;
+    }
+
+    [Fact]
+    public void GetChildElementById_ReturnsNull_WhenIdIsNullOrEmpty()
+    {
+        var element = new TestUIElement("parent", new List<IUIElement>());
+        Assert.Null(element.GetChildElementById(null!));
+        Assert.Null(element.GetChildElementById(string.Empty));
+    }
+
+    [Fact]
+    public void GetChildElementById_ReturnsNull_WhenChildNotFound()
+    {
+        var child = new Mock<IUIElement>();
+        child.Setup(c => c.Id).Returns("child1");
+
+        var element = new TestUIElement("parent", new List<IUIElement> { child.Object });
+        Assert.Null(element.GetChildElementById("child2"));
+    }
+
+    [Fact]
+    public void GetChildElementById_ReturnsChild_WhenChildFound()
+    {
+        var child = new Mock<IUIElement>();
+        child.Setup(c => c.Id).Returns("child1");
+
+        var element = new TestUIElement("parent", new List<IUIElement> { child.Object });
+        Assert.Equal(child.Object, element.GetChildElementById("child1"));
+    }
+
+    [Fact]
+    public void GetChildElementById_ReturnsNestedChild_WhenNestedChildFound()
+    {
+        var nestedChild = new Mock<IUIElement>();
+        nestedChild.Setup(c => c.Id).Returns("nestedChild");
+
+        var child = new Mock<IUIElementWithChildren>();
+        child.Setup(c => c.Id).Returns("child1");
+        child.Setup(c => c.GetChildElementById("nestedChild")).Returns(nestedChild.Object);
+
+        var element = new TestUIElement("parent", new List<IUIElement> { child.Object });
+        Assert.Equal(nestedChild.Object, element.GetChildElementById("nestedChild"));
+    }
+
+    [Fact]
+    public void GetChildElementById_ReturnsFirstMatchingChild_WhenMultipleChildrenWithSameId()
+    {
+        var child1 = new Mock<IUIElement>();
+        child1.Setup(c => c.Id).Returns("child");
+
+        var child2 = new Mock<IUIElement>();
+        child2.Setup(c => c.Id).Returns("child");
+
+        var element = new TestUIElement(
+            "parent",
+            new List<IUIElement> { child1.Object, child2.Object }
+        );
+        Assert.Equal(child1.Object, element.GetChildElementById("child"));
+    }
+}