added guarding test to make sure no triversing back up tree in visitor

composeApp/src/commonMain/kotlin/model/ItemVisitor.kt

@@ -0,0 +1,8 @@
+package model
+
+class ItemVisitor(val action: ItemVisitorAction) {
+    fun visit(item: Item) {
+        action.accept(item)
+        item.subItems.forEach { visit(it) }
+    }
+}

composeApp/src/commonMain/kotlin/model/ItemVisitorAction.kt

@@ -0,0 +1,5 @@
+package model
+
+interface ItemVisitorAction {
+    fun accept(item: Item)
+}

composeApp/src/commonTest/kotlin/model/ItemVisitorCollector.kt

@@ -0,0 +1,10 @@
+package model
+
+class ItemVisitorCollector : ItemVisitorAction {
+    val items = mutableListOf<Item>()
+
+    override fun accept(item: Item) {
+        items.add(item)
+    }
+
+}

composeApp/src/commonTest/kotlin/model/ItemVisitorVisitsAllSubItemsTest.kt

@@ -0,0 +1,49 @@
+package model
+
+import io.kotest.matchers.collections.shouldContainExactlyInAnyOrder
+import kotlin.test.Test
+
+class ItemVisitorVisitsAllSubItemsTest {
+
+    @Test
+    fun `ItemVisitor visits all sub items`() {
+        // Given
+        val item = Item("parent")
+        val child = Item("child")
+        val sibling = Item("sibling")
+        val grandChild = Item("grandChild")
+        child.add(grandChild)
+        item.add(child)
+        item.add(sibling)
+
+        val itemCollector = ItemVisitorCollector()
+        val visitor = ItemVisitor(itemCollector)
+
+        // When
+        visitor.visit(item)
+
+        // Then
+        itemCollector.items shouldContainExactlyInAnyOrder  listOf(item, child, sibling, grandChild)
+    }
+
+    @Test
+    fun `ItemVisitor starting at child does not get other branches`() {
+        // Given
+        val item = Item("parent")
+        val child = Item("child")
+        val sibling = Item("sibling")
+        val grandChild = Item("grandChild")
+        child.add(grandChild)
+        item.add(child)
+        item.add(sibling)
+
+        val itemCollector = ItemVisitorCollector()
+        val visitor = ItemVisitor(itemCollector)
+
+        // When
+        visitor.visit(child)
+
+        // Then
+        itemCollector.items shouldContainExactlyInAnyOrder  listOf(child, grandChild)
+    }
+}