Forbid class types for ItemContent and HeaderFooterContent

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Added
 
+- [Assert that `ItemContent` and `HeaderFooterContent` are value types](https://github.com/kyleve/Listable/pull/243). This is assumed by the framework and should be validated.
+
 ### Removed
 
 ### Changed

ListableUI/Sources/HeaderFooter/HeaderFooter.swift

@@ -45,6 +45,8 @@ public struct HeaderFooter<Content:HeaderFooterContent> : AnyHeaderFooter
         layout : HeaderFooterLayout = HeaderFooterLayout(),
         onTap : OnTap? = nil
     ) {
+        assertIsValueType(Content.self)
+
         self.content = content
 
         self.sizing = sizing

ListableUI/Sources/Internal/Validations.swift

@@ -0,0 +1,30 @@
+//
+//  Validations.swift
+//  ListableUI
+//
+//  Created by Kyle Van Essen on 12/15/20.
+//
+
+import Foundation
+
+
+/// Validates that the provided object is not a class type.
+func assertIsValueType<Value>(_ valueType : Value.Type) {
+
+    #if !DEBUG
+    return
+    #endif
+
+    precondition(
+        valueType is AnyClass == false,
+        {
+            let typeName = String(describing: Value.self)
+
+            return """
+            `\(typeName)` must be a value type to work properly with Listable and value semantics. Instead, it was a class.
+
+            Please convert your `\(typeName)` class from a `class` to a `struct` type.
+            """
+        }()
+    )
+}

ListableUI/Sources/Item/Item.swift

@@ -70,6 +70,8 @@ public struct Item<Content:ItemContent> : AnyItem
         onMove : OnMove.Callback? = nil,
         onUpdate : OnUpdate.Callback? = nil
     ) {
+        assertIsValueType(Content.self)
+
         self.content = content
 
         if let sizing = sizing {