From 7e3a797a694a04700ce2c03a23465d201fac7bc7 Mon Sep 17 00:00:00 2001
From: Kyle Van Essen <k@squareup.com>
Date: Tue, 15 Dec 2020 18:26:18 -0800
Subject: [PATCH] Forbid class types for ItemContent and HeaderFooterContent

---
 CHANGELOG.md                                  |  2 ++
 .../Sources/HeaderFooter/HeaderFooter.swift   |  2 ++
 ListableUI/Sources/Internal/Validations.swift | 30 +++++++++++++++++++
 ListableUI/Sources/Item/Item.swift            |  2 ++
 4 files changed, 36 insertions(+)
 create mode 100644 ListableUI/Sources/Internal/Validations.swift

diff --git a/CHANGELOG.md b/CHANGELOG.md
index d44684371..3e28deac8 100644
--- a/CHANGELOG.md
+++ b/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
diff --git a/ListableUI/Sources/HeaderFooter/HeaderFooter.swift b/ListableUI/Sources/HeaderFooter/HeaderFooter.swift
index 36934c00b..7bd6b16d4 100644
--- a/ListableUI/Sources/HeaderFooter/HeaderFooter.swift
+++ b/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
diff --git a/ListableUI/Sources/Internal/Validations.swift b/ListableUI/Sources/Internal/Validations.swift
new file mode 100644
index 000000000..af05fa61d
--- /dev/null
+++ b/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.
+            """
+        }()
+    )
+}
diff --git a/ListableUI/Sources/Item/Item.swift b/ListableUI/Sources/Item/Item.swift
index 99a8f346d..92b8ab7bf 100644
--- a/ListableUI/Sources/Item/Item.swift
+++ b/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 {