Add ViewTraitCollection implementation

Author: Kyle-Ye

Sources/OpenSwiftUI/Core/View/ViewTrait/ViewTraitCollection.swift

@@ -0,0 +1,63 @@
+//
+//  ViewTraitCollection.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: WIP
+//  ID: 9929B476764059557433A108298EE66F
+
+struct ViewTraitCollection {
+    private var storage: [any AnyViewTrait]
+    
+    private struct AnyTrait<Key: _ViewTraitKey>: AnyViewTrait {
+        var value: Key.Value
+        
+        init(value: Key.Value) {
+            self.value = value
+        }
+        
+        var id: ObjectIdentifier { ObjectIdentifier(Key.self) }
+        
+        subscript<V>() -> V {
+            get { value as! V }
+            set { value = newValue as! Key.Value }
+        }
+    }
+    
+    subscript<Key: _ViewTraitKey>(_: Key.Type) -> Key.Value {
+        get {
+            value(for: Key.self)
+        }
+        set {
+            if let index = storage.firstIndex(where: { $0.id == ObjectIdentifier(Key.self) }) {
+                storage[index][] = newValue
+            } else {
+                storage.append(AnyTrait<Key>(value: newValue))
+            }
+        }
+    }
+    
+    func value<Key: _ViewTraitKey>(for _: Key.Type, defaultValue: Key.Value = Key.defaultValue) -> Key.Value {
+        storage.first { $0.id == ObjectIdentifier(Key.self) }?[] ?? defaultValue
+    }
+    
+    mutating func setValueIfUnset<Key: _ViewTraitKey>(_ value: Key.Value, for _: Key.Type) {
+        guard !storage.contains(where: { $0.id == ObjectIdentifier(Key.self) }) else {
+            return
+        }
+        storage.append(AnyTrait<Key>(value: value))
+    }
+    
+//    func insertInteraction(for: OnInsertInteraction.Strategy) -> OnInsertInteraction? {
+//        fatalError("TODO")
+//    }
+//    
+//    var optionalTransition: AnyTransition? {
+//        fatalError("TODO")
+//    }
+}
+
+private protocol AnyViewTrait {
+    var id: ObjectIdentifier { get }
+    subscript<V>() -> V { get set }
+}

Sources/OpenSwiftUI/Core/View/ViewTrait/ViewTraitKey.swift

@@ -0,0 +1,11 @@
+//
+//  ViewTraitKey.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: Complete
+
+protocol _ViewTraitKey {
+    associatedtype Value
+    static var defaultValue: Value { get }
+}

Sources/OpenSwiftUI/Core/View/ViewTrait/ViewTraitKeys.swift

@@ -0,0 +1,19 @@
+//
+//  ViewTraitKeys.swift
+//  OpenSwiftUI
+//
+//  Audited for RELEASE_2021
+//  Status: Complete
+
+struct ViewTraitKeys {
+    var types: Set<ObjectIdentifier>
+    var isDataDependent: Bool
+    
+    mutating func insert<Key: _ViewTraitKey>(_ type: Key.Type) {
+        types.insert(ObjectIdentifier(type))
+    }
+    
+    func contains<Key: _ViewTraitKey>(_ type: Key.Type) -> Bool {
+        types.contains(ObjectIdentifier(type))
+    }
+}