Add initial FocusState support (#34)

* Add FocusState.Binding

* Add FocusState implementation

* Fix cast compile warning

Author: Kyle-Ye

Sources/OpenSwiftUI/EventHandling/Focus/FocusState.swift

@@ -0,0 +1,103 @@
+//
+//  FocusState.swift
+//  OpenSwiftUI
+//
+//  Created by Kyle on 2024/2/1.
+//  Lastest Version: iOS 15.5
+//  Status: WIP
+//  ID: 274D264A38B51DC68ACC48A91353B7D0
+
+@frozen
+@propertyWrapper
+public struct FocusState<Value>: DynamicProperty where Value: Hashable {
+    @frozen
+    @propertyWrapper
+    public struct Binding {
+        @OpenSwiftUI.Binding
+        private var binding: Value
+        
+        init(binding: OpenSwiftUI.Binding<Value>) {
+            _binding = binding
+        }
+        
+        public var wrappedValue: Value {
+            get { binding }
+            nonmutating set { binding = newValue }
+        }
+
+        public var projectedValue: FocusState<Value>.Binding {
+            self
+        }
+        
+        var propertyID: ObjectIdentifier {
+            if let location = _binding.location as? FocusStoreLocation<Value> {
+                location.id
+            } else {
+                #if canImport(ObjectiveC)
+                ObjectIdentifier(PrivateType.self)
+                #else
+                ObjectIdentifier(unsafeBitCast(0, to: AnyObject.self))
+                #endif
+            }
+        }
+
+        private enum PrivateType {}
+    }
+
+    var value: Value
+    var location: AnyLocation<Value>?
+    var resetValue: Value
+    public var wrappedValue: Value {
+        get {
+            getValue(forReading: true)
+        }
+        nonmutating set {
+            guard let location else {
+                return
+            }
+            location.set(newValue, transaction: Transaction())
+        }
+    }
+
+    public var projectedValue: FocusState<Value>.Binding {
+        let value = getValue(forReading: false)
+        let binding: OpenSwiftUI.Binding<Value>
+        if let location {
+            binding = OpenSwiftUI.Binding(value: value, location: location)
+        } else {
+            Log.runtimeIssues("Accessing FocusState's value outside of the body of a View. This will result in a constant Binding of the initial value and will not update.")
+            binding = .constant(value)
+        }
+        return Binding(binding: binding)
+    }
+    
+    public static func _makeProperty<V>(in buffer: inout _DynamicPropertyBuffer, container: _GraphValue<V>, fieldOffset: Int, inputs: inout _GraphInputs) {
+        // TODO
+    }
+    
+    public init() where Value == Bool {
+        value = false
+        location = nil
+        resetValue = false
+    }
+    
+    public init<T>() where Value == T?, T: Hashable {
+        value = nil
+        location = nil
+        resetValue = nil
+    }
+    
+    private func getValue(forReading: Bool) -> Value {
+        guard let location else {
+            return value
+        }
+        if GraphHost.isUpdating {
+            if forReading {
+                location.wasRead = true
+            }
+            return value
+        } else {
+            return location.get()
+        }
+    }
+}

Sources/OpenSwiftUI/EventHandling/Focus/FocusStore.swift

@@ -0,0 +1,23 @@
+// FIXME
+protocol ResponderNode {}
+
+struct FocusStore {
+    var seed: VersionSeed
+    var focusedResponders: ContiguousArray<ResponderNode>
+    var plists: [ObjectIdentifier : PropertyList]
+}
+
+// MARK: - FocusStore.Key
+
+extension FocusStore {
+    struct Key<V: Hashable>: PropertyKey {
+        static var defaultValue: Entry<V>? { nil }
+    }
+}
+
+// MARK: - FocusStore.Item
+extension FocusStore {
+    struct Entry<Value: Hashable> {
+        
+    }
+}

Sources/OpenSwiftUI/EventHandling/Focus/FocusStoreLocation.swift

@@ -0,0 +1,33 @@
+class FocusStoreLocation<A: Hashable>: AnyLocation<A> {
+    override var wasRead: Bool {
+        get {
+            _wasRead
+        }
+        set {
+            _wasRead = newValue
+        }
+    }
+    
+    override func get() -> A {
+        fatalError("TODO")
+    }
+    
+    override func set(_ value: A, transaction: Transaction) {
+        fatalError("TODO")
+    }
+    
+    typealias Value = A
+    
+    override init() { fatalError() }
+    
+    var store: FocusStore
+    weak var host: GraphHost?
+    var resetValue: A
+    var focusSeed: VersionSeed
+    var failedAssignment: (A, VersionSeed)?
+    var resolvedEntry: FocusStore.Entry<A>?
+    var resolvedSeed: VersionSeed
+    var _wasRead: Bool
+    
+    var id: ObjectIdentifier { ObjectIdentifier(self) }
+}

Sources/OpenSwiftUI/EventHandling/Focus/FocusedValueKey.swift

@@ -0,0 +1,11 @@
+//
+//  FocusedValueKey.swift
+//  OpenSwiftUI
+//
+//  Created by Kyle on 2024/2/1.
+//  Lastest Version: iOS 15.5
+//  Status: Complete
+
+public protocol FocusedValueKey {
+    associatedtype Value
+}