Improve Constant to include var and let variables (#72)

Sources/AppState/PropertyWrappers/Constant.swift

@@ -2,15 +2,15 @@ import Combine
 import SwiftUI
 
 /// A property wrapper that provides access to a specific part of the AppState's state.
-@propertyWrapper public struct Constant<SlicedState: MutableApplicationState, Value, SliceValue> where SlicedState.Value == Value {
+@propertyWrapper public struct Constant<SlicedState: MutableApplicationState, Value, SliceValue, SliceKeyPath: KeyPath<Value, SliceValue>> where SlicedState.Value == Value {
     /// Holds the singleton instance of `Application`.
     @ObservedObject private var app: Application = Application.shared
 
     /// Path for accessing `State` from Application.
     private let stateKeyPath: KeyPath<Application, SlicedState>
 
     /// Path for accessing `SliceValue` from `Value`.
-    private let valueKeyPath: KeyPath<Value, SliceValue>
+    private let valueKeyPath: SliceKeyPath
 
     private let fileID: StaticString
     private let function: StaticString
@@ -44,7 +44,38 @@ import SwiftUI
         _ function: StaticString = #function,
         _ line: Int = #line,
         _ column: Int = #column
-    ) {
+    ) where SliceKeyPath == KeyPath<Value, SliceValue> {
+        self.stateKeyPath = stateKeyPath
+        self.valueKeyPath = valueKeyPath
+        self.fileID = fileID
+        self.function = function
+        self.line = line
+        self.column = column
+
+        let stateKeyPathString = String(describing: stateKeyPath)
+        let valueTypeCharacterCount = String(describing: Value.self).count
+        var valueKeyPathString = String(describing: valueKeyPath)
+
+        valueKeyPathString.removeFirst(valueTypeCharacterCount + 1)
+
+        self.sliceKeyPath = "\(stateKeyPathString)\(valueKeyPathString)"
+    }
+
+    /**
+     Initializes a Slice with the provided parameters. This constructor is used to create a Slice that provides access and modification to a specific part of an AppState's state. It provides granular control over the AppState.
+
+     - Parameters:
+         - stateKeyPath: A KeyPath that points to the state in AppState that should be sliced.
+         - valueKeyPath: A WritableKeyPath that points to the specific part of the state that should be accessed.
+     */
+    public init(
+        _ stateKeyPath: KeyPath<Application, SlicedState>,
+        _ valueKeyPath: WritableKeyPath<Value, SliceValue>,
+        _ fileID: StaticString = #fileID,
+        _ function: StaticString = #function,
+        _ line: Int = #line,
+        _ column: Int = #column
+    ) where SliceKeyPath == WritableKeyPath<Value, SliceValue> {
         self.stateKeyPath = stateKeyPath
         self.valueKeyPath = valueKeyPath
         self.fileID = fileID

Tests/AppStateTests/SliceTests.swift

@@ -6,6 +6,7 @@ fileprivate struct ExampleValue {
     var username: String?
     var isLoading: Bool
     let value: String
+    var mutableValue: String
 }
 
 fileprivate extension Application {
@@ -14,7 +15,8 @@ fileprivate extension Application {
             initial: ExampleValue(
                 username: "Leif",
                 isLoading: false,
-                value: "value"
+                value: "value",
+                mutableValue: ""
             )
         )
     }
@@ -32,13 +34,14 @@ fileprivate class ExampleViewModel: ObservableObject {
 fileprivate struct ExampleView: View {
     @Slice(\.exampleValue, \.username) var username
     @Slice(\.exampleValue, \.isLoading) var isLoading
-
+    @Constant(\.exampleValue, \.mutableValue) var constantMutableValue
+
     var body: some View { fatalError() }
 
     func testPropertyWrappers() {
         username = "Hello, ExampleView"
         _ = Toggle(isOn: $isLoading) {
-            Text("Is Loading")
+            Text(constantMutableValue)
         }
     }
 }