Working on version 2 of the MQTT client.

Sources/Models/TrackedChanges.swift

@@ -0,0 +1,64 @@
+
+@propertyWrapper
+public struct TrackedChanges<Value> {
+  
+  private var tracking: TrackingState
+  private var value: Value
+  private var isEqual: (Value, Value) -> Bool
+  
+  public var wrappedValue: Value {
+    get { value }
+    set {
+      // Check if the new value is equal to the old value.
+      guard !isEqual(newValue, value) else { return }
+      // If it's not equal then set it, as well as set the tracking to `.needsProcessed`.
+      value = newValue
+      tracking = .needsProcessed
+    }
+  }
+  
+  public init(
+    wrappedValue: Value,
+    needsProcessed: Bool = false,
+    isEqual: @escaping (Value, Value) -> Bool
+  ) {
+    self.value = wrappedValue
+    self.tracking = needsProcessed ? .needsProcessed : .hasProcessed
+    self.isEqual = isEqual
+  }
+  
+  enum TrackingState {
+    case hasProcessed
+    case needsProcessed
+  }
+  
+  public var needsProcessed: Bool {
+    get { tracking == .needsProcessed }
+    set {
+      if newValue {
+        tracking = .needsProcessed
+      } else {
+        tracking = .hasProcessed
+      }
+    }
+  }
+  
+  public var projectedValue: Self {
+    get { self }
+    set { self = newValue }
+  }
+}
+
+extension TrackedChanges: Equatable where Value: Equatable {
+  public static func == (lhs: TrackedChanges<Value>, rhs: TrackedChanges<Value>) -> Bool {
+    lhs.wrappedValue == rhs.wrappedValue
+      && lhs.needsProcessed == rhs.needsProcessed
+  }
+  
+  public init(
+    wrappedValue: Value,
+    needsProcessed: Bool = false
+  ) {
+    self.init(wrappedValue: wrappedValue, needsProcessed: needsProcessed, isEqual: ==)
+  }
+}