Adds @Managed property wrapper

• Keeps strong reference to wrapped managed object’s context.
• Projected value simplifies accessing properties on context’s thread.

AltStore/Types/Managed.swift

@@ -0,0 +1,49 @@
+//
+//  Managed.swift
+//  AltStore
+//
+//  Created by Riley Testut on 10/5/20.
+//  Copyright © 2020 Riley Testut. All rights reserved.
+//
+
+import Foundation
+import CoreData
+
+@propertyWrapper @dynamicMemberLookup
+struct Managed<ManagedObject: NSManagedObject>
+{
+    var wrappedValue: ManagedObject {
+        didSet {
+            self.managedObjectContext = self.wrappedValue.managedObjectContext
+        }
+    }
+    private var managedObjectContext: NSManagedObjectContext?
+    
+    var projectedValue: Managed<ManagedObject> {
+        return self
+    }
+    
+    init(wrappedValue: ManagedObject)
+    {
+        self.wrappedValue = wrappedValue
+        self.managedObjectContext = wrappedValue.managedObjectContext
+    }
+    
+    subscript<T>(dynamicMember keyPath: KeyPath<ManagedObject, T>) -> T
+    {
+        var result: T!
+        
+        if let context = self.managedObjectContext
+        {
+            context.performAndWait {
+                result = self.wrappedValue[keyPath: keyPath]
+            }
+        }
+        else
+        {
+            result = self.wrappedValue[keyPath: keyPath]
+        }
+        
+        return result
+    }
+}