Migrate setting and key stores to shared UserDefaults

This is needed to allow the notification extension to process push notifications, respect user's notification settings, and decrypt DMs on the push notification

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Models/DamusUserDefaults.swift

@@ -0,0 +1,67 @@
+//
+//  DamusUserDefaults.swift
+//  damus
+//
+//  Created by Daniel D’Aquino on 2023-11-25.
+//
+
+import Foundation
+
+/// DamusUserDefaults
+/// This struct acts like a drop-in replacement for `UserDefaults.standard`
+/// for cases where we want to store such items in a UserDefaults that is shared among the Damus app group
+/// so that they can be accessed from other target (e.g. The notification extension target).
+///
+/// This struct handles migration automatically to the new shared UserDefaults
+struct DamusUserDefaults {
+    static let shared: DamusUserDefaults = DamusUserDefaults()
+    private static let default_suite_name: String = "group.com.damus"  // Shared defaults for this app group
+    
+    private let suite_name: String
+    private let defaults: UserDefaults
+    
+    // MARK: - Initializers
+    
+    init() {
+        self.init(suite_name: Self.default_suite_name)! // Pretty low risk to force-unwrap given that the default suite name is a constant.
+    }
+    
+    init?(suite_name: String = Self.default_suite_name) {
+        self.suite_name = suite_name
+        guard let defaults = UserDefaults(suiteName: suite_name) else {
+            return nil
+        }
+        self.defaults = defaults
+    }
+    
+    // MARK: - Functions for feature parity with UserDefaults.standard
+    
+    func string(forKey defaultName: String) -> String? {
+        if let value = self.defaults.string(forKey: defaultName) {
+            return value
+        }
+        let fallback_value = UserDefaults.standard.string(forKey: defaultName)
+        self.defaults.set(fallback_value, forKey: defaultName)  // Migrate
+        return fallback_value
+    }
+    
+    func set(_ value: Any?, forKey defaultName: String) {
+        self.defaults.set(value, forKey: defaultName)
+    }
+    
+    func removeObject(forKey defaultName: String) {
+        self.defaults.removeObject(forKey: defaultName)
+        // Remove from standard UserDefaults to avoid it coming back as a fallback_value when we fetch it next time
+        UserDefaults.standard.removeObject(forKey: defaultName)
+    }
+    
+    func object(forKey defaultName: String) -> Any? {
+        if let value = self.defaults.object(forKey: defaultName) {
+            return value
+        }
+        let fallback_value = UserDefaults.standard.string(forKey: defaultName)
+        self.defaults.set(fallback_value, forKey: defaultName)  // Migrate
+        return fallback_value
+    }
+    
+}