Backup improvements & example

XXMessengerClient library

• Fix race condition issues when starting the backup and setting initial parameters.
• Add BackupStorage utility that can store the backup.
• Store BackupStorage.Backup on disk.

xx-messenger example

• Start backup
• Resume backup
• Stop backup
• Store backups in storage
• Observe backup storage
• Export backup from storage
• Resume backup on launch

Sources/XXMessengerClient/Utils/MessengerFileManager.swift

@@ -5,6 +5,9 @@ public struct MessengerFileManager {
   public var isDirectoryEmpty: (String) -> Bool
   public var removeItem: (String) throws -> Void
   public var createDirectory: (String) throws -> Void
+  public var saveFile: (String, Data) throws -> Void
+  public var loadFile: (String) throws -> Data?
+  public var modifiedTime: (String) throws -> Date?
 }
 
 extension MessengerFileManager {
@@ -26,6 +29,16 @@ extension MessengerFileManager {
           atPath: path,
           withIntermediateDirectories: true
         )
+      },
+      saveFile: { path, data in
+        try data.write(to: URL(fileURLWithPath: path))
+      },
+      loadFile: { path in
+        try Data(contentsOf: URL(fileURLWithPath: path))
+      },
+      modifiedTime: { path in
+        let attributes = try fileManager.attributesOfItem(atPath: path)
+        return attributes[.modificationDate] as? Date
       }
     )
   }
@@ -35,6 +48,9 @@ extension MessengerFileManager {
   public static let unimplemented = MessengerFileManager(
     isDirectoryEmpty: XCTUnimplemented("\(Self.self).isDirectoryEmpty", placeholder: false),
     removeItem: XCTUnimplemented("\(Self.self).removeItem"),
-    createDirectory: XCTUnimplemented("\(Self.self).createDirectory")
+    createDirectory: XCTUnimplemented("\(Self.self).createDirectory"),
+    saveFile: XCTUnimplemented("\(Self.self).saveFile"),
+    loadFile: XCTUnimplemented("\(Self.self).loadFile"),
+    modifiedTime: XCTUnimplemented("\(Self.self).modifiedTime")
   )
 }