IOS-651 Add FriendlyError protocol

Sources/Errors/FriendlyError.swift

@@ -0,0 +1,84 @@
+//
+//  Created by Ettore Pasquini on 3/13/24.
+//  Copyright © 2024 The New York Public Library. All Rights Reserved.
+//
+
+/// A protocol that all `Error` subclass should extend to support user and
+/// developer friendly error messaging (and logging) inside the NYPL eReader.
+///
+/// - Important: different errors -- with different underlying causes and
+/// developer friendly descriptions / errorIDs -- may have the same user
+/// friendly description.
+public protocol FriendlyError: Error {
+
+    /// A localized description of the error that will be understandable by
+    /// most users of the app.
+    var userFriendlyMessage: String {get}
+
+    /// A very short lowercase string common to related errors.
+    var domain: String {get}
+
+    /// A very short lowercase string identifying the error within the `domain`.
+    var code: String {get}
+
+    // MARK: Properties with default implementations
+
+    /// A localized description of the error that will be understandable by
+    /// most users of the app and that provides the `errorID`.
+    ///
+    /// **Default implementation provided.**
+    ///
+    /// - Important: there's a 1-many relation between this and
+    /// `devFriendlyDescription` / `errorID`.
+    var userFriendlyDescription: String {get}
+
+    /// A detailed description of the error in English. 
+    ///
+    /// **Default implementation provided.**
+    ///
+    /// - Important: there's a 1-1 relation between this and `errorID`.
+    var devFriendlyDescription: String {get}
+
+    /// A short identifier of the error that may be displayed to the user.
+    /// This error ID should be unique across server, web client, native client.
+    ///
+    /// **Default implementation provided.**
+    ///
+    /// This MUST include the `domain` string + a `code` unique to the domain.
+    /// This combination will help in identifying the underlying cause of a
+    /// problem reported by a user.
+    var errorID: String {get}
+}
+
+// MARK: - Default Implementations
+
+public extension FriendlyError {
+    /// Default implementation.
+    ///
+    /// This default implementation provides, in order:
+    /// 1. The localized message to the user;
+    /// 2. `The errorID` that will help identify/debug the error.
+    var userFriendlyDescription: String {
+        "\(userFriendlyMessage) [\(errorID)]"
+    }
+
+    /// Default implementation.
+    ///
+    /// This default implementation provides, in order:
+    /// 1. The type of the error
+    /// 2. String interpolation of `self` (doesn't seem to include the type,
+    ///    but it does include interpolation of nested errors, if present)
+    /// 3. `The errorID`
+    /// 4. The system `localizedDescription` of the error. This is sometimes
+    /// redundant but it's provided as additional context.
+    var devFriendlyDescription: String {
+        "[\(type(of: self)): \(self)] (\(errorID)) \(localizedDescription)"
+    }
+
+    /// Default implementation.
+    ///
+    /// This default implementation provides the error `domain` and `code`.
+    var errorID: String {
+        "\(domain): \(code)"
+    }
+}