chore: init auth provider as base class

Author: russellwheatley

FirebaseSwiftUI/FirebaseAuthSwiftUI/Sources/FirebaseAuthSwiftUI/FirebaseAuthSwiftUI.swift

@@ -1,2 +1,98 @@
 // The Swift Programming Language
 // https://docs.swift.org/swift-book
+import SwiftUI
+import FirebaseAuth
+import Combine
+
+
+protocol AuthListenerProtocol {
+    func onError(_ error: Error)
+    func onBeforeSignIn()
+    func onSignedIn(_ user: User)
+    func onCanceled()
+    // TODO - add when I get to this point
+    // func onCredentialReceived(_ credential: AuthCredential)
+    // func onCredentialLinked(_ credential: AuthCredential)
+    // func onMFARequired(_ resolver: MultiFactorResolver)
+}
+
+protocol AuthProviderProtocol {
+    associatedtype Listener: AuthListenerProtocol
+    associatedtype Credential: AuthCredential
+    
+    var auth: Auth { get }
+    var authListener: Listener { get set }
+    var providerId: String { get }
+    
+    func signInWithCredential(_ credential: Credential)
+    func linkWithCredential(_ credential: Credential)
+}
+
+enum AuthAction {
+    /// Performs user sign in
+    case signIn
+
+    /// Creates a new account with for a provided credential
+    case signUp
+
+    /// Links a provided credential with currently signed in user account
+    case link
+
+    /// Disables automatic credential handling.
+    /// It's up to the user to decide what to do with the obtained credential.
+    case none
+}
+
+class AuthProvider<Listener: AuthListenerProtocol, Credential: AuthCredential>: AuthProviderProtocol {
+    var auth: Auth = Auth.auth()
+    var authListener: Listener
+  // add providerId to classes that extend this
+//    var providerId: String = "default_provider_id"
+    
+    init(listener: Listener) {
+        self.authListener = listener
+    }
+    
+    func signInWithCredential(_ credential: Credential) {
+        authListener.onBeforeSignIn()
+        auth.signIn(with: credential) { [weak self] result, error in
+            if let error = error {
+                self?.authListener.onError(error)
+            } else if let user = result?.user {
+                self?.authListener.onSignedIn(user)
+            }
+        }
+    }
+    
+    func linkWithCredential(_ credential: Credential) {
+        authListener.onCredentialReceived(credential)
+        guard let user = auth.currentUser else { return }
+        
+        user.link(with: credential) { [weak self] result, error in
+            if let error = error {
+                self?.authListener.onError(error)
+            } else {
+                self?.authListener.onCredentialLinked(credential)
+            }
+        }
+    }
+  
+  func onCredentialReceived(credential: AuthCredential, action: AuthAction) {
+      switch action {
+      case .link:
+          linkWithCredential(credential: credential)
+      case .signIn, .signUp:
+          // Only email provider has a different action for sign in and sign up
+          // and implements its own sign up logic.
+          if shouldUpgradeAnonymous {
+              linkWithCredential(credential: credential)
+          } else {
+              signInWithCredential(credential: credential)
+          }
+      case .none:
+          authListener.onCredentialReceived(credential: credential)
+      }
+  }
+    // TODO - fetchSignInMethodsForEmail/fetchProvidersForEmail is deprecated
+}
+