Better login error handling

Reader/Screens/Login/LoginViewController.swift

@@ -9,6 +9,7 @@ import UIKit
 import AuthenticationServices
 import Fervor
 
+@MainActor
 protocol LoginViewControllerDelegate: AnyObject {
     func didLogin(with controller: FervorController)
 }
@@ -18,6 +19,7 @@ class LoginViewController: UIViewController {
     weak var delegate: LoginViewControllerDelegate?
     
     private var textField: UITextField!
+    private var activityIndicator: UIActivityIndicatorView!
     
     private var authSession: ASWebAuthenticationSession?
 
@@ -37,10 +39,18 @@ class LoginViewController: UIViewController {
         textField.placeholder = "example.com"
         textField.addTarget(self, action: #selector(doEnteredURL), for: .primaryActionTriggered)
         view.addSubview(textField)
+        
+        activityIndicator = UIActivityIndicatorView(style: .medium)
+        activityIndicator.translatesAutoresizingMaskIntoConstraints = false
+        view.addSubview(activityIndicator)
+        
         NSLayoutConstraint.activate([
             textField.centerYAnchor.constraint(equalTo: view.centerYAnchor),
             textField.centerXAnchor.constraint(equalTo: view.centerXAnchor),
             textField.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.75),
+            
+            activityIndicator.centerXAnchor.constraint(equalTo: view.centerXAnchor),
+            activityIndicator.topAnchor.constraint(equalTo: textField.bottomAnchor, constant: 8),
         ])
     }
     
@@ -59,14 +69,22 @@ class LoginViewController: UIViewController {
             return
         }
         
+        textField.isEnabled = false
+        activityIndicator.startAnimating()
+        
         let controller = FervorController(instanceURL: components.url!)
         
         let registration: ClientRegistration
         do {
             registration = try await controller.register()
         } catch {
-            let alert = UIAlertController(title: "Unable to Register Client", message: error.localizedDescription, preferredStyle: .alert)
+            let alert = UIAlertController(title: "Unable to register client", message: error.localizedDescription, preferredStyle: .alert)
+            alert.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
             present(alert, animated: true)
+            
+            textField.isEnabled = true
+            activityIndicator.stopAnimating()
+            
             return
         }
         
@@ -81,13 +99,29 @@ class LoginViewController: UIViewController {
             let components = URLComponents(url: callbackURL!, resolvingAgainstBaseURL: false)
             guard let codeItem = components?.queryItems?.first(where: { $0.name == "code" }),
                   let codeValue = codeItem.value else {
-                      fatalError()
+                      DispatchQueue.main.async {
+                          let alert = UIAlertController(title: "Unable to retrieve authorization code", message: error?.localizedDescription ?? "Unknown Error", preferredStyle: .alert)
+                          alert.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
+                          self.present(alert, animated: true)
+                          
+                          self.textField.isEnabled = true
+                          self.activityIndicator.stopAnimating()
+                      }
+                      return
                   }
-            Task {
-                try! await controller.getToken(authCode: codeValue)
-                DispatchQueue.main.async {
-                    self.delegate?.didLogin(with: controller)
+            Task { @MainActor in
+                do {
+                    try await controller.getToken(authCode: codeValue)
+                } catch {
+                    let alert = UIAlertController(title: "Unable to retrieve access token", message: error.localizedDescription, preferredStyle: .alert)
+                    alert.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
+                    self.present(alert, animated: true)
+                    
+                    self.textField.isEnabled = true
+                    self.activityIndicator.stopAnimating()
+                    return
                 }
+                self.delegate?.didLogin(with: controller)
             }
         }