Add support for Face ID/Touch ID to app lock. (#1966)

* Fix biometrics with low grace period and backgrounding before unlocked.

* Trigger permissions alert when enabling Face ID.

ElementX/Sources/FlowCoordinators/AppLockFlowCoordinator.swift

@@ -32,6 +32,8 @@ class AppLockFlowCoordinator: CoordinatorProtocol {
     let userIndicatorController: UserIndicatorController
     let navigationCoordinator: NavigationRootCoordinator
     
+    /// A task used to await biometric unlock before showing the PIN screen.
+    @CancellableTask private var unlockTask: Task<Void, Never>?
     private var cancellables: Set<AnyCancellable> = []
     
     private let actionsSubject: PassthroughSubject<AppLockFlowCoordinatorAction, Never> = .init()
@@ -44,6 +46,9 @@ class AppLockFlowCoordinator: CoordinatorProtocol {
         self.userIndicatorController = userIndicatorController
         self.navigationCoordinator = navigationCoordinator
         
+        // Set the initial background state.
+        showPlaceholder()
+        
         appLockService.disabledPublisher
             .sink { [weak self] _ in
                 // When the service is disabled via a force logout, we need to remove the activity indicator.
@@ -71,6 +76,8 @@ class AppLockFlowCoordinator: CoordinatorProtocol {
     // MARK: - App unlock
     
     private func applicationDidEnterBackground() {
+        unlockTask = nil
+        
         guard appLockService.isEnabled else { return }
         
         appLockService.applicationDidEnterBackground()
@@ -80,12 +87,33 @@ class AppLockFlowCoordinator: CoordinatorProtocol {
     private func applicationWillEnterForeground() {
         guard appLockService.isEnabled else { return }
         
-        if appLockService.computeNeedsUnlock(willEnterForegroundAt: .now) {
-            showUnlockScreen()
-        } else {
+        guard appLockService.computeNeedsUnlock(willEnterForegroundAt: .now) else {
             // Reveal the app again if within the grace period.
             actionsSubject.send(.unlockApp)
+            return
         }
+        
+        // Show the relevant unlock mechanism.
+        unlockTask = Task { [weak self] in
+            guard let self else { return }
+            await startUnlockFlow()
+        }
+    }
+    
+    /// Runs the unlock flow, showing Touch ID/Face ID if available, transitioning to PIN unlock if it fails or isn't available.
+    private func startUnlockFlow() async {
+        if appLockService.biometricUnlockEnabled, appLockService.biometricUnlockTrusted {
+            showPlaceholder() // For the unlock background.
+            
+            if await appLockService.unlockWithBiometrics() {
+                actionsSubject.send(.unlockApp)
+                return
+            }
+        }
+        
+        guard !Task.isCancelled else { return }
+        
+        showUnlockScreen()
     }
     
     /// Displays the unlock flow with the app's placeholder view to hide obscure the view hierarchy in the app switcher.

ElementX/Sources/FlowCoordinators/AppLockSetupFlowCoordinator.swift

@@ -140,6 +140,9 @@ class AppLockSetupFlowCoordinator: FlowCoordinatorProtocol {
                 showBiometricsPrompt()
             case (.createPIN, .settings):
                 navigationStackCoordinator.setSheetCoordinator(nil)
+            // The above is fine for change pin, but not create PIN when biometrics are unavailable.
+            // Need to track the two flows differently to call the alternative below.
+            // showSettings()
             case (.biometricsPrompt, .settings):
                 showSettings()
             case (.settings, .createPIN):