feat: add pin lock / fix performance on history view

lib/viewmodels/pin_lock_view_model.dart

@@ -0,0 +1,134 @@
+import 'package:flutter/foundation.dart';
+
+import '../services/pin_lock_service.dart';
+
+class PinLockViewModel extends ChangeNotifier {
+  final PinLockService pinLockService;
+
+  PinLockViewModel({required this.pinLockService});
+
+  bool _isPinSet = false;
+  bool _isUnlocked = false;
+  bool _isLoading = false;
+  bool _hasLoaded = false;
+  String? _errorMessage;
+
+  bool get isPinSet => _isPinSet;
+
+  bool get isUnlocked => _isUnlocked;
+
+  bool get isLoading => _isLoading;
+
+  bool get hasLoaded => _hasLoaded;
+
+  String? get errorMessage => _errorMessage;
+
+  Future<void> ensureLoaded() async {
+    if (_hasLoaded || _isLoading) return;
+
+    await load();
+  }
+
+  /// Checks whether a PIN has already been configured on this device.
+  Future<void> load() async {
+    if (_isLoading) return;
+
+    _isLoading = true;
+    _errorMessage = null;
+    notifyListeners();
+
+    try {
+      _isPinSet = await pinLockService.hasPin();
+    } catch (_) {
+      _errorMessage = 'Could not check PIN status.';
+    } finally {
+      _hasLoaded = true;
+      _isLoading = false;
+      notifyListeners();
+    }
+  }
+
+  /// Creates a new PIN (first-time setup, or after disabling an old one).
+  /// Unlocks the app immediately on success, since the person just proved
+  /// they know it by typing it.
+  Future<bool> setPin(String pin) async {
+    _errorMessage = null;
+
+    try {
+      await pinLockService.setPin(pin);
+      _isPinSet = true;
+      _isUnlocked = true;
+      notifyListeners();
+      return true;
+    } catch (_) {
+      _errorMessage = 'Could not save PIN.';
+      notifyListeners();
+      return false;
+    }
+  }
+
+  /// Checks an entered PIN against the stored one, unlocking on match.
+  Future<bool> verify(String pin) async {
+    _errorMessage = null;
+
+    try {
+      final matches = await pinLockService.verifyPin(pin);
+
+      if (matches) {
+        _isUnlocked = true;
+        notifyListeners();
+        return true;
+      }
+
+      _errorMessage = 'Incorrect PIN.';
+      notifyListeners();
+      return false;
+    } catch (_) {
+      _errorMessage = 'Could not verify PIN.';
+      notifyListeners();
+      return false;
+    }
+  }
+
+  /// Replaces the current PIN. Requires the current PIN to match first.
+  Future<bool> changePin({
+    required String currentPin,
+    required String newPin,
+  }) async {
+    final currentMatches = await pinLockService.verifyPin(currentPin);
+
+    if (!currentMatches) {
+      _errorMessage = 'Current PIN is incorrect.';
+      notifyListeners();
+      return false;
+    }
+
+    return setPin(newPin);
+  }
+
+  /// Removes the PIN entirely. Requires the current PIN to confirm.
+  Future<bool> disablePin(String currentPin) async {
+    final matches = await pinLockService.verifyPin(currentPin);
+
+    if (!matches) {
+      _errorMessage = 'Current PIN is incorrect.';
+      notifyListeners();
+      return false;
+    }
+
+    await pinLockService.clearPin();
+    _isPinSet = false;
+    notifyListeners();
+    return true;
+  }
+
+  /// Re-locks the app. Call this on app backgrounding (e.g. from a
+  /// WidgetsBindingObserver on AppLifecycleState.paused) if you want the
+  /// PIN required again after the app is put away, not just on cold start.
+  void lock() {
+    if (!_isUnlocked) return;
+
+    _isUnlocked = false;
+    notifyListeners();
+  }
+}