import 'dart:io';
import 'dart:convert';

import 'snapshot_store.dart';
import '../domain/app_snapshot.dart';

class FileSnapshotStore extends SnapshotStore {
  FileSnapshotStore(this.directory);
  final Directory directory;
  File get _file => File('${directory.path}/hepan_snapshot.json');
  File get _backup => File('${directory.path}/hepan_snapshot.bak');
  Future<AppSnapshot> _read(File file) async {
    final value = jsonDecode(await file.readAsString());
    if (value is! Map<String, dynamic>) throw const FormatException('保存文件损坏');
    return AppSnapshot.fromJson(value);
  }

  @override
  Future<AppSnapshot?> load() async =>
      await _file.exists() ? await _read(_file) : null;
  @override
  Future<void> save(AppSnapshot snapshot) async {
    await directory.create(recursive: true);
    final text = jsonEncode(snapshot.toJson());
    // Validate before touching the last successfully saved version.
    AppSnapshot.fromJson(jsonDecode(text) as Map<String, dynamic>);
    if (await _file.exists()) {
      try {
        await _read(_file);
        await _file.copy(_backup.path);
      } on FormatException {
        /* Keep an existing valid backup. */
      }
    }
    final temporary = File('${_file.path}.tmp');
    await temporary.writeAsString(text, flush: true);
    await temporary.rename(_file.path);
  }

  @override
  Future<AppSnapshot> restoreBackup() async {
    if (!await _backup.exists()) throw StateError('没有可恢复的备份');
    final previous = await _read(_backup);
    final temporary = File('${_file.path}.tmp');
    await temporary.writeAsString(jsonEncode(previous.toJson()), flush: true);
    await temporary.rename(_file.path);
    return previous;
  }
}