Add simple desktop app

desktop/lib/main.dart

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+import 'package:flutter/widgets.dart';
+import 'package:musicus_common/musicus_common.dart';
+import 'package:path/path.dart' as p;
+import 'package:path_provider/path_provider.dart' as pp;
+
+import 'settings.dart';
+import 'platform.dart';
+import 'playback.dart';
+
+Future<void> main() async {
+  WidgetsFlutterBinding.ensureInitialized();
+
+  final dir = await pp.getApplicationSupportDirectory();
+  final dbPath = p.join(dir.path, 'db.sqlite');
+
+  runApp(MusicusApp(
+    dbPath: dbPath,
+    settingsStorage: SettingsStorage(),
+    platform: MusicusDesktopPlatform(),
+    playback: MusicusDesktopPlayback(),
+  ));
+}

desktop/lib/platform.dart

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+import 'dart:io';
+
+import 'package:file_chooser/file_chooser.dart';
+import 'package:musicus_common/musicus_common.dart';
+import 'package:path/path.dart' as p;
+
+class MusicusDesktopPlatform extends MusicusPlatform {
+  @override
+  Future<String> chooseBasePath() async {
+    final result = await showOpenPanel(
+      canSelectDirectories: true,
+    );
+
+    if (result != null && !result.canceled) {
+      return result.paths.first;
+    } else {
+      return null;
+    }
+  }
+
+  @override
+  Future<List<Document>> getChildren(String parentId) async {
+    final List<Document> result = [];
+
+    final parent = Directory(parentId ?? basePath);
+    await for (final fse in parent.list()) {
+      result.add(Document(
+        id: fse.path,
+        name: p.basename(fse.path),
+        parent: parentId,
+        isDirectory: fse is Directory,
+      ));
+    }
+
+    return result;
+  }
+
+  @override
+  Future<String> getIdentifier(String parentId, String fileName) async {
+    return p.absolute(parentId, fileName);
+  }
+
+  @override
+  Future<String> readDocument(String id) async {
+    try {
+      return await File(id).readAsString();
+    } on FileSystemException {
+      return null;
+    }
+  }
+
+  @override
+  Future<String> readDocumentByName(String parentId, String fileName) async {
+    try {
+      return await File(p.absolute(parentId, fileName)).readAsString();
+    } on FileSystemException {
+      return null;
+    }
+  }
+
+  @override
+  Future<void> writeDocumentByName(
+      String parentId, String fileName, String contents) async {
+    await File(p.absolute(parentId, fileName)).writeAsString(contents);
+  }
+}

desktop/lib/playback.dart

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+import 'package:musicus_common/musicus_common.dart';
+
+class MusicusDesktopPlayback extends MusicusPlayback {
+  @override
+  Future<void> addTracks(List<InternalTrack> tracks) async {
+    // TODO: implement addTracks
+  }
+
+  @override
+  Future<void> playPause() async {
+    // TODO: implement playPause
+  }
+
+  @override
+  Future<void> removeTrack(int index) async {
+    // TODO: implement removeTrack
+  }
+
+  @override
+  Future<void> seekTo(double pos) async {
+    // TODO: implement seekTo
+  }
+
+  @override
+  Future<void> setup() async {
+    // TODO: implement setup
+  }
+
+  @override
+  Future<void> skipTo(int index) async {
+    // TODO: implement skipTo
+  }
+
+  @override
+  Future<void> skipToNext() async {
+    // TODO: implement skipToNext
+  }
+
+  @override
+  Future<void> skipToPrevious() async {
+    // TODO: implement skipToPrevious
+  }
+}

desktop/lib/settings.dart

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+import 'package:musicus_common/musicus_common.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+class SettingsStorage extends MusicusSettingsStorage {
+  SharedPreferences _pref;
+
+  Future<void> load() async {
+    _pref = await SharedPreferences.getInstance();
+  }
+
+  @override
+  Future<int> getInt(String key) {
+    return Future.value(_pref.getInt(key));
+  }
+
+  @override
+  Future<String> getString(String key) {
+    return Future.value(_pref.getString(key));
+  }
+
+  @override
+  Future<void> setInt(String key, int value) async {
+    await _pref.setInt(key, value);
+  }
+
+  @override
+  Future<void> setString(String key, String value) async {
+    await _pref.setString(key, value);
+  }
+}