问题出在 FutureBuilder 小部件部分。我试图从 FutureBuilder 中的快照获取数据，但收到错误，因为“类型‘Null’不是类型转换中‘DatabaseNotes’类型的子类型”。我尝试将 _note 字段声明为 Late Final 但它仍然抛出相同的错误。我已经通过添加 ? 来处理空安全部分。在数据类型之后（这里是DatabaseNotes）。我还是不明白这里出了什么问题。
以下是NewNoteView小部件的代码：
class NewNoteView extends StatefulWidget {
const NewNoteView({Key? key}) : super(key: key);
@override
State<NewNoteView> createState() => _NewNoteViewState();
}
class _NewNoteViewState extends State<NewNoteView> {
DatabaseNotes? _note;
late final NotesService _notesService;
late final TextEditingController _textController;
@override
void initState() {
_notesService = NotesService();
_textController = TextEditingController();
super.initState();
}
void _textControllerListener() async {
final note = _note;
if (note == null) {
return;
}
final text = _textController.text;
await _notesService.updateNote(
note: note,
text: text,
);
}
void _setupTextControllerListener() {
_textController.removeListener(_textControllerListener);
_textController.addListener(_textControllerListener);
}
Future<DatabaseNotes> createNewNote() async {
final existingNote = _note;
if (existingNote != null) {
return existingNote;
}
final currentUser = AuthService.firebase().currentUser!;
final email = currentUser.email!;
final owner = await _notesService.getUser(email: email);
return await _notesService.createNote(owner: owner);
}
void _deleteNoteIfTextIsEmpty() {
final note = _note;
if (_textController.text.isEmpty && note != null) {
_notesService.deleteNote(id: note.id);
}
}
void _saveNoteIfTextNotEmpty() async {
final note = _note;
final text = _textController.text;
if (note != null && text.isNotEmpty) {
await _notesService.updateNote(
note: note,
text: text,
);
}
}
@override
void dispose() {
_deleteNoteIfTextIsEmpty();
_saveNoteIfTextNotEmpty();
_textController.dispose();
super.dispose();
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('New note'),
),
body: FutureBuilder(
future: createNewNote(),
builder: (context, snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.done:
_note = snapshot.data as DatabaseNotes;
_setupTextControllerListener();
return TextField(
controller: _textController,
keyboardType: TextInputType.multiline,
maxLines: null,
decoration: const InputDecoration(
hintText: 'Start typing your note...',
),
);
default:
return const CircularProgressIndicator();
}
},
));
}
}
以下是NotesService的代码：
class DatabaseAlreadyOpenException implements Exception {}
class NotesService {
Database? _db;
List<DatabaseNotes> _notes = [];
static final NotesService _shared = NotesService._sharedInstance();
NotesService._sharedInstance();
factory NotesService() => _shared;
final _notesStreamController =
StreamController<List<DatabaseNotes>>.broadcast();
Stream<List<DatabaseNotes>> get allNotes => _notesStreamController.stream;
Future<DatabaseUser> getOrCreateUser({required String email}) async {
try {
final user = getUser(email: email);
return user;
} on CouldNotFindUser {
final createdUser = createUser(email: email);
return createdUser;
} catch (e) {
rethrow;
}
}
Future<void> _cacheNotes() async {
final allNotes = await getAllNotes();
_notes = allNotes.toList();
_notesStreamController.add(_notes);
}
Future<DatabaseNotes> updateNote({
required DatabaseNotes note,
required String text,
}) async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
// make sure note exists
await getNote(id: note.id);
// update DB
final updatesCount = await db.update(notesTable, {
textColumn: text,
isSyncedWithCloudColumn: 0,
});
if (updatesCount == 0) {
throw CouldNotUpdateNote();
} else {
final updatedNote = await getNote(id: note.id);
_notes.removeWhere((note) => note.id == updatedNote.id);
_notes.add(updatedNote);
_notesStreamController.add(_notes);
return updatedNote;
}
}
Future<Iterable<DatabaseNotes>> getAllNotes() async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
final notes = await db.query(
notesTable,
);
return notes.map((noteRow) => DatabaseNotes.fromRow(noteRow));
}
Future<DatabaseNotes> getNote({required int id}) async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
final notes = await db.query(
notesTable,
limit: 1,
where: 'id = ?',
whereArgs: [id],
);
if (notes.isEmpty) {
throw CouldNotFindNote();
} else {
final note = DatabaseNotes.fromRow(notes.first);
_notes.removeWhere((note) => note.id == id);
_notes.add(note);
_notesStreamController.add(_notes);
return note;
}
}
Future<int> deleteAllNotes() async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
final numberOfDeletions = await db.delete(notesTable);
_notes = [];
_notesStreamController.add(_notes);
return numberOfDeletions;
}
Future<void> deleteNote({required int id}) async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
final deletedCount = await db.delete(
notesTable,
where: 'id = ?',
whereArgs: [id],
);
if (deletedCount == 0) {
throw CouldNotDeleteNote();
} else {
_notes.removeWhere((note) => note.id == id);
_notesStreamController.add(_notes);
}
}
Future<DatabaseNotes> createNote({required DatabaseUser owner}) async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
// make sure the owner exists in the database with the correct id
final dbUser = await getUser(email: owner.email);
if (dbUser != owner) {
throw CouldNotFindUser();
}
const text = '';
// create the note
final noteId = await db.insert(notesTable, {
userIdColumn: owner.id,
textColumn: text,
isSyncedWithCloudColumn: 1,
});
final note = DatabaseNotes(
id: noteId,
userId: owner.id,
text: text,
isSyncedWithCloud: true,
);
_notes.add(note);
_notesStreamController.add(_notes);
return note;
}
Future<DatabaseUser> getUser({required String email}) async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
final results = await db.query(
userTable,
limit: 1,
where: 'email = ?',
whereArgs: [email.toLowerCase()],
);
if (results.isEmpty) {
throw CouldNotFindUser();
} else {
return DatabaseUser.fromRow(results.first);
}
}
Future<DatabaseUser> createUser({required String email}) async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
final results = await db.query(
userTable,
limit: 1,
where: 'email = ?',
whereArgs: [email.toLowerCase()],
);
if (results.isNotEmpty) {
throw UserAlreadyExists();
}
final userId = await db.insert(userTable, {
emailColumn: email.toLowerCase(),
});
return DatabaseUser(
id: userId,
email: email,
);
}
Future<void> deleteUser({required String email}) async {
await _ensureDbIsOpen();
final db = _getDatabaseOrThrow();
final deletedCount = await db.delete(
userTable,
where: 'email = ?',
whereArgs: [email.toLowerCase()],
);
if (deletedCount != 1) {
throw CouldNotDeleteUser();
}
}
Database _getDatabaseOrThrow() {
final db = _db;
if (db == null) {
throw DatabaseIsNotOpen();
} else {
return db;
}
}
Future<void> close() async {
final db = _db;
if (db == null) {
throw DatabaseIsNotOpen();
} else {
await db.close();
_db = null;
}
}
Future<void> _ensureDbIsOpen() async {
try {
await open();
} on DatabaseAlreadyOpenException {
//empty block
}
}
Future<void> open() async {
if (_db != null) {
throw DatabaseAlreadyOpenException;
}
try {
final docsPath = await getApplicationDocumentsDirectory();
final dbPath = join(docsPath.path, dbName);
final db = await openDatabase(dbPath);
_db = db;
// create the user table
await db.execute(createUserTable);
// create the notes table
await db.execute(createNotesTable);
await _cacheNotes();
} on MissingPlatformDirectoryException {
throw UnableToGetDocumentsDirectory;
}
}
}
@immutable
class DatabaseUser {
final int id;
final String email;
const DatabaseUser({
required this.id,
required this.email,
});
DatabaseUser.fromRow(Map<String, Object?> map)
: id = map[idColumn] as int,
email = map[emailColumn] as String;
@override
String toString() => 'Person id = $id, email = $email';
@override
bool operator ==(covariant DatabaseUser other) => id == other.id;
@override
int get hashCode => id.hashCode;
}
class DatabaseNotes {
final int id;
final int userId;
final String text;
final bool isSyncedWithCloud;
DatabaseNotes({
required this.id,
required this.userId,
required this.text,
required this.isSyncedWithCloud,
});
DatabaseNotes.fromRow(Map<String, Object?> map)
: id = map[idColumn] as int,
userId = map[userIdColumn] as int,
text = map[textColumn] as String,
isSyncedWithCloud = (map[isSyncedWithCloudColumn]) == 1 ? true : false;
@override
String toString() =>
'Note, ID = $id, userId = $userId, isSyncedWithCloud = $isSyncedWithCloud, text = $text';
@override
bool operator ==(covariant DatabaseNotes other) => id == other.id;
@override
int get hashCode => id.hashCode;
}
您可以放置“？”在 DatabaseNotes 之后（如果您更准确地遵循 FreeCodeCamp 教程，则为 DatabaseNote）。然后，您的 IDE 可能会告诉您不需要强制转换为 DatabaseNotes。然后我完全删除了“as DatabaseNotes”并且它起作用了。最后一行代码如下。
_note = snapshot.data;
试试这个
FutureBuilder<DatabaseNotes>(your code);
我发现了错误，结果发现我根本没有处理空安全；我觉得自己像个白痴。 我创建了一个 'DatabaseNotes?' 的实例_note，但是当我在 _note = snapshot.data as DatabaseNotes 行中将 FutureBuilder 内的快照数据分配给 DatabaseNotes 时，我忘记在 DatabaseNotes 后面添加 ?。 该语句应该是： _note = snapshot.data as DatabaseNotes?;
谢谢大家的回答和建议。我非常感谢您的努力。
之后
_note = snapshot.data as DatabaseNotes;
就放
？
这个无效的安全标志。并且您的代码将无错误地运行。我的代码因这个“该死的”标志而受到影响。
有任何建议请