我开始学习Flutter。我正在使用它开发一个简单的应用程序。现在，我正在开发一项功能，其中我的应用程序将显示SQLite数据库中的记录，并且用户将新记录添加到SQLite数据库中。但是我的ListView显示空白屏幕。
我有一个名为DatabaseHelper的类，其中包含以下代码。
class DatabaseHelper {
static DatabaseHelper _databaseHelper;
Database _database;
String noteTable = 'note_table';
String colId = 'id';
String colTitle = 'title';
String colDescription = 'description';
String colPriority = 'priority';
String colDate = 'date';
DatabaseHelper._createInstance();
factory DatabaseHelper() {
if (_databaseHelper == null) {
_databaseHelper = DatabaseHelper._createInstance();
}
return _databaseHelper;
}
Future<Database> get database async {
if (_database == null) {
_database = await initializeDatabase();
}
return _database;
}
Future<Database> initializeDatabase() async {
Directory directory = await getApplicationDocumentsDirectory();
String path = directory.path + 'notes.db';
var notesDatabase = await openDatabase(path, version: 1, onCreate: _createDB);
return notesDatabase;
}
void _createDB(Database db, int newVersion) async {
await db.execute('CREATE TABLE $noteTable($colId INTEGER PRIMARY KEY AUTOINCREMENT, $colTitle TEXT, $colDescription TEXT, $colPriority INTEGER, $colDate TEXT)');
}
Future<List<Map<String, dynamic>>> getNoteMapList() async {
Database db = await this.database;
return await db.query(noteTable, orderBy: '$colPriority ASC');
}
Future<int> insertNote(Note note) async {
Database db = await this.database;
return await db.insert(noteTable, note.toMap());
}
Future<int> updateNote(Note note) async {
var db = await this.database;
return await db.update(noteTable, note.toMap(), where: '$colId = ?', whereArgs: [note.id]);
}
Future<int> deleteNote(int id) async {
var db = await this.database;
return await db.rawDelete('DELETE FROM $noteTable WHERE $colId = $id');
}
Future<int> getCount() async {
Database db = await this.database;
List<Map<String, dynamic>> x = await db.rawQuery('SELECT COUNT(*) FROM $noteTable');
return Sqflite.firstIntValue(x);
}
}
然后，我有一个名为NoteList的小部件，其中包含以下代码，其中显示了项目列表。
class NoteList extends StatefulWidget {
@override
State<StatefulWidget> createState() {
return _NoteListState();
}
}
class _NoteListState extends State<NoteList> {
List<Note> _notes = [];
int _count = 0;
DatabaseHelper _databaseHelper = DatabaseHelper();
_NoteListState() {
this._notes = getNotes();
this._count = _notes.length;
}
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text("Notes"),),
body: Container(
child: getListView(context),
),
floatingActionButton: FloatingActionButton(
child: Icon(Icons.add),
onPressed: () {
navigateToNoteForm("Add Note");
},
),
);
}
Widget getListView(BuildContext context) {
return ListView.builder(
itemCount: _count,
itemBuilder: (context, index) {
return ListTile(
leading: CircleAvatar(
backgroundColor: _notes[index].priority == 1? Colors.yellow: Colors.red,
child: Icon(_notes[index].priority == 1 ? Icons.arrow_right : Icons.add),
),
title: Text(_notes[index].title),
subtitle: Text(_notes[index].date),
trailing: Icon(Icons.delete),
onTap: () {
navigateToNoteForm("Edit Note", _notes[index]);
},
);
});
}
void navigateToNoteForm(String pageTitle, [Note note]) async {
bool result = await Navigator.push(context, MaterialPageRoute(builder: (context) {
return NoteForm(pageTitle, note);
}));
if (result) {
setState(() {
debugPrint("Updating list");
_notes = getNotes();
_count = _notes.length;
});
}
}
List<Note> getNotes() {
List<Note> notes = List<Note>();
Future<List<Map<String, dynamic>>> notesFuture = _databaseHelper.getNoteMapList();
notesFuture.then((notesMap) {
debugPrint("Total notes found in the database ${notesMap.length}");
notesMap.forEach((map) {
notes.add(Note.fromMapObject(map));
});
});
return notes;
}
}
Then I also have another widget class called NoteForm with the following code.
class NoteForm extends StatefulWidget {
String _title = "";
Note _note = null;
NoteForm(String title, [Note note]) {
this._title = title;
this._note = note;
}
@override
State<StatefulWidget> createState() {
return _NoteFormState();
}
}
class _NoteFormState extends State<NoteForm> {
double _minimumPadding = 15.0;
var _priorities = [ 1, 2 ];
var _titleController = TextEditingController();
var _descriptionController = TextEditingController();
var _dateController = TextEditingController();
DatabaseHelper _databaseHelper = DatabaseHelper();
var _selectedPriority = 1;
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text(widget._title),),
body: Builder(
builder: (scaffoldContext) => Form(
child: Column(
children: <Widget>[
Container(
child: Padding(
padding: EdgeInsets.all(_minimumPadding),
child: TextFormField(
controller: _titleController,
decoration: InputDecoration(
labelText: "Title",
hintText: "Enter title"
),
),
),
),
Container(
child: Padding(
padding: EdgeInsets.all(_minimumPadding),
child: TextFormField(
controller: _descriptionController,
decoration: InputDecoration(
labelText: "Description",
hintText: "Enter description"
),
),
)
),
Container(
child: Padding(
padding: EdgeInsets.all(_minimumPadding),
child: TextFormField(
controller: _dateController,
decoration: InputDecoration(
labelText: "Date",
hintText: "Enter date"
),
),
),
),
Container(
child: Padding(
padding: EdgeInsets.all(_minimumPadding),
child: DropdownButton<int>(
value: _selectedPriority,
items: _priorities.map((dropdownItem) {
return DropdownMenuItem<int>(
value: dropdownItem,
child: Text(dropdownItem == 1? "Low": "High"),
);
}).toList(),
onChanged: (int newSelectedValue) {
setState(() {
_selectedPriority = newSelectedValue;
});
},
),
),
),
Container(
child: Padding(
padding: EdgeInsets.all(_minimumPadding),
child: RaisedButton(
child: Text(
"Save"
),
onPressed: () {
_save(scaffoldContext);
},
),
),
)
],
),
),
)
);
}
void _save(BuildContext context) async {
Note note = Note();
note.title = _titleController.text;
note.description = _descriptionController.text;
note.date = _dateController.text;
note.priority = _selectedPriority;
if (widget._note != null && widget._note.id!=null) {
//update
_databaseHelper.updateNote(note);
this.showSnackBar(context, "Note has been updated.");
} else {
//create
_databaseHelper.insertNote(note);
this.showSnackBar(context, "Note has been added.");
}
closeForm(context);
}
void showSnackBar(BuildContext context, String message) {
var snackBar = SnackBar(
content: Text(message),
action: SnackBarAction(
label: "UNDO",
onPressed: () {
},
),
);
Scaffold.of(context).showSnackBar(snackBar);
}
void closeForm(BuildContext context) {
Navigator.pop(context, true);
}
}
当我运行我的应用程序时，它仅显示空白屏幕，如下所示。
如您所见，我正在使用debugPrint方法注销从数据库返回的记录数。就是说数据库中有6条记录。它只是不显示记录。我的代码有什么问题，我该如何解决？
最佳答案
正如我在评论中提到的那样，由于异步任务而发生的事情需要花费一些时间来执行，如果不保持异步，则setState函数将在实际数据加载或设置之前执行。
因此，进行以下更改可以解决您的问题。
使getNotes async method和
getNotes().then((noteresponce){ setState((){ _notes=noteresponce; _count = _notes.length;} });
关于sqlite - 列表项更改时Flutter ListView不会更新，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/61022155/