我正在尝试从 Internet 获取一些数据，将其存储到我的 sqlite 数据库并使用ListView.
当我第一次获取数据时，一切正常并且我能够在屏幕上看到数据，数据也插入到 sqlite 数据库中，但是当我重新打开应用程序时，我收到一条错误消息
flutter: The following NoSuchMethodError was thrown building StreamBuilder<StudentModel>(dirty, state:
flutter: _StreamBuilderBaseState<StudentModel, AsyncSnapshot<StudentModel>>#3f888):
flutter: The getter 'studentData' was called on null.
flutter: Receiver: null
flutter: Tried calling: studentData
这是我的模型课
class StudentModel {
int status;
String msg;
StudentModelData studentModelData;
StudentModel({this.status, this.msg, this.studentModelData});
StudentModel.fromJson(Map<String, dynamic> json) {
status = json['status'];
msg = json['msg'];
studentModelData = json['data'] != null ? new StudentModelData.fromJson(json['data']) : null;
}
StudentModel.fromDb(Map<String, dynamic> parsedJson) {
status = parsedJson['status'];
msg = parsedJson['msg'];
studentModelData = parsedJson['data'] != null ? new StudentModelData.fromJson(parsedJson['data']) : null;
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['status'] = this.status;
data['msg'] = this.msg;
if (this.studentModelData != null) {
data['data'] = this.studentModelData.toJson();
}
return data;
}
}
class StudentModelData {
int lastIndex;
List<StudentData> studentData;
StudentModelData({this.lastIndex, this.studentData});
StudentModelData.fromJson(Map<String, dynamic> json) {
lastIndex = json['lastIndex'];
if (json['studentData'] != null) {
studentData = new List<StudentData>();
json['studentData'].forEach((v) {
studentData.add(new StudentData.fromJson(v));
});
}
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['lastIndex'] = this.lastIndex;
if (this.studentData != null) {
data['studentData'] = this.studentData.map((v) => v.toJson()).toList();
}
return data;
}
}
class StudentData {
String studentId;
String studName;
String studProfilepic;
String studentEmail;
String studentMobile;
String courseName;
String classCode;
int minAvg;
int avg;
StudentData(
{this.studentId,
this.studName,
this.studProfilepic,
this.studentEmail,
this.studentMobile,
this.courseName,
this.classCode,
this.minAvg,
this.avg});
StudentData.fromJson(Map<String, dynamic> json) {
studentId = json['student_id'];
studName = json['stud_name'];
studProfilepic = json['stud_profilepic'];
studentEmail = json['student_email'];
studentMobile = json['student_mobile'];
courseName = json['course_name'];
classCode = json['class_code'];
minAvg = json['minAvg'];
avg = json['avg'];
}
StudentData.fromDb(Map<String, dynamic> parsedJson){
studentId = parsedJson['student_id'];
studName = parsedJson['stud_name'];
studProfilepic = parsedJson['stud_profilepic'];
studentEmail = parsedJson['student_email'];
studentMobile = parsedJson['student_mobile'];
courseName = parsedJson['course_name'];
classCode = parsedJson['class_code'];
minAvg = parsedJson['minAvg'];
avg = parsedJson['avg'];
}
Map<String, dynamic> toJson() {
final Map<String, dynamic> data = new Map<String, dynamic>();
data['student_id'] = this.studentId;
data['stud_name'] = this.studName;
data['stud_profilepic'] = this.studProfilepic;
data['student_email'] = this.studentEmail;
data['student_mobile'] = this.studentMobile;
data['course_name'] = this.courseName;
data['class_code'] = this.classCode;
data['minAvg'] = this.minAvg;
data['avg'] = this.avg;
return data;
}
}
以下是我的存储库类
class StudentDbProvider implements Source, Cache {
Database db;
void init() async {
print("database initialized");
Directory documentsDirectory = await getApplicationDocumentsDirectory();
final path = join(documentsDirectory.path, "students.db");
db = await openDatabase(path, version: 1,
onCreate: (Database newDb, int version) {
newDb.execute("""
CREATE TABLE STUDENTS(
id INTEGER PRIMARY KEY,
student_id TEXT,
stud_name TEXT,
stud_profilepic TEXT,
student_email TEXT,
student_mobile TEXT,
course_name TEXT,
class_code TEXT,
minAvg TEXT,
avg TEXT
)
""");
});
}
@override
Future<int> clear() {
return db.delete("STUDENTS");
}
@override
Future<StudentModel> fetchStudents(String disciplineId, String schoolId,
String year_id, String lastIndex) async {
print("PritishSawant${db==null}");
final maps =
await db.query("STUDENTS");
if (maps.length > 0) {
return StudentModel.fromDb(maps.first);
}
return null;
}
@override
Future<int> addStudent(StudentData studentData) {
return db.insert("STUDENTS", studentData.toJson(),
conflictAlgorithm: ConflictAlgorithm.ignore);
}
}
final studentDbProvider = StudentDbProvider();
以下是我的集团课程
class StudentsBloc {
final _repository = Repository();
final _students = PublishSubject<StudentModel>();
Observable<StudentModel> get students => _students.stream;
fetchStudents(String disciplineId,String schoolId,String year_id,String lastIndex) async {
await studentDbProvider.init();
final student = await _repository.fetchStudents(disciplineId, schoolId, year_id, lastIndex);
_students.sink.add(student);
}
clearCache(){
return _repository.clearCache();
}
dispose(){
_students.close();
}
}
据我所知，错误一定是由于数据库初始化不当而发生的，但是当我进行第一次网络请求时，一切正常，我在控制台中没有收到任何错误，并且数据库也已初始化。我无法理解为什么该错误第二次发生？