长笛/飞镖-方法'setStringList'在null上调用

Question

我是编程新手。我尝试了一些简单的事情，但我犯了一个错误，我不能再做下去了。这是我得到的NoSuchMethodError：

对null调用了方法"setStringList"。接收方：空尝试调用：setStringList("title"，"_GrowableList"的实例(长度：1((

我现在将向您展示我认为错误的代码：

import 'dart:collection';
import 'dart:convert';
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:pro_todo/models/task.dart';
class TodosModel extends ChangeNotifier {
List<Task> _tasks = [];
SharedPreferences sharedPreferences;
saveSharedPreferences() async {
sharedPreferences = await SharedPreferences.getInstance();
loadData();
}
UnmodifiableListView<Task> get allTasks => UnmodifiableListView(_tasks);
UnmodifiableListView<Task> get incompleteTasks =>
UnmodifiableListView(_tasks.where((task) => !task.completed));
UnmodifiableListView<Task> get completedTasks =>
UnmodifiableListView(_tasks.where((task) => task.completed));
void addTodo(Task task) {
_tasks.add(task);
saveData();
notifyListeners();
}
void toggleTodo(Task task) {
final taskIndex = _tasks.indexOf(task);
_tasks[taskIndex].toggleCompleted();
notifyListeners();
}
void deleteTodo(Task task) {
_tasks.remove(task);
notifyListeners();
}
void saveData() {
List<String> spList =
_tasks.map((task) => json.encode(task.toMap())).toList();
sharedPreferences.setStringList('title', spList);
print(spList);
}
void loadData() {
List<String> spList = sharedPreferences.getStringList('task');
_tasks = spList.map((task) => Task.fromMap(json.decode(task))).toList();
notifyListeners();
}
}

at void saveData=>sharedPreferences.setStringList('title'，spList(是错误。如果我把它标记为注释，程序不会给我任何错误，但也不会保存数据。

我认为要理解_tasks列表，您需要查看以下代码：

import 'package:flutter/material.dart';
class Task {
String title;
bool completed;
Task({@required this.title, this.completed = false});
void toggleCompleted() {
completed = !completed;
}
Task.fromMap(Map map)
: this.title = map['title'],
this.completed = map['completed'];
Map toMap() {
return {
'title': this.title,
'completed': this.completed,
};
}
}

这就是调用addTask方法的方法：

void onAdd() {
final String textVal = taskTitleController.text;
final bool completed = completedStatus;
if (textVal.isNotEmpty) {
final Task task = Task(
title: textVal,
completed: completed,
);
Provider.of<TodosModel>(context, listen: false).addTodo(task);
Navigator.pop(context);
}
}

我希望任何人都能帮助并提前感谢你！

Answer 1

调用方法saveData时，SharedPreferences的实例未初始化在设置列表之前，必须使用sharedPreferences = await SharedPreferences.getInstance();对其进行初始化。

SharedPreferences sharedPreferences;
SharedPreferences  _getSharedPreferences() async {
if(sharedPreferences == null){
sharedPreferences = await SharedPreferences.getInstance();
}
return sharedPreferences;
}
void saveData() async {
List<String> spList =
_tasks.map((task) => json.encode(task.toMap())).toList();
_getSharedPreferences().setStringList('title', spList);
print(spList);
}
void loadData() {
List<String> spList = _getSharedPreferences().getStringList('task');
_tasks = spList.map((task) => Task.fromMap(json.decode(task))).toList();
notifyListeners();
}