我正在尝试读取一个JSON文件，并从最后一个条目中获取一个值，以便在构建小部件时显示在屏幕上。JSON文件存储在本地，并被添加到pubspec.yaml中。每次我转到my test页面查看是否显示了该值时，我都会得到下面的错误截图。我不知道我做错了什么。
这是我的podo:
import 'dart:convert';
List<HistoryData> historyDataFromJson(String str) => List<HistoryData>.from(json.decode(str).map((x) => HistoryData.fromJson(x)));
String historyDataToJson(List<HistoryData> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));
class HistoryData {
HistoryData({
this.date,
this.weight,
this.loss,
this.change,
});
String date;
String weight;
String loss;
String change;
factory HistoryData.fromJson(Map<String, dynamic> json) => HistoryData(
date: json["date"],
weight: json["weight"],
loss: json["loss"],
change: json["change"],
);
Map<String, dynamic> toJson() => {
"date": date,
"weight": weight,
"loss": loss,
"change": change,
};
}
这个小部件将创建我的屏幕:
class Test extends StatefulWidget {
@override
_TestState createState() => _TestState();
}
class _TestState extends State<Test> {
String current = '';
void initState() {
super.initState();
current = getCurrentWeight();
}
@override
Widget build(BuildContext context) {
return Container(
child: Center(
child: Text(current)
),
);
}
String getCurrentWeight() {
List<HistoryData> historyList = historyDataFromJson(rootBundle.loadString('json_files/history.json').toString());
var history = historyList[historyList.length-1];
String current = history.weight;
return current;
}
}
更新:根据要求，这里是整个JSON文件。
[
{
"date" : "17/06/2020",
"weight" : "95.0",
"loss" : "+0.0",
"change" : "+0.0"
},
{
"date" : "18/06/2020",
"weight" : "96.0",
"loss" : "+1.0",
"change" : "+1.1"
},
{
"date" : "19/06/2020",
"weight" : "95.1",
"loss" : "-0.9",
"change" : "-0.9"
},
{
"date" : "20/06/2020",
"weight" : "94.2",
"loss" : "-0.9",
"change" : "-0.9"
},
{
"date" : "21/06/2020",
"weight" : "92.0",
"loss" : "-2.2",
"change" : "-2.3"
},
{
"date" : "22/06/2020",
"weight" : "90.6",
"loss" : "-1.4",
"change" : "-1.5"
},
{
"date" : "23/06/2020",
"weight" : "89.6",
"loss" : "-1.0",
"change" : "-1.1"
},
{
"date" : "24/06/2020",
"weight" : "89.4",
"loss" : "-0.2",
"change" : "-0.2"
},
{
"date" : "25/06/2020",
"weight" : "87.8",
"loss" : "-1.6",
"change" : "-1.8"
},
{
"date" : "26/06/2020",
"weight" : "86.1",
"loss" : "-1.7",
"change" : "-1.9"
}
]
RootBundle.LoadString()返回一个future，就像错误所暗示的那样。然后对它执行toString，它是...的实例，由于它不是JSON，因此导致了特定的错误。
您需要awaitrootBundle.loadString('json_files/history.json'):
Future<String> getCurrentWeight() async {
List<HistoryData> historyList = historyDataFromJson(await rootBundle.loadString('json_files/history.json'));
var history = historyList[historyList.length-1];
String current = history.weight;
return current;
}
然后，您必须修改您的小部件，以便用FutureBuilder正确处理和显示这些未来数据。
class _TestState extends State<Test> {
Future<String> current;
@override
void initState() {
super.initState();
current = getCurrentWeight();//Obtain your future
}
@override
Widget build(BuildContext context) {
return FutureBuilder(
future: current,//Pass the future
builder: (context, snapshot) {
if(snapshot.hasData) {//Show data only when it's available
return Container(
child: Center(
child: Text(snapshot.data)//Obtain data here
),
);
}
return CircularProgressIndicator();//Show this otherwise
}
);
}
Future<String> getCurrentWeight() async {
List<HistoryData> historyList = historyDataFromJson(await rootBundle.loadString('json_files/history.json'));
var history = historyList[historyList.length-1];
String current = history.weight;
return current;
}
}
