【发布时间】：2020-07-17 12:39:58
【问题描述】：
我对 Flutter / Dart 还是有点陌生，而且 Ive 已经在解析 JSON 方面苦苦挣扎了一段时间。这对我来说似乎是一项艰巨的任务，尽管我认为我的 JSON 结构并不复杂。
非常感谢您的帮助。
这是我要解析的 JSON：
{
"predictionICL":{
"openingTimeToday":"8:00 - 23:00",
"openingTimeTomorrow":"8:00 - 23:00",
"percentagesToday":[
3,
5,
11,
17,
20,
23,
25,
26,
25,
29,
30,
32,
31,
35,
40,
43,
44,
46,
49,
53,
50,
56,
54,
60,
62,
61,
69,
70,
75,
76,
84,
90,
94,
100,
93,
81,
72,
70,
73,
71,
63,
59,
55,
56,
51,
49,
50,
45,
43,
40,
38,
35,
31,
27,
25,
23,
20,
20,
19,
17,
12,
9,
8,
2,
1
],
"percentagesTomorrow":[
0,
0,
1,
7,
14,
20,
22,
21,
21,
22,
20,
25,
27,
31,
30,
31,
32,
33,
30,
34,
35,
33,
35,
37,
39,
40,
40,
39,
38,
40,
41,
38,
34,
37,
34,
35,
33,
32,
31,
30,
33,
30,
31,
30,
29,
30,
27,
28,
26,
23,
20,
19,
16,
17,
15,
12,
10,
7,
5,
1,
1,
0,
0,
0,
0
]
},
"predictionRandwyck":{
"openingTimeToday":"8:00 - 23:00",
"openingTimeTomorrow":"8:00 - 23:00",
"percentagesToday":[
3,
5,
11,
17,
20,
23,
25,
26,
25,
29,
30,
32,
31,
35,
40,
43,
44,
46,
49,
53,
50,
56,
54,
60,
62,
61,
69,
70,
75,
76,
84,
90,
94,
100,
93,
81,
72,
70,
73,
71,
63,
59,
55,
56,
51,
49,
50,
45,
43,
40,
38,
35,
31,
27,
25,
23,
20,
20,
19,
17,
12,
9,
8,
2,
1
],
"percentagesTomorrow":[
0,
0,
1,
7,
14,
20,
22,
21,
21,
22,
20,
25,
27,
31,
30,
31,
32,
33,
30,
34,
35,
33,
35,
37,
39,
40,
40,
39,
38,
40,
41,
38,
34,
37,
34,
35,
33,
32,
31,
30,
33,
30,
31,
30,
29,
30,
27,
28,
26,
23,
20,
19,
16,
17,
15,
12,
10,
7,
5,
1,
1,
0,
0,
0,
0
]
},
"predictionTapijn":{
"openingTimeToday":"8:00 - 23:00",
"openingTimeTomorrow":"8:00 - 23:00",
"percentagesToday":[
3,
5,
11,
17,
20,
23,
25,
26,
25,
29,
30,
32,
31,
35,
40,
43,
44,
46,
49,
53,
50,
56,
54,
60,
62,
61,
69,
70,
75,
76,
84,
90,
94,
100,
93,
81,
72,
70,
73,
71,
63,
59,
55,
56,
51,
49,
50,
45,
43,
40,
38,
35,
31,
27,
25,
23,
20,
20,
19,
17,
12,
9,
8,
2,
1
],
"percentagesTomorrow":[
0,
0,
1,
7,
14,
20,
22,
21,
21,
22,
20,
25,
27,
31,
30,
31,
32,
33,
30,
34,
35,
33,
35,
37,
39,
40,
40,
39,
38,
40,
41,
38,
34,
37,
34,
35,
33,
32,
31,
30,
33,
30,
31,
30,
29,
30,
27,
28,
26,
23,
20,
19,
16,
17,
15,
12,
10,
7,
5,
1,
1,
0,
0,
0,
0
]
},
"message":"optionalmessageString"
}
它基本上只是数据类型 LibraryPrediction 的三个实例和一个可选的消息字符串。
数据类型 LibraryPrediction 由一个字符串“openingTimeToday”、一个字符串“openingTimeTomorrow”和两个双精度数组“percentagesToday”和“percentagesTomorrow”组成。
现在，我正在尝试从磁盘上解析 json，因为我的服务器尚未运行。 到目前为止，这是我的代码：
我有一个服务文件：
import 'dart:convert';
import 'package:flutter/services.dart';
import 'package:test_app/models/predictions_update_model.dart';
PredictionsUpdate parseUpdate(String responseBody) {
final jsonResponse = json.decode(responseBody).cast<Map<String, dynamic>>();
PredictionsUpdate update = jsonResponse.map<PredictionsUpdate>((json) => PredictionsUpdate.fromJson(json));
return update;
}
Future<PredictionsUpdate> fetchUpdate() async {
final response = await rootBundle.loadString('lib/testJson/data.json');
return parseUpdate(response);
}
还有一个模型文件：
class PredictionsUpdate {
final LibraryPrediction predictionICL;
final LibraryPrediction predictionRandwyck;
final LibraryPrediction predictionTapijn;
final String message;
PredictionsUpdate({
this.predictionICL,
this.predictionRandwyck,
this.predictionTapijn,
this.message,
});
factory PredictionsUpdate.fromJson(Map<String, dynamic> parsedJson){
return PredictionsUpdate(
predictionICL: LibraryPrediction.fromJson(parsedJson['predictionICL']),
predictionRandwyck: LibraryPrediction.fromJson(parsedJson['predictionRandwyck']),
predictionTapijn: LibraryPrediction.fromJson(parsedJson['predictionTapijn']),
message: parsedJson['message'] as String,
);
}
}
class LibraryPrediction {
final String openingTimeToday;
final String openingTimeTomorrow;
final List<double> percentagesToday;
final List<double> percentagesTomorrow;
LibraryPrediction({
this.openingTimeToday,
this.openingTimeTomorrow,
this.percentagesToday,
this.percentagesTomorrow,
});
factory LibraryPrediction.fromJson(Map<String, dynamic> json){
return LibraryPrediction(
openingTimeToday: json['openingTimeToday'] as String,
openingTimeTomorrow: json['openingTimeTomorrow'] as String,
percentagesToday: json['percentagesToday'] as List<double>,
percentagesTomorrow: json['percentagesTomorrow'] as List<double>,
);
}
}
这就是我调用函数的方式：
Row(
children: <Widget>[
RaisedButton(
child: Text('update'),
onPressed: () {
Future<PredictionsUpdate> futureUpdate = fetchUpdate();
futureUpdate.then((update)=> widget.currentNumbers = update)
.catchError((error) => print(error));
},
),
],
),
每当我尝试解析 JSON 时，都会收到以下错误：
flutter: NoSuchMethodError: Class '_InternalLinkedHashMap<String, dynamic>' has no instance method 'cast' with matching arguments.
Receiver: _LinkedHashMap len:4
Tried calling: cast<Map<String, dynamic>>()
Found: cast<RK, RV>() => Map<RK, RV>
我有这种感觉，当我尝试解析双数组“percentagesToday”或“percentagesTomorrow”时，错误源自某处，但我不能肯定地说，我无法从错误消息中获得更多线索。
如果我能帮我找出错误所在，我将不胜感激。
【问题讨论】：