如何定义一个具有属性的模型，该属性可以是null，也可以是Dart中的另一个模型

Question

我正在构建一个Flutter应用程序，在定义模型时遇到了一些问题。我有一个模型，它有一些属性。其中一个属性可以是null，也可以是另一个模型。

import 'package:proef/models/worked_time.dart';
class Shift {
String uuid;
DateTime start_at;
DateTime end_at;
String title;
String comment;
WorkedTime worked_time;
Shift(
{this.uuid,
this.start_at,
this.end_at,
this.title,
this.comment,
this.worked_time});
factory Shift.fromData(Map<String, dynamic> parsedJson) {
print(parsedJson);
return Shift(
uuid: parsedJson['uuid'],
start_at: DateTime.parse(parsedJson['start_at']).toLocal(),
end_at: DateTime.parse(parsedJson['end_at']).toLocal(),
title: parsedJson['title'],
comment: parsedJson['comment'],
worked_time: parsedJson['worked_time'] == null
? null
: parsedJson['worked_time']);
}
}

这不起作用。当我使用这个模型时，它会给我带来以下错误：

Unhandled Exception: type '_InternalLinkedHashMap<String, dynamic>' is not a subtype of type 'WorkedTime'

我不知道如何解决这个问题，因为我对飞镖和长笛还很陌生。

工作时间模型：

class WorkedTime {
String uuid;
String comment;
int status;
int took_break;
DateTime start_at;
DateTime end_at;
WorkedTime({
this.uuid,
this.comment,
this.status,
this.took_break,
this.start_at,
this.end_at,
});
factory WorkedTime.fromData(Map<String, dynamic> parsedJson) {
print(parsedJson);
return WorkedTime(
uuid: parsedJson['uuid'],
comment: parsedJson['comment'],
status: parsedJson['status'],
took_break: parsedJson['took_break'],
start_at: DateTime.parse(parsedJson['start_at']),
end_at: DateTime.parse(parsedJson['end_at']));
}
}

带有worked_time 的JSON

[
{
"uuid": "706f40e7-d57c-470c-9023-b0c58e2c7c3a",
"start_at": "2020-09-01T08:00:00.000000Z",
"end_at": "2020-09-01T16:00:00.000000Z",
"title": "Test",
"comment": "Test",
"worked_time": {
"uuid": "6e73b4aa-d6e1-41f7-86cb-09745d2db033",
"comment": "Test",
"status": 0,
"break": 0,
"start_at": "2020-09-01T08:00:00.000000Z",
"end_at": "2020-09-01T16:00:00.000000Z",
"took_break": 0
}
}
]

没有worked_time 的JSON

[
{
"uuid": "706f40e7-d57c-470c-9023-b0c58e2c7c3a",
"start_at": "2020-09-01T08:00:00.000000Z",
"end_at": "2020-09-01T16:00:00.000000Z",
"title": "Test",
"comment": "Test",
"worked_time": null
}
]

Answer 1

你能试试这个型号的吗

// To parse this JSON data, do
//
//     final shift = shiftFromJson(jsonString);
import 'dart:convert';
List<Shift> shiftFromJson(String str) => List<Shift>.from(json.decode(str).map((x) => Shift.fromJson(x)));
String shiftToJson(List<Shift> data) => json.encode(List<dynamic>.from(data.map((x) => x.toJson())));
class Shift {
Shift({
this.uuid,
this.startAt,
this.endAt,
this.title,
this.comment,
this.workedTime,
});
String uuid;
DateTime startAt;
DateTime endAt;
String title;
String comment;
WorkedTime workedTime;
factory Shift.fromJson(Map<String, dynamic> json) => Shift(
uuid: json["uuid"] == null ? null : json["uuid"],
startAt: json["start_at"] == null ? null : DateTime.parse(json["start_at"]),
endAt: json["end_at"] == null ? null : DateTime.parse(json["end_at"]),
title: json["title"] == null ? null : json["title"],
comment: json["comment"] == null ? null : json["comment"],
workedTime: json["worked_time"] == null ? null : WorkedTime.fromJson(json["worked_time"]),
);
Map<String, dynamic> toJson() => {
"uuid": uuid == null ? null : uuid,
"start_at": startAt == null ? null : startAt.toIso8601String(),
"end_at": endAt == null ? null : endAt.toIso8601String(),
"title": title == null ? null : title,
"comment": comment == null ? null : comment,
"worked_time": workedTime == null ? null : workedTime.toJson(),
};
}
class WorkedTime {
WorkedTime({
this.uuid,
this.comment,
this.status,
this.workedTimeBreak,
this.startAt,
this.endAt,
this.tookBreak,
});
String uuid;
String comment;
int status;
int workedTimeBreak;
DateTime startAt;
DateTime endAt;
int tookBreak;
factory WorkedTime.fromJson(Map<String, dynamic> json) => WorkedTime(
uuid: json["uuid"] == null ? null : json["uuid"],
comment: json["comment"] == null ? null : json["comment"],
status: json["status"] == null ? null : json["status"],
workedTimeBreak: json["break"] == null ? null : json["break"],
startAt: json["start_at"] == null ? null : DateTime.parse(json["start_at"]),
endAt: json["end_at"] == null ? null : DateTime.parse(json["end_at"]),
tookBreak: json["took_break"] == null ? null : json["took_break"],
);
Map<String, dynamic> toJson() => {
"uuid": uuid == null ? null : uuid,
"comment": comment == null ? null : comment,
"status": status == null ? null : status,
"break": workedTimeBreak == null ? null : workedTimeBreak,
"start_at": startAt == null ? null : startAt.toIso8601String(),
"end_at": endAt == null ? null : endAt.toIso8601String(),
"took_break": tookBreak == null ? null : tookBreak,
};
}

Answer 2

编译器不会抱怨将dynamic变量分配给类型化变量，因为检查是在运行时完成的。因此，你可以写：

void main() {
dynamic value = "something";
String string = value; // ok, value holds a String; runtime is fine
WorkedTime time = value; // evaluated at runtime; exception
}

这个问题出现在Shift的工厂构造函数中

worked_time: parsedJson['worked_time'] == null
? null                        // true: can assign null to WorkedTime
: parsedJson['worked_time']); // false: assign dynamic to WorkedTime

您正试图将值为parsedJson['worked_time']的dynamic类型分配给您的WorkedTime类型。这些是不兼容的类型。相反，必须使用WorkedTime的构造函数从JSON映射中构造实例。以下是您问题的缩小示例：

class WorkedTime {
String comment;
WorkedTime.fromData(Map<String, dynamic> json) : comment = json['comment'];
}
class Shift {
String uuid;
WorkedTime worked_time;
Shift.fromData(Map<String, dynamic> json)
: uuid = json['uuid'],
// next line causes the exception; cannot assign dynamic to WorkedTime
// fix: use WorkedTime.fromData(json['worked_time'])
worked_time = json['worked_time'];
}

为了解决此问题，请使用WorkedTime.fromData(...)的构造函数。

class Shift {
String uuid;
WorkedTime worked_time;
Shift.fromData(Map<String, dynamic> json)
: uuid = json['uuid'],
worked_time = json['worked_time'] != null
? WorkedTime.fromData(json['worked_time']) // parse WorkedTime's json
: null;
}
void main() {
const json = <String, dynamic>{
'uuid': '706f40e7-d57c-470c-9023-b0c58e2c7c3a',
'worked_time': <String, dynamic>{'comment': 'on break'}
};
final shift = Shift.fromData(json);
// check that shift.worked_time is not null, etc.
print(shift.worked_time.comment);
}