在我的flutter项目中，用于空值的空检查运算符

Question

日历中用于空值的空检查运算符！我正在为分配日历？标记，则发生此null运算符错误，并且当我添加延迟日历LateInitializationError:Field时_calendar@94028380'尚未初始化错误正在获取

class MedicationListChild extends StatefulWidget {
final String? medicationName;
final String? medicationUID;
final String? childUid;
final String? childName;
final Calendar? calendar;
const MedicationListChild({
Key? key,
this.medicationName,
this.medicationUID,
this.childUid,
this.childName,
this.calendar,
}) : super(key: key);
@override
_MedicationListChildState createState() => _MedicationListChildState();
}
class _MedicationListChildState extends State<MedicationListChild> {
Calendar? _calendar;
@override
void initState() => super.initState();
@override
Widget build(BuildContext context) {
return Padding(
padding: const EdgeInsets.only(top: 7.0),
child: Card(
elevation: 3.0,
shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(7.0)),
child: InkWell(
splashColor: Colors.blue,
highlightColor: Colors.green,
onTap: () {
Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => CalendarEventPage(_calendar!),
),
);
},

Answer 1

不要使用_calendar，而是使用widget.calendar:

Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => CalendarEventPage(widget.calendar!),
),
);

Answer 2

您没有在_calendar上分配任何值，因此它保持为null。

与其直接使用!，不如先进行空检查。

onTap: () {
if(_calendar!=null) {
Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => CalendarEventPage(_calendar),
),
);
},
}

如果你只是喜欢访问小部件变量做

onTap: () {
if(widget.calendar!=null) {
Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => CalendarEventPage(widget.calendar),
),
);
},
}

Answer 3

尝试以下代码：

class MedicationListChild extends StatefulWidget {
final String? medicationName;
final String? medicationUID;
final String? childUid;
final String? childName;
final Calendar? calendar;
const MedicationListChild({
Key? key,
this.medicationName,
this.medicationUID,
this.childUid,
this.childName,
this.calendar,
}) : super(key: key);
@override
_MedicationListChildState createState() => _MedicationListChildState();
}
class _MedicationListChildState extends State<MedicationListChild> {
@override
void initState() => super.initState();
@override
Widget build(BuildContext context) {
return Padding(
padding: const EdgeInsets.only(top: 7.0),
child: Card(
elevation: 3.0,
shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(7.0)),
child: InkWell(
splashColor: Colors.blue,
highlightColor: Colors.green,
onTap: () {
Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => CalendarEventPage(widget.calendar!),
),
);
},