// 正确示例intmeasureMessage(String?message){if(message!=null&&message.isNotEmpty){// message is promoted to String.returnmessage.length;}return0;}// 错误示例intmeasureMessage(String?message){if(message?.isNotEmpty??false){// message is not promoted to String.returnmessage!.length;}return0;}...
if (VerifyHelper.empty(userMobile)) return ctx.showError('用户手机号不能为空'); if (VerifyHelper.empty(userPassword)) return ctx.showError('用户密码不能为空'); if (VerifyHelper.empty(userNickname)) return ctx.showError('用户昵称不能为空'); if (VerifyHelper.empty(userAvatar)) return ctx...
}classEmployeeextendsPerson{// Person does not have a default constructor;// you must call super.fromJson(data).Employee.fromJson(Mapdata) :super.fromJson(data) {print('in Employee'); } }main() {varemp =newEmployee.fromJson({});// Prints:// in Person// in Employeeif(emp isPerson)...
if(!_scaffoldKey.currentState!.isDrawerOpen){_scaffoldKey.currentState!.openDrawer();} 避空运算符?.根据运行时的语义化规定,如果接收者是null,那么右侧的属性访问就会被跳过,表达式将被作为null看待。 // Without null safety:StringnotAString=null;print(notAString?.length); 这段代码将打印 “null”,而...
if(computation ==null&& !typeAcceptsNull<T>) { throwArgumentError.value(null,"computation","The type parameter is not nullable"); } _Future<T> result =new_Future<T>; newTimer(duration, { if(computation ==null) { result._complete(nullasT); ...
// null safety后需要不为空才可以设置 if (_cookie != null) { options.headers['Cookie'] = _cookie; } return super.onRequest(options, handler); } 总结 从编码的角度来说,null safety特性实际上增加了编码的工作量。但是null safety更像是一个强制的约定,要求接口或类明确参数或属性的是否为空,从而...
if (replacement != null) { future._asyncCompleteError( _nonNullError(replacement.error), replacement.stackTrace); } else { future._asyncCompleteError(error, stackTrace); } return future; } } 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. ...
如果扩展操作符右边可能为null,使用空感知扩展操作符(...?)可以避免产生异常 varlist=null;// ...varlist2 = [0, ...?list];assert(list2.length ==1); 创建List时使用if或for 如下,使用collection if创建List,可能包含3个或4个元素。 varnav = ['主页','发现','阅读',if(promoActive)'个人中心'...
String mysteryMessage(String who, [String what, String where]){ var message = '$who'; if(what != null && where == null){ message = '$message said $what'; } else if (where != null){ message = '$message said $what at $where'; } return message; } main() { var result = my...
not. Regardless, the compiler should not crash. The Dart team would greatly appreciate if you would take a moment to report this problem athttp://dartbug.com/new. Please include the following information: the name and version of your operating system, ...