什么是？？ Dart 中的双问号？

Question

The following line of code has two question marks:

final myStringList = prefs.getStringList('my_string_list_key') ?? [];

What is the meaning?

Answer 1

The ?? double question mark operator means "if null". Take the following expression, for example.

String a = b ?? 'hello';

This means a equals b , but if b is null then a equals 'hello' .

Another related operator is ??= . For example:

b ??= 'hello';

This means if b is null then set it equal to hello . Otherwise, don't change it.

Reference

• A Tour of the Dart Language: Operators
• Null-aware operators in Dart

Terms

The Dart 1.12 release news collectively referred to the following as null-aware operators :

• ?? -- if null operator
• ??= -- null-aware assignment
• x?.p -- null-aware access
• x?.m() -- null-aware method invocation

Answer 2

Dart offers some handy operators for dealing with values that might be null. One is the ??= assignment operator, which assigns a value to a variable only if that variable is currently null:

int a; // The initial value of a is null.
a ??= 3;
print(a); // <-- Prints 3.

a ??= 5;
print(a); // <-- Still prints 3.

Another null-aware operator is ?? , which returns the expression on its left unless that expression's value is null, in which case it evaluates and returns the expression on its right:

print(1 ?? 3); // <-- Prints 1.
print(null ?? 12); // <-- Prints 12.

Answer 3

This is especially useful in the copyWith method which is used in flutter very often to override. Here is an example below:

import './color.dart';
import './colors.dart';

class CoreState {
  final int counter;
  final Color backgroundColor;

  const CoreState({
    this.counter = 0,
    this.backgroundColor = Colors.white,
  });

  CoreState copyWith({
    int? counter,
    Color? backgroundColor,
  }) =>
      CoreState(
        counter: counter ?? this.counter,
        backgroundColor: backgroundColor ?? this.backgroundColor,
      );

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
          other is CoreState &&
              runtimeType == other.runtimeType &&
              counter == other.counter &&
              backgroundColor == other.backgroundColor;

  @override
  int get hashCode => counter.hashCode ^ backgroundColor.hashCode;


  @override
  String toString() {
    return "counter: $counter\n"
            "color:$backgroundColor";
  }
}

Answer 4

?? is a null-aware operator, the expression means that, if the value of "my_string_list_key" in the prefs instance is null then assign an empty list to myStringList .

Answer 5

(??) null operator , it returns the expression on its left when the it's not null , otherwise it'll return the right expression.

(??=) Null-aware assignment - This operator assigns value to the variable on its left, only if that variable is currently null.

(?.) Null-Aware access This operator prevents you from crashing your app by trying to access a property or a method of an object that might be null. For example,

String x;
print(x.toUpperCase());   // WILL GIVE AN ERROR
print(x?.toUpperCase()); // OUTPUT WILL BE NULL

(...?) Null-Aware spread operator - This operator prevents you from adding null elements using spread operator.