这个运算符“？”是什么？在飞镖？

Question

我使用 flutter 框架这部分代码使用了一个操作“？”。但我不明白

 if (state is WeatherLoaded) {
          final weather = state.weather;
          final themeBloc = BlocProvider.of<ThemeBloc>(context);
          themeBloc.dispatch(WeatherChanged(condition: weather.condition));

          _refreshCompleter?.complete();
          _refreshCompleter = Completer();

全部代码thislink

Answer 1

检查此 link：Language tour

?.

Conditional member access

Like ., but the leftmost operand can be null; example: foo?.bar selects property bar from expression foo unless foo is null (in which case the value of foo?.bar is null)

Answer 2

证明这一点的最好方法是一个简单的例子。
我有一个对象 SomeObject 和一个方法 username。

我做了2个实例：

aeonObject 这不是 null
someOtherObject 即 null

class SomeObject {
  String username() => "aeon";
}

void main() {
  final aeonObject = SomeObject();
  print(aeonObject.username());

  SomeObject someOtherObject;
  print(someOtherObject.username());
}

如果我执行此代码段，您将看到以下输出。
程序会崩溃，因为我们试图在 null 引用上执行一个方法。

dart lib/main.dart lib/main.dart: Warning: Interpreting this as package URI, 'package:sample/main.dart'.

aeon

Unhandled exception: NoSuchMethodError: The method 'username' was called on null.

Receiver: null Tried calling: username()

但是，如果我用 ?. 又名 Conditional member access operator.

调用 print 语句

print(someOtherObject?.username());

我们反而得到了。

null

这个运算符“？”是什么？在飞镖？

What is this operator "?." in Dart?

operators

dart

flutter