飞镖/颤振中的一元后缀是什么？

Question

i saw this unary postfix in Dart/flutter code: ?.

like this:

videoController?.dispose();

and i want to know how it work...

Answer 1

This is a great feature in Dart

meaning is if and only if that object is not null otherwise, return null .

Simple example:

void main() {
  Person p1 = new Person("Joe");
  print(p1?.getName); // Joe

  Person p2;
  print(p2?.getName); // null

  //print(p2.getName); // this will give you an error because you cannot invoke a method or getter from a null
}

class Person {
  Person(this.name);
  String name;

  String get getName => name;
}

There are other cool null aware operators like ?? . Read my QnA to find out more about null-aware operators.

Answer 2

It tests for null,

https://www.dartlang.org/guides/language/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 3

It is a null-aware operator . It is the short form of the following.

Code

 ((obj) => obj == null ? null : x.method())(object)
 // is equal to
 object?.method()

You can find out more about null-aware operators here .

Explanation

Reads as:

• Only execute method if object is not null

• If object is null return null (otherwise evaluation from method )