flutter - 什么是 flutter 中的 "?."运算符

Question

我正在使用图书馆，并且在很多地方？。
使用了运算符，我无法理解它的用途。

Timer _debounceTimer;
  @override
  initState() {
    _textController.addListener(() {
      // We debounce the listener as sometimes the caret position is updated after the listener
      // this assures us we get an accurate caret position.
      if (_debounceTimer?.isActive ?? false) _debounceTimer.cancel();

Answer 1

?. [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)

from Dart Language Tour (Other Operators)

TLDR:它只是一个 null在访问成员(member)之前检查。如果运算符的左侧不为空，则它的工作方式简单地为 .如果是 null值(value)那么整个事情是 null .

在您的示例中: _debounceTimer?.isActive - 如果 _debounceTimer为空然后 _debounceTimer?.isActive <-> null如果 _debounceTimer不为空然后 _debounceTimer?.isActive <-> _debounceTimer.isActive .

同时检查: Dart Language tour (Conditional Expressions)为 ??和 ?运算符(operator)。