flutter - Whats ?? = Dart中的运算符

Question

这是我在Flutter源代码中看到的新赋值运算符:

splashFactory ??= InkSplash.splashFactory;
textSelectionColor ??= isDark ? accentColor : primarySwatch[200];

这个赋值运算符是什么意思？
example in Flutter source code

Answer 1

?? =是一个新的空感知运算符。具体地说，?? =是可识别空值的赋值运算符。

?? if null operator. expr1 ?? expr2 evaluates to expr1 if not null, otherwise expr2.

??= null-aware assignment. v ??= expr causes v to be assigned expr only if v is null.

?. null-aware access. x?.p evaluates to x.p if x is not null, otherwise evaluates to null.