I'm upgrading a personal package that is based on the Flutter framework. I noticed here in the Flutter Text widget source code that there is a null check:
if (textSpan != null) {
properties.add(textSpan!.toDiagnosticsNode(name: 'textSpan', style: DiagnosticsTreeStyle.transition));
}
However, textSpan!
is still using the !
operator. Shouldn't textSpan
be promoted to a non-nullable type without having to use the !
operator? However, trying to remove the operator gives the following error:
An expression whose value can be 'null' must be null-checked before it can be dereferenced. Try checking that the value isn't 'null' before dereferencing it.
Here is a self-contained example:
class MyClass {
String? _myString;
String get myString {
if (_myString == null) {
return '';
}
return _myString; // <-- error here
}
}
I get a compile-time error:
Error: A value of type 'String?' can't be returned from function 'myString' because it has a return type of 'String'.
Or if I try to get _mySting.length
I get the following error:
The property 'length' can't be unconditionally accessed because the receiver can be 'null'.
I thought doing the null check would promote _myString
to a non-nullable type. Why doesn't it?
My question was solved on GitHub so I'm posting an answer below.
Dart engineer Erik Ernst says on GitHub:
Type promotion is only applicable to local variables. ... Promotion of an instance variable is not sound, because it could be overridden by a getter that runs a computation and returns a different object each time it is invoked. Cf. dart-lang/language#1188 for discussions about a mechanism which is similar to type promotion but based on dynamic checks, with some links to related discussions.
So local type promotion works:
String myMethod(String? myString) {
if (myString == null) {
return '';
}
return myString;
}
But instance variables don't promote. For that you need to manually tell Dart that you are sure that the instance variable isn't null in this case by using the !
operator:
class MyClass {
String? _myString;
String myMethod() {
if (_myString == null) {
return '';
}
return _myString!;
}
}