Dart: How do I use "optional chaining" or a default value?-CodePudding

I'm coming from TypeScript to Dart because of Flutter and It's incredible how I can't do the simplest of things.

I have style?.p? as double? and I would like to read its value or use 0.0 as a default. Like this:

EdgeInsets.all(style?.p != null ? style.p : 0.0))

... but Dart is saying double? can't be assigned to double. Well, I'm using this ternary expression to check for null, but I think Dart is not as smart as TypeScript in type inference.

Any idea?

CodePudding user response：

try this you can give the optional value like (optionalVaribaleValue ?? DefaultValue) so if OptionalValue null then DefaultValue set

EdgeInsets.all(style?.p ?? 0.0)

CodePudding user response：

The reason why EdgeInsets.all(style?.p != null ? style.p : 0.0)) doesn't work is because only local variables can be type-promoted, and style.p is not a local variable. style.p therefore remains a double?, causing the evaluated type of the conditional ternary expression to also be double?.

Assigning style?.p to a local variable would work:

@override
void build(BuildContext context) {
  var p = style?.p;

  return ...
    padding: EdgeInsets.all(p != null ? p : 0));
}

but as others have noted (and as the Dart linter recommends), you should prefer using a dedicated null operator:

padding: EdgeInsets.all(style?.p ?? 0);

where, because style?.p will not be evaluated multiple times (avoiding the potential for returning different values), ?? can evaluate to a non-nullable type.

CodePudding user response：

Your style attribute is nullable plus the p property is nullable too, try doing this:

padding: style != null
            ? EdgeInsets.all(style!.p != null ? style!.p! : 0.0)
            : const EdgeInsets.all(0.0)