CODEWITHSUNDEEP
flutterdart
jon
jon

Reputation: 3600

Confusing function declaration syntax

In the flutter_platform_widgets package, the following declaration appears:

final PlatformAppBar Function(BuildContext context, int index)? appBarBuilder;

I'm sorry for the beginner question, but I am confused by this syntax. I understand the final and the name of the instance is appBarBuilder.

I am used to seeing a declaration like:

final Classname? classInstance;

Where in this declaration, the classInstance can also be null. But it seems to see this declaration has two types: PlatformAppBar and Function(BuildContext context, int index)?. I don't think something can have two different types, so what is going on here?

My final goal is to pass a null function.

Here is how I am using it. Some of the pages need an app bar and others do not, which is why I am trying to return a null for some pages.

return PlatformTabScaffold(
  appBarBuilder: (context, index) {
    if (index == 1) return SearchPage.appBar(context);
    if (index == 3) return CalculatorPage.appBar(context);
    return null;  // Gives error The return type 'Null' isn't a 'PlatformAppBar', as required by the closure's context.
  },
  // Other parameters...
);

Upvotes: 0

Views: 239

Answers (1)

jamesdlin
jamesdlin

Reputation: 89965

final PlatformAppBar Function(BuildContext context, int index)? appBarBuilder;

declares a variable named appBarBuilder of type:

PlatformAppBar Function(BuildContext context, int index)?

appBarBuilder therefore either is null or is a function that takes arguments of type BuildContext and int and that returns a PlatformAppBar.

Here is how I am using it. Some of the pages need an app bar and others do not, which is why I am trying to return a null for some pages.

return PlatformTabScaffold(
  appBarBuilder: (context, index) {
    if (index == 1) return SearchPage.appBar(context);
    if (index == 3) return CalculatorPage.appBar(context);
    return null;  // Gives error The return type 'Null' isn't a 'PlatformAppBar', as required by the closure's context.
  },
  // Other parameters...
);

Your code is not passing null to appBarBuilder. You are passing an anonymous function that returns null when invoked. Consider:

int Function()? f = null;
int? Function()? g = () => null;

print(f == null); // Prints: true
print(g == null); // Prints: false

print(g() == null); // Prints: true
f(); // Throws a runtime exception.

In your case, it does not seem to me that you actually want appBarBuilder itself to be null; you instead want invoking it to conditionally do nothing based on index.

I'm not familiar with package:flutter_platform_widgets, but I suspect that there might not be a way to do what you want. You probably should contact the package author and ask if there's a preferred way to make the callback do nothing, and if not, perhaps change the declaration of appBarBuilder to PlatformAppBar? Function(BuildContext context, int index)? instead so that it allows a null return value.

Upvotes: 1

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