Oncecall is a memoization tool for the Flutter Widget build method. It allows your code to be called only once in the build method, even if the Widget is rebuilt.

Why do we need it?

We always want to construct use cases that have dynamic properties and conform to const Widgets, or we have some logic that must be placed in the build method.

Oncecall allows you to write code in build that runs only once.

Installation

We use the following command:

flutter pub add oncecall

Or add in pubspec.yaml:

dependencies:
  oncecall: latest

Basic Usage

class MyWidget extends StatelessWidget {
    const MyWidget({super.key});

    Widget build(BuildContext context) {
        oncecall(context, () => print('Will only run once'));

        return ...;
    }
}

This doesn't seem to be any different. But without oncecall, you would write the following code:

class MyWidget extends StatelessWidget {
    MyWidget({super.key}) {
        print('Will only run once'); // Widget rebuild might execute again
    }

    Widget build(BuildContext context) {
        return ...;
    }
}

For One-time Calculations

The oncecall() function, in addition to being used for one-time functions, can also perform some object initialization work:

class Counter extends StatelessWidget {
    const Counter({super.key});

    Widget build(BuildContext context) {
        final value = oncecall(context, () {
            ...
        });

        return ...;
    }
}

More

As a foundational part of the Odroe ecosystem, you'll see Oncecall everywhere in other packages. It is a very low-level and simple API, and as its name suggests, it is only used for value memoization and initialization work that depends on context.