Welcome to this guide on simplifying flow management in Flutter applications using the Flow Builder library. This powerful tool allows developers to create user-friendly navigation stacks driven by state changes. Let’s dive into how to define a flow state, create a FlowBuilder, update the flow state, and complete the flow.

Define a Flow State

The flow state is crucial as it drives the flow of your application. Whenever this state changes, a new navigation stack is generated based on the updated flow state. Think of it as building a house where each room represents a different state of your application.

dart
class Profile {
  const Profile(this.name, this.age, this.weight);
  final String? name;
  final int? age;
  final int? weight;

  Profile copyWith(String? name, int? age, int? weight) {
    return Profile(
      name: name ?? this.name,
      age: age ?? this.age,
      weight: weight ?? this.weight,
    );
  }
}

Create a Flow Builder

A FlowBuilder is a widget that creates a navigation stack in response to changes in the flow state. Every time the state shifts, the onGeneratePages method is invoked to return a new list of pages based on the current state. Here’s how it works:

dart
FlowBuilder(
  state: const Profile(),
  onGeneratePages: (profile, pages) {
    return [
      MaterialPage(child: NameForm()),
      if (profile.name != null) MaterialPage(child: AgeForm()),
    ];
  },
);

Update the Flow State

You can update the state of the flow with ease via the context. When a user provides new information, simply use the built-in methods to trigger state updates:

dart
class NameForm extends StatefulWidget {
  @override
  _NameFormState createState() => _NameFormState();
}

class _NameFormState extends State {
  var _name = '';

  void _continuePressed() {
    context.flow().update((profile) => profile.copyWith(name: _name));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Name')),
      body: Center(
        child: Column(
          children: [
            TextField(
              onChanged: (value) => setState(() => _name = value),
              decoration: InputDecoration(
                labelText: 'Name',
                hintText: 'John Doe',
              ),
            ),
            RaisedButton(
              child: const Text('Continue'),
              onPressed: _name.isNotEmpty ? _continuePressed : null,
            )
          ],
        ),
      ),
    );
  }
}

Complete the Flow

Completing the flow can be achieved in a similar fashion. Use the context to complete the current flow state with the required data:

dart
class AgeForm extends StatefulWidget {
  @override
  _AgeFormState createState() => _AgeFormState();
}

class _AgeFormState extends State {
  int? _age;

  void _continuePressed() {
    context
        .flow()
        .complete((profile) => profile.copyWith(age: _age));
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Age')),
      body: Center(
        child: Column(
          children: [
            TextField(
              onChanged: (value) => setState(() => _age = int.parse(value)),
              decoration: InputDecoration(
                labelText: 'Age',
                hintText: '42',
              ),
              keyboardType: TextInputType.number,
            ),
            RaisedButton(
              child: const Text('Continue'),
              onPressed: _age != null ? _continuePressed : null,
            )
          ],
        ),
      ),
    );
  }
}

Using Flow Controller

If you need to manipulate the flow from outside of the sub-tree, you can create a custom FlowController:

dart
class MyFlow extends StatefulWidget {
  @override
  State createState() => _MyFlowState();
}

class _MyFlowState extends State {
  late FlowController _controller;

  @override
  void initState() {
    super.initState();
    _controller = FlowController(const Profile());
  }

  @override
  Widget build(BuildContext context) {
    return FlowBuilder(
      controller: _controller,
      onGeneratePages: ...
    );
  }

  @override
  dispose() {
    _controller.dispose();
    super.dispose();
  }
}

Troubleshooting

If you encounter issues with your FlowBuilder or have questions about navigating states, consider the following troubleshooting steps:

• Ensure that your Profile state is correctly set up and is being passed accurately at each state change.
• If the navigation is not functioning as expected, check the conditions in your onGeneratePages method to verify that the logic flows correctly.
• Make sure there are no mismatches in the expected data types, especially when updating the flow state.

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