Result Command

Result Command is a lightweight package that implements the Command Pattern in Flutter, enabling encapsulation of actions, state tracking, and result management.

Installation

Add the package to your pubspec.yaml:

dependencies:
  result_command: x.x.x
  result_dart: 2.x.x

How to Use

1. Creating a Command

Commands encapsulate actions and manage their lifecycle. Depending on the number of parameters your action requires, you can use:

  • Command0: For actions with no parameters.
  • Command1: For actions with one parameter.
  • Command2: For actions with two parameters.

Example:

final fetchGreetingCommand = Command0<String>(
  () async {
    await Future.delayed(Duration(seconds: 2));
    return Success('Hello, World!');
  },
);

final calculateSquareCommand = Command1<int, int>(
  (number) async {
    if (number < 0) {
      return Failure(Exception('Negative numbers are not allowed.'));
    }
    return Success(number * number);
  },
);

2. Listening to a Command

Commands are Listenable, meaning you can react to their state changes:

Using addListener

fetchGreetingCommand.addListener(() {
  final status = fetchGreetingCommand.value;
  if (status is SuccessCommand<String>) {
    print('Success: ${status.value}');
  } else if (status is FailureCommand<String>) {
    print('Failure: ${status.error}');
  }
});

Using ValueListenableBuilder

Widget build(BuildContext context) {
  return ListenableBuilder(
    listenable: fetchGreetingCommand,
    builder: (context, _) {
      return switch (state) {
        RunningCommand<String>() => CircularProgressIndicator(),
        SuccessCommand<String>(:final value) => Text('Success: $value'),
        FailureCommand<String>(:final error) => Text('Failure: $error'),
        _ => ElevatedButton(
          onPressed: () => fetchGreetingCommand.execute(),
          child: Text('Fetch Greeting'),
        ),
      };
    },
  );
}

Using when for Simplified State Handling

The when method simplifies state management by mapping each state to a specific action or value:

fetchGreetingCommand.addListener(() {
  final status = fetchGreetingCommand.value;
  final message = status.when(
    data: (value) => 'Success: $value',
    failure: (exception) => 'Error: ${exception?.message}',
    running: () => 'Fetching...',
    orElse: () => 'Idle',
  );

  print(message);
});

This approach ensures type safety and provides a clean way to handle all possible states of a command.

3. Executing a Command

The execute() method triggers the action encapsulated by the command. During execution, the command transitions through the following states:

  1. Idle: The initial state, indicating the command is ready to execute.
  2. Running: The state while the action is being executed.
  3. Success: The state when the action completes successfully.
  4. Failure: The state when the action fails.
  5. Cancelled: The state when the action is cancelled.

Each command can only be executed one at a time. If another call to execute() is made while the command is already running, it will be ignored until the current execution finishes or is cancelled.

Example:

fetchGreetingCommand.execute();

4. Cancelling a Command

Commands can be cancelled if they are in the Running state. When cancelled, the command transitions to the Cancelled state and invokes the optional onCancel callback.

Example:

final uploadCommand = Command0<void>(
  () async {
    await Future.delayed(Duration(seconds: 5));
  },
  onCancel: () {
    print('Upload cancelled');
  },
);

uploadCommand.execute();

Future.delayed(Duration(seconds: 2), () {
  uploadCommand.cancel();
});

5. Facilitators

To simplify interaction with commands, several helper methods and getters are available:

State Check Getters

These getters allow you to easily check the current state of a command:

  if (command.isRunning) {
    print('Command is idle and ready to execute.');
  }

Cached Values

To avoid flickering or unnecessary updates in the UI, commands cache their last success and failure states:

  • getCachedSuccess(): Retrieves the cached value of the last successful execution, or null if no success is cached. 
    final successValue = command.getCachedSuccess();
if (successValue != null) {
  print('Last successful value: $successValue');
}
  • getCachedFailure(): Retrieves the cached exception of the last failed execution, or null if no failure is cached. 
    final failureException = command.getCachedFailure();
if (failureException != null) {
  print('Last failure exception: $failureException');
}

These facilitators improve code readability and make it easier to manage command states and results efficiently.

6. Filtering Command State

The filter method allows you to derive a new value from the command's state using a transformation function. This is useful for creating filtered or transformed views of the command's state.

Example:

final filteredValue = command.filter<String>(
  'Default Value',
  (state) {
    if (state is SuccessCommand<String>) {
      return 'Success: ${state.value}';
    } else if (state is FailureCommand<String>) {
      return 'Error: ${state.error}';
    }
    return null; // Ignore other states.
  },
);

filteredValue.addListener(() {
  print('Filtered Value: ${filteredValue.value}');
});

This method simplifies state management by allowing you to focus on specific aspects of the command's state.

7. CommandRef

The CommandRef class allows you to create commands that listen to changes in one or more ValueListenables and execute actions based on derived values.

Example:

final listenable = ValueNotifier<int>(0);

final commandRef = CommandRef<int, int>(
  (ref) => ref(listenable),
  (value) async => Success(value * 2),
);

commandRef.addListener(() {
  final status = commandRef.value;
  if (status is SuccessCommand<int>) {
    print('Result: ${status.value}');
  }
});

listenable.value = 5; // Executes the command with the value 5.

Features:

  • Automatically listens to changes in ValueListenables.
  • Executes the action whenever the derived value changes.
  • Cleans up listeners when disposed.

This class is ideal for scenarios where commands need to react dynamically to external state changes.

Documentation

For advanced examples and detailed documentation, visit:

Contribute

We welcome contributions! Feel free to report issues, suggest features, or submit pull requests.

Libraries

result_command
Result Command