flutter_command

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flutter_command is a way to manage your state based on ValueListenable and the Command design pattern. Sounds scary uh? Ok lets try it a different way. A Command is an object that wraps a function that can be executed by calling the command, therefore decoupling your UI from the wrapped function.

It's not that easy to define what exactly state management is (see https://medium.com/super-declarative/understanding-state-management-and-why-you-never-will-dd84b624d0e). For me it's how the UI triggers processes in the model/business layer of your app and how to get back the results of these processes to display them. For both aspects flutter_command offers solution plus some nice extras. So in a way it offers the same that BLoC does but in a more logical way.

This readme might seem very long, but it will guide you easily step by step through all features of flutter_command.

Why Commands

When I started Flutter the most often recommended way to manage your state was BLoC. What never appealed to me was that in order to execute a process in your model layer you had to push an object into a StreamController which just didn't feel right. For me triggering a process should feel like calling a function. Coming from the .Net world I was used to use Commands for this, which had an additional nice feature that the Button that triggered the command would automatically disable for the duration, the command was running and by this, preventing a double execution at the same time. I also learned to love a special breed of .Net commands called ReactiveCommands which emitted the result of the called function on their own Stream interface (the ReactiveUI community might oversee that I don't talk of Observables here.) As I wanted to have something similar I ported ReactiveCommands to Dart with my rx_command. But somehow they did not get much attention because 1. I didn't call them state management and 2. they had to do with Streams and even had that scary rx in the name and probably the readme wasn't as good to start as I thought.

Remi Rousselet talked to me about that ValueNotifier and how much easier they are than using Streams. So what you have here is my second attempt to warm the hearts of the Flutter community for the Command metaphor absolutely free of Streams

A first careful encounter

Let's start with the (in)famous counter example but by using a Command. As said before a Command wraps a function and can publish the result in a way that can be consumed by the UI. It does this by implementing the ValueListenable interface which means a command behaves like ValueNotifier. A command has the type:

Command<TParam,TResult>

at which TParam is the type of the parameter which the wrapped function expects as argument and TResult is the type of the result of it, which means the Command behaves like a ValueNotifier<TResult>. In the included project counter_example the command is defined as:

class _MyHomePageState extends State<MyHomePage> {
  int counter = 0;
  /// This command does not expect any parameters when called therefore TParam 
  /// is void and publishes its results as String
  Command<void, String> _incrementCounterCommand;

  _MyHomePageState() {
    _incrementCounterCommand = Command.createSyncNoParam(() {
      counter++;
      return counter.toString();
    }, '0');
  }

To create a Command the Command class offers different static functions depending on the signature of the wrapped function. In this case we want to use a synchronous function without any parameters.

Our widget tree now looks like this:


  body: Center(
    child: Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: <Widget>[
        Text(
          'You have pushed the button this many times:',
        ),
        ValueListenableBuilder<String>(
            valueListenable: _incrementCounterCommand,
            builder: (context, val, _) {
              return Text(
                val,
                style: Theme.of(context).textTheme.headline4,
              );
            }),
      ],
    ),
  ),
  floatingActionButton: FloatingActionButton(
    onPressed: _incrementCounterCommand,
    tooltip: 'Increment',
    child: Icon(Icons.add),
  ), // This trailing comma makes auto-formatting nicer for build methods.
);

As Command is a callable class, so we can pass it directly to the onPressed handler of the FloatingActionButton and it will execute the wrapped function. The result of the function will get assigned to the Command.value so that the ValueListenableBuilder updates automatically.

Commands in full power mode

So far the command did not do more than what you could do with BLoC, besides that you could call it like a function and didn't need a Stream. But Command can do more than that. It allows us to:

  • Update the UI based on if the Command is executing
  • React on Exceptions in the wrapped functions
  • Control when a Command can be executed

Let's explore this features by examining the included example app which queries an open weather service and displays a list of cities with the current weather.

The app uses a WeatherViewModel which contains the Command to update the ListView by making a REST call:

Command<String, List<WeatherEntry>> updateWeatherCommand;

The updateWeatherCommand expects a search term and will return a list of WeatherEntry. The Command gets initialized in the constructor of the WeatherViewModel:

updateWeatherCommand = Command.createAsync<String, List<WeatherEntry>>(
    update, // Wrapped function
    [],     // Initial value
    restriction: setExecutionStateCommand, //please ignore for the moment
)   

update is the asynchronous function that queries the weather service, therefore we create an async version of Command using the createAsync constructor.

Updating the ListView

In listview.dart:

class WeatherListView extends StatelessWidget {
  WeatherListView();
  @override
  Widget build(BuildContext context) {
    return ValueListenableBuilder<List<WeatherEntry>>(
      valueListenable: TheViewModel.of(context).updateWeatherCommand,
      builder: (BuildContext context, List<WeatherEntry> data, _) {
        // only if we get data
        return ListView.builder(
          itemCount: data.length,
    ....

Reacting on changes of the function execution state

Command has a property

ValueListenable<bool> isExecuting;

that has the value of false while the wrapped function isn't executed and true when it is. So we use this in the UI in homepage.dart to display a progress indicator while the app waits for the result of the REST call:

child: ValueListenableBuilder<bool>(
    valueListenable:
        TheViewModel.of(context).updateWeatherCommand.isExecuting,
    builder: (BuildContext context, bool isRunning, _) {
    // if true we show a buys Spinner otherwise the ListView
    if (isRunning == true) {
        return Center(
        child: SizedBox(
            width: 50.0,
            height: 50.0,
            child: CircularProgressIndicator(),
          ),
        );
    } else {
        return WeatherListView();
    }
  },
),

:triangular_flag_on_post: As it's not possible to update the UI while a synchronous function is being executed Commands that wrap a synchronous function don't support isExecuting and will throw an assertion if you try to access it.

Update the UI on change of the search field

As we don't want to send a new HTTP request on every keypress in the search field we don't directly wire the onChanged event to the updateWeatherCommand. Instead we use a second Command to convert the onChanged event to a ValueListenable so that we can use the debounce and listen function of my extension function package functional_listener:

For this a synchronous Command is sufficient:

// in weather_viewmodel.dart:
Command<String, String> textChangedCommand;
// and in the constructor:

// Will be called on every change of the searchfield
textChangedCommand = Command.createSync((s) => s, '');

// 
// make sure we start processing only if the user make a short pause typing
textChangedCommand.debounce(Duration(milliseconds: 500)).listen(
    (filterText, _) {
    // I could omit the execute because Command is a callable
    // class  but here it makes the intention clearer
    updateWeatherCommand.execute(filterText);
    },
);

In the homepage.dart:

child: TextField(
    /// I omitted some properties from the example here
    onChanged: TheViewModel.of(context).textChangedCommand,
),

Restricting command execution

Sometimes it is desirable to make the execution of a Command depending on some other state. For this you can pass a ValueListenable<bool> as restriction parameter, when you create a command. If you do so the command will only be executed if the value of the passed listenable is true. In the example app we can restrict the execution by changing the state of a Switch. To handle changes of the Switch we use..., you guessed it, another command in the WeatherViewModel:

WeatherViewModel() {
    // Command expects a bool value when executed and sets it as its own value
    setExecutionStateCommand = Command.createSync<bool, bool>((b) => b, true);

    // We pass the result of switchChangedCommand as restriction to the updateWeatherCommand
    updateWeatherCommand = Command.createAsync<String, List<WeatherEntry>>(
    update, // Wrapped function
    [], // Initial value
    restriction: setExecutionStateCommand,
  );
...

To update the Switch we use again a ValueListenableBuilder:

ValueListenableBuilder<bool>(
    valueListenable:
        TheViewModel.of(context).setExecutionStateCommand,
    builder: (context, value, _) {
        return Switch(
        value: value,
        onChanged:
            TheViewModel.of(context).setExecutionStateCommand,
        );
    })

Disabling the update button while another update is in progress

The update button should not be active while an update is running or when the Switch deactivates it. We could achieve this, again by using the isExecuting property of Command but we would have to somehow combine it with the value of setExecutionStateCommand which is cumbersome. Luckily Command has another property canExecute which reflects a combined value of !isExecuting && restriction.

So we can easily solve this requirement with another....wait for it...ValueListenableBuilder

child: ValueListenableBuilder<bool>(
  valueListenable: TheViewModel.of(context)
      .updateWeatherCommand
      .canExecute,
  builder: (BuildContext context, bool canExecute, _) {
    // Depending on the value of canExecute we set or clear the handler
    final handler = canExecute
        ? TheViewModel.of(context).updateWeatherCommand
        : null;
    return RaisedButton(
      child: Text("Update"),
      color: Color.fromARGB(255, 33, 150, 243),
      textColor: Color.fromARGB(255, 255, 255, 255),
      onPressed: handler,
    );
  },
),

Error Handling

If the wrapped function inside a Command throws an Exception the Command catches it so your App won't crash. Instead it will wrap the caught error together with the value that was passed when the command was executed in a CommandError object and assign it to the Command's thrownExeceptions property which is a ValueListenable<CommandError>. So to react on occurring error you can register your handler with addListener or use my listen extension function from functional_listener as it is done in the example:

/// in HomePage.dart
@override
void didChangeDependencies() {
  errorSubscription ??= TheViewModel.of(context)
      .updateWeatherCommand
      .thrownExceptions
      .where((x) => x != null) // filter out the error value reset
      .listen((error, _) {
    showDialog(
        context: context,
        builder: (context) => AlertDialog(
              title: const Text('An error has occured!'),
              content: Text(error.toString()),
            ));
  });
  super.didChangeDependencies();
}

Unfortunately its not possible to reset the value of a ValueNotifier without triggering its listeners. So if you have registered a listener you will get it called at every start of a Command execution with a value of null and clear all previous errors. If you use functional_listener you can do it easily by using the where extension.

Error handling the fine print

You can tweak the behaviour of the error handling by passing a catchAlways parameter to the factory functions. If you pass false Exceptions will only be caught if there is a listener on thrownExceptions or on results (see next chapter). You can also change the default behaviour of all Command in your app by changing the value of the catchAlwaysDefault property. During development its a good idea to set it to false to find any non handled exception. In production, setting it to true might be the better decision to prevent hard crashes. Note that catchAlwaysDefault property will be implicitly ignored if the catchAlways parameter for a command is set.

Command also offers a static global Exception handler:

static void Function(String commandName, CommandError<Object> error) globalExceptionHandler;

If you assign a handler function to it, it will be called for all Exceptions thrown by any Command in your app independent of the value of catchAlways if the Command has no listeners on thrownExceptions or on results.

The overall work flow of exception handling in flutter_command is depicted in the following diagram.

Getting all data at once

isExecuting and thrownExceptions are great properties but what if you don't want to use separate ValueListenableBuilders for each of them plus one for the data? Command got you covered with the results property that is an ValueListenable<CommandResult> which combines all needed data and is updated several times during a Command execution.

/// Combined execution state of an `Command`
/// Will be updated for any state change of any of the fields
/// 1. If the command was just newly created `results.value` has the value:
///    `param data,null, null, false` (paramData,data, error, isExecuting)
/// 2. When calling execute: `param data, null, null, true`
/// 3. When execution finishes: `param data, the result, null, false`
/// If an error occurs: `param data, null, error, false`
/// `param data` is the data that you pass as parameter when calling the command
class CommandResult<TParam, TResult> {
  final TParam paramData;
  final TResult data;
  final Object error;
  final bool isExecuting;

  bool get hasData => data != null;
  bool get hasError => error != null;

  /// This is a stripped down version of the class. Please see the source
}

You can find a Version of the Weather app that uses this approach in example_command_results. There the homepage.dart looks like:

child: ValueListenableBuilder<
    CommandResult<String, List<WeatherEntry>>>(
  valueListenable:
      TheViewModel.of(context).updateWeatherCommand.results,
  builder: (BuildContext context, result, _) {
    if (result.isExecuting) {
      return Center(
        child: SizedBox(
          width: 50.0,
          height: 50.0,
          child: CircularProgressIndicator(),
        ),
      );
    } else if (result.hasData) {
      return WeatherListView(result.data);
    } else {
      assert(result.hasError);
      return Column(
        children: [
          Text('An Error has occurred!'),
          Text(result.error.toString()),
          if (result.error != null)
            Text('For search term: ${result.paramData}')
        ],
      );
    }
  },
),

Even if you use results the other properties are updated as before, so you can mix both approaches as you need it. For instance use results as above but additionally listening to thrownExceptions for logging.

If you want to be able to always display data (while loading or in case of an error) you can pass includeLastResultInCommandResults=true, the last successful result will be included as data unless a new result is available.

CommandBuilder, reducing boilerplate

flutter_command includes a CommandBuilder widget which makes the code above a bit nicer:

child: CommandBuilder<String, List<WeatherEntry>>(
  command: TheViewModel.of(context).updateWeatherCommand,
  whileExecuting: (context, _) => Center(
    child: SizedBox(
      width: 50.0,
      height: 50.0,
      child: CircularProgressIndicator(),
    ),
  ),
  onData: (context, data, _) => WeatherListView(data),
  onError: (context, error, param) => Column(
    children: [
      Text('An Error has occurred!'),
      Text(error.toString()),
      if (error != null) Text('For search term: $param')
    ],
  ),
),

How to create Commands

┬┤Command┬┤ offers different static factory functions for the different function types you want to wrap:

  /// for syncronous functions with no parameter and no result
  static Command<void, void> createSyncNoParamNoResult(
    void Function() action, {
    ValueListenable<bool> restriction,
    bool catchAlways,
    String debugName
  }) 
  /// for syncronous functions with one parameter and no result
  static Command<TParam, void> createSyncNoResult<TParam>(
    void Function(TParam x) action, {
    ValueListenable<bool> restriction,
    bool catchAlways,
    String debugName
  }) 
  /// for syncronous functions with no parameter and but a result
  static Command<void, TResult> createSyncNoParam<TResult>(
    TResult Function() func,
    TResult initialValue, {
    ValueListenable<bool> restriction,
    bool includeLastResultInCommandResults = false,
    bool catchAlways,
    String debugName
  })
  /// for syncronous functions with one parameter and result
  static Command<TParam, TResult> createSync<TParam, TResult>(
    TResult Function(TParam x) func,
    TResult initialValue, {
    ValueListenable<bool> restriction,
    bool includeLastResultInCommandResults = false,
    bool catchAlways,
    String debugName
  }) 

  /// and for Async functions:
  static Command<void, void> createAsyncNoParamNoResult(
    Future Function() action, {
    ValueListenable<bool> restriction,
    bool catchAlways,
    String debugName
  }) 
  static Command<TParam, void> createAsyncNoResult<TParam>(
    Future Function(TParam x) action, {
    ValueListenable<bool> restriction,
    bool catchAlways,
    String debugName
  }) 
  static Command<void, TResult> createAsyncNoParam<TResult>(
    Future<TResult> Function() func,
    TResult initialValue, {
    ValueListenable<bool> restriction,
    bool includeLastResultInCommandResults = false,
    bool catchAlways,
    String debugName
  })
  static Command<TParam, TResult> createAsync<TParam, TResult>(
    Future<TResult> Function(TParam x) func,
    TResult initialValue, {
    ValueListenable<bool> restriction,
    bool includeLastResultInCommandResults = false,
    bool catchAlways,
    String debugName
  })

For detailed information on the parameters of these functions consult the API docs or the source code documentation.

Reacting on Functions with no results

Even if your wrapped function doesn't return a value, you can react on the end of the function execution by registering a listener to the Command. The command Value will be void but your handler is ensured to be called.

Logging

If you are not sure what's going on in your App you can register an handler function to

static void Function(String commandName, CommandResult result) loggingHandler;

It will get executed on every Command execution in your App. commandName is the optional debugName that you can pass when creating a command.

Libraries

code_for_docs
command_builder
flutter_command