RxCommand

You can find a tutorial on how to use RxCommands in this blog post Making Flutter more Reactive

RxCommand is an Reactive Extensions (Rx) based abstraction for event handlers. It is based on ReactiveCommand for the ReactiveUI framework. It makes heavy use of the RxDart package.

PRs are always welcome ;-)

MAYBE BREAKING CHANGE in 2.0.0: Till now the results Observable and the RxCommand itself behaved like a BehaviourSubjects. This can lead to problems when using with Flutter. From now on the default is PublishSubject. If you need BehaviourSubject behaviour, meaning every new listener gets the last received value, you can set emitsLastValueToNewSubscriptions = true when creating RxCommand.

If you don't know Rx think of it as Dart Streams on steroids. RxCommand capsules a given handler function that can then be executed by its execute method or directly assigned to a widget's handler because it's a callable class. The result of this method is then published through its results Observable (Observable wrap Dart Streams). Additionally it offers Observables for it's current execution state, if the command can be executed and for all possibly thrown exceptions during command execution.

A very simple example

final command = RxCommand.createSync3<int, String>((myInt) => "$myInt");

command.results.listen((s) => print(s)); // Setup the listener that now waits for events, not doing anything

// Somwhere else
command.execute(10); // the listener will print "10"

Getting a bit more impressive:

// This command will be executed everytime the text in a TextField changes
final textChangedCommand = RxCommand.createSync3((s) => s);

// handler for results
textChangedCommand.results
  .debounce( new Duration(milliseconds: 500))  // Rx magic: make sure we start processing 
                                               // only if the user make a short pause typing 
    .listen( (filterText)
    {
      updateWeatherCommand.execute( filterText); // I could omit he execute because RxCommand is a callable class but here it 
                                                  //  makes the intention clearer
    });

Getting Started

Add to your pubspec.yaml dependencies to rxdartand rx_command.

An RxCommand is a generic class of type RxCommand<TParam, TRESULT> where TPARAM is the type of data that is passed when calling execute and TResult denotes the return type of the handler function. To signal that a handler doesn't take a parameter or returns a Null value.

An example of the declaration from the included sample App

RxCommand<String,List<WeatherEntry>>  updateWeatherCommand;
RxCommand<bool,bool>  switchChangedCommand;

updateWeatherCommand expects a handler that takes a String as parameter and returns a List<WeatherEntry>. switchChangedCommand expects and returns a bool value

Creating RxCommands

For the different variations of possible handler methods RxCommand offers several factory methods for synchronous and asynchronous handlers. Due to the limitation that Dart doesn't allow method overloading they are numbered and look like this.

  /// Creates  a RxCommand for a synchronous handler function with no parameter and no return type 
  /// `action`: handler function
  /// `canExecute` : observable that can bve used to enable/diable the command based on some other state change
  /// if omitted the command can be executed always except it's already executing
  static RxCommand<Null, Null> createSync(Action action,[Observable<bool> canExecute])

createFromStream

Creates a RxCommand from an "one time" observable. This is handy if used together with a Stream generator function.
provider: provider function that returns a new Stream that will be subscribed on the call of execute canExecute : observable that can be used to enable/disable the command based on some other state change If omitted the command can be executed always except it's already executing

  static RxCommand<TParam, TResult> createFromStream<TParam, TResult>(StreamProvider<TParam, TResult> provider, [Observable<bool> canExecute])

Example

The sample App contains a Switch widget that enables/disables the update command. The switch itself is bound to the switchChangedCommand that's result is then used as canExcecute of the updateWeatherCommand:

switchChangedCommand = RxCommand.createSync3<bool,bool>((b)=>b);

// We pass the result of switchChangedCommand as canExecute Observable to the upDateWeatherCommand
updateWeatherCommand = RxCommand.createAsync3<String,List<WeatherEntry>>(update,switchChangedCommand.results);

As the Update Button's building is based on a StreamBuilderthat listens on the canExecute Observable of the updateWeatherCommand the buttons enabled/disabled state gets automatically updated when the Switch's state changes

Using RxCommands

RxCommand is typically used in a ViewModel of a Page, which is made accessible to the Widgets via an InheritedWidget. Its executemethod can then directly be assigned as event handler of the Widgets.

The result of the command is best used with a StreamBuilder or inside a StatefulWidget.

By subscribing (listening) to the isExecuting property of a RxCommand you can react on any execution state change of the command. E.g. show a spinner while the command is running.

By subscribing to the canExecute property of a RxCommand you can react on any state change of the executability of the command.

As RxCommand is a callable class you can assign it directly to handler functions of Flutter widgets like:

new TextField(onChanged: TheViewModel.of(context).textChangedCommand,)

Listening for CommandResults

The original ReactiveCommand from ReactiveUI separates the state information of the command into four Observables (result, thrownExceptions, isExecuting, canExecute) this works great in an environment that doesn't rebuild the whole screen on state change. Flutter it's often desirable when working with a StreamBuilder to have all this information at one place so that you can decide what to display depending on the returned state. Therefore RxCommand itself is an Observable emitting CommandResultobjects:

class CommandResult<T>
{
  final T         data;
  final Exception error;
  final bool      isExecuting;

  const CommandResult(this.data, this.error, this.isExecuting);

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

isExecuting will issue a bool value on each state change. Even if you subscribe to a newly created command it will issue false. When listening for CommandResult this normally doesn't make sense, so no initial CommandResult will be emitted. If you want to get an initial Result with data==null, error==null, isExceuting==false pass emitInitialCommandResult=true when creating a command.

Accessing the last result

RxCommand.lastResult gives you access to the last successful result of the commands execution.

If you want to get the last result included in the CommandResult events while executing or in case of and error you can pass emitInitialCommandResult=true when creating the command.

Disposing subscriptions (listeners)

When subscribing to an Observable with .listen you should store the returned StreamSubscription and call .cancel on it if you want to cancel this subscription to a later point or if the object where the subscription is made is getting destroyed to avoid memory leaks. RxCommand has a dispose function that will cancel all active subscriptions on its observables. Calling disposebefore a command gets out of scope is a good practise.

Exploring the sample App

The best way to understand how RxCommand is used is to look at the supplied sample app which is a simple app that queries a REST API for weather data.

The ViewModel

It follow the MVVM design pattern so all business logic is bundled in the WeatherViewModel class in weather_viewmodel.dart.

It is made accessible to the Widgets by using an InheritedWidget which is defined in main.dart and returns and instance of WeatherViewModelwhen used like TheViewModel.of(context)

The view model publishes two commands

updateWeatherCommand which makes a call to the weather API and filters the result based on a string that is passed to execute. Its result will be bound to a StreamBuilderin your View.
switchChangedCommand which will be bound to a Switch widget to enable/disable the `updateWeatherCommand.

The View

main.dart creates the ViewModel and places it at the very base of the app`s widget tree.

homepage.dart creates a Column with a

TextField where you can enter a filter text which binds to the ViewModels textChangedCommand.
a middle block which can either be a ListView (WeatherListView) or a busy spinner. It is created by a StreamBuilder which listens to <br/> TheViewModel.of(context).updateWeatherCommand.isExecuting<br/>
A row with the Update Button and a Switch that toggles if an update should be possible or not by binding to TheViewModel.of(context).switchChangedCommand). To change the enabled state of the button the button is build by a StreamBuilder that listens to the TheViewModel.of(context).updateWeatherCommand.canExecute

listview.dart implements WeatherListView which consists again of a StreamBuilder which updates automatically by listening on TheViewModel.of(context).updateWeatherCommand.results

Mocking RxCommands

When writing UI Tests with Flutter its often better not to work with the real commands in the ViewModel but to use a MockCommand to have better control over the data a command receives and emits.

For this the MockCommand class is for. It behaves almost like a normal RxCommand

It's created by

/// Factory constructor that can take an optional observable to control if the command can be executet
factory MockCommand({Observable<bool> canExecute} )

You don't pass a handler function because this should be controlled from the outside. To control the outcome of the Command execution you can inspect these properties:

/// the last value that was passed when execute or the command directly was called
TParam lastPassedValueToExecute;

/// Number of times execute or the command directly was called
int executionCount = 0;

To simulate a certain data output after calling the command use:

/// to be able to simulate any output of the command when it is called you can here queue the output data for the next exeution call
queueResultsForNextExecuteCall(List<CommandResult<TResult>> values)

To execute the command you can either call the command instance directly or call execute

/// Can either be called directly or by calling the object itself because RxCommands are callable classes
/// Will increase [executionCount] and assign [lastPassedValueToExecute] the value of [param]
/// If you have queued a result with [queueResultsForNextExecuteCall] it will be copies tho the output stream.
/// [isExecuting], [canExceute] and [results] will work as with a real command.  
execute([TParam param])

Here an example from the rx_widgets example App

testWidgets('Tapping update button updates the weather', (tester) async {
  final model = new MockModel(); // using mockito
  final command = new MockCommand<String,List<WeatherEntry>>();
    final widget = new ModelProvider(
                          model: model,
                          child: new MaterialApp(home: new HomePage()),
                      );

  // to make the mocked model use the MockCommand instance. 
  when(model.updateWeatherCommand).thenReturn(command);
  // if your App does not only access the command but also calls
  // it directly you have to register the call too:
  when(model.updateWeatherCommand()).thenAnswer((_)=>command());

  command.queueResultsForNextExecuteCall([CommandResult<List<WeatherEntry>>(
              [WeatherEntry("London", 10.0, 30.0, "sunny", 12)],null, false)]);

  expect(command, emitsInOrder([ crm(null, false, false), // default value that will be emited at startup 
                                 crm([WeatherEntry("London", 10.0, 30.0, "sunny", 12)], // data
                                  false, false) ]));

  await tester.pumpWidget(widget); // Build initial State
  await tester.pump(); // Build after Stream delivers value

  await tester.tap(find.byKey(AppKeys.updateButtonEnabled));


});

To verify the changing states of the command e.g. to check if linked UI controls are created or in a certain state use:

/// For a more fine grained control to simulate the different states of an `RxCommand`
/// there are these functions
/// [startExecution] will issue a `CommandResult` with
/// data: null
/// error: null
/// isExecuting : true
void startExecution()

/// [endExecutionWithData] will issue a `CommandResult` with
/// data: [data]
/// error: null
/// isExecuting : false
void endExecutionWithData(TResult data)

/// [endExecutionWithData] will issue a `CommandResult` with
/// data: null
/// error: Exeption([message])
/// isExecuting : false
void endExecutionWithError(String message)

/// [endExecutionWithData] will issue a `CommandResult` with
/// data: null
/// error: null
/// isExecuting : false
void endExecutionNoData()

Also an example from rx_widgets

testWidgets('Shows a loading spinner and disables the button while executing 
            and shows the ListView on data arrival', (tester) async {
  final model = new MockModel();
  final command = new MockCommand<String,List<WeatherEntry>>();
  final widget = new ModelProvider(
                        model: model,
                        child: new MaterialApp(home: new HomePage()),
                      );

  // Link MockCommand instance to mocked field in model
  when(model.updateWeatherCommand).thenReturn(command);


  await tester.pumpWidget(widget);// Build initial State
  await tester.pump(); 

  expect(find.byKey(AppKeys.loadingSpinner), findsNothing);
  expect(find.byKey(AppKeys.updateButtonDisabled), findsNothing);
  expect(find.byKey(AppKeys.updateButtonEnabled), findsOneWidget);
  expect(find.byKey(AppKeys.weatherList), findsNothing);
  expect(find.byKey(AppKeys.loaderError), findsNothing);
  expect(find.byKey(AppKeys.loaderPlaceHolder), findsOneWidget);


  command.startExecution();
  await tester.pump(); 
  // because there are two streams involded it seems we have to pump 
  // twice so that both streambuilders can work
  await tester.pump();  

  expect(find.byKey(AppKeys.loadingSpinner), findsOneWidget);
  expect(find.byKey(AppKeys.updateButtonDisabled), findsOneWidget);
  expect(find.byKey(AppKeys.updateButtonEnabled), findsNothing);
  expect(find.byKey(AppKeys.weatherList), findsNothing);
  expect(find.byKey(AppKeys.loaderError), findsNothing);
  expect(find.byKey(AppKeys.loaderPlaceHolder), findsNothing);

  command.endExecutionWithData([new WeatherEntry("London", 10.0, 30.0, "sunny", 12)]);
  await tester.pump(); // Build after Stream delivers value

  expect(find.byKey(AppKeys.loadingSpinner), findsNothing);
  expect(find.byKey(AppKeys.updateButtonDisabled), findsNothing);
  expect(find.byKey(AppKeys.updateButtonEnabled), findsOneWidget);
  expect(find.byKey(AppKeys.weatherList), findsOneWidget);
  expect(find.byKey(AppKeys.loaderError), findsNothing);
  expect(find.byKey(AppKeys.loaderPlaceHolder), findsNothing);
});

Libraries

rx_command

rx_command package