RxCommand
BREAKING CHANGE with V5.0 RxCommand no longer works with Observables but with plain Dart Streams because the latest RxDart version now uses extension methods on Streams instead of Observables:
static RxCommand<TParam, TResult> createSync<TParam, TResult>(Func1<TParam, TResult> func,...
static RxCommand<void, TResult> createSyncNoParam<TResult>(Func<TResult> func,...
static RxCommand<TParam, void> createSyncNoResult<TParam>(Action1<TParam> action,...
static RxCommand<void, void> createSyncNoParamNoResult(Action action,...
static RxCommand<TParam, TResult> createAsync<TParam, TResult>(AsyncFunc1<TParam, TResult> func,...
static RxCommand<void, TResult> createAsyncNoParam<TResult>(AsyncFunc<TResult> func,...
static RxCommand<TParam, void> createAsyncNoResult<TParam>(AsyncAction1<TParam> action,...
static RxCommand<void, void> createAsyncNoParamNoResult(AsyncAction action,...
IMPORTANT: As of V3.0
CommandResultobjects are now emitted on the.resultsproperty and the pure results of the wrapped function on the RxCommand itself. So I switched the two because while working on RxVMS it turned out that I use the pure result much more often. Also the name of.resultsmatches much better withCommandResult. If you don't want to change your code you can just stay on 2.06 if you don't need any of V 3.0 features.
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
resultsStream and theRxCommanditself behaved like aBehaviourSubjects. This can lead to problems when using with Flutter. From now on the default isPublishSubject. If you needBehaviourSubjectbehaviour, meaning every new listener gets the last received value, you can setemitsLastValueToNewSubscriptions = truewhen creatingRxCommand.
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 Stream interface. Additionally it offers Streams 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.createSync<int, String>((myInt) => "$myInt");
command.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.createSync((s) => s);
// handler for results
textChangedCommand
.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 rxdart and 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 use void as type.
Even if you create a RxCommand<void,void> you will receive a null value when the wrapped function finishes so you can listen for the successful completion.
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. They look like this.
/// Creates a RxCommand for a synchronous handler function with no parameter and no return type
/// `action`: handler function
/// `canExecute` : Stream 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<void, void> createSyncNoParamNoResult(Action action,[Stream<bool> canExecute])
There are these variants:
static RxCommand<TParam, TResult> createSync<TParam, TResult>(Func1<TParam, TResult> func,...
static RxCommand<void, TResult> createSyncNoParam<TResult>(Func<TResult> func,...
static RxCommand<TParam, void> createSyncNoResult<TParam>(Action1<TParam> action,...
static RxCommand<void, void> createSyncNoParamNoResult(Action action,...
static RxCommand<TParam, TResult> createAsync<TParam, TResult>(AsyncFunc1<TParam, TResult> func,...
static RxCommand<void, TResult> createAsyncNoParam<TResult>(AsyncFunc<TResult> func,...
static RxCommand<TParam, void> createAsyncNoResult<TParam>(AsyncAction1<TParam> action,...
static RxCommand<void, void> createAsyncNoParamNoResult(AsyncAction action,...
Please check the API docs for detailed description of all parameters
createFromStream
Creates a RxCommand from an "one time" Stream. 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 : Stream 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, [Stream<bool> canExecute])
You can pass in an additional Stream<bool> as canExceute that determines if command can be executed.
Example for canExceute
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.createSync<bool,bool>((b)=>b);
// We pass the result of switchChangedCommand as canExecute Stream to the upDateWeatherCommand
updateWeatherCommand = RxCommand.createAsync<String,List<WeatherEntry>>(update,switchChangedCommand.results);
As the Update Button's building is based on a StreamBuilderthat listens on the canExecute Stream of the updateWeatherCommand the buttons enabled/disabled state gets automatically updated when the Switch's state changes
Error handling with RxCommands
By default all exceptions thrown by the wrapped function will be caught and swallowed. If you want to react on the, you can listen on the thrownException property.
If you want to force RxCommand not to catch Exceptions set throwExceptions=true.
Using RxCommands in a Flutter App
RxCommand is typically used in a ViewModel of a Page, which is made accessible to the Widgets via an InheritedWidget or GetIt. 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. Like changing the appearance of a Button.
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 Streams (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 offer the .results Stream emitting CommandResultobjects:
class CommandResult<T>
{
final T data;
final dynamic 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.
If you want to assign an initialValue to .lastResult e.g. if you use it with a StreamBuilder's initialData you can pass it with the initialLastResult parameter when creating the command.
Disposing subscriptions (listeners)
When subscribing to an Stream 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 Streams. 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
updateWeatherCommandwhich 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 aStreamBuilderin your View.switchChangedCommandwhich will be bound to aSwitchwidget 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
-
TextFieldwhere you can enter a filter text which binds to the ViewModelstextChangedCommand. -
a middle block which can either be a
ListView(WeatherListView) or a busy spinner. It is created by aStreamBuilderwhich listens to <br/>TheViewModel.of(context).updateWeatherCommand.isExecuting<br/> -
A row with the Update
Buttonand aSwitchthat toggles if an update should be possible or not by binding toTheViewModel.of(context).switchChangedCommand). To change the enabled state of the button the button is build by aStreamBuilderthat listens to theTheViewModel.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
Making live easier with RxCommandListeners
If you want to react on more than one Stream of one command the listening and freeing of multiple of subscriptions makes the code less readable and you have to be careful not to forget to cancel all of them.
RxCommandListener makes this handling much easier. Its constructor takes a command and direct handler functions for the different state changes:
class RxCommandListener<TParam, TResult> {
final RxCommand<TParam, TResult> command;
// Is called on every emitted value of the command
final void Function(TResult value) onValue;
// Is called when isExceuting changes
final void Function(bool isBusy) onIsBusyChange;
// Is called on exceptions in the wrapped command function
final void Function(dynamic ex) onError;
// Is called when canExecute changes
final void Function(bool state) onCanExecuteChange;
// is called with the vealue of the .results Stream of the command
final void Function(CommandResult<TResult> result) onResult;
// to make the handling of busy states even easier these are called on their respective states
final void Function() onIsBusy;
final void Function() onNotBusy;
// optional you can directly pass in a debounce duration for the values of the command
final Duration debounceDuration;
RxCommandListener(this.command,{
this.onValue,
this.onIsBusyChange,
this.onIsBusy,
this.onNotBusy,
this.onError,
this.onCanExecuteChange,
this.onResult,
this.debounceDuration,}
)
void dispose();
You don't have to pass all handler functions. they all are optional so you can just pass the ones you need. You only have to dispose the RxCommandListener in your dispose function and it will cancel all internally uses subscriptions.
Let's compare the same code with and without RxCommandListener in some real app code. The selectAndUploadImageCommand here is used in a chat screen where the user can upload images to the chat. When the command is called an ImagePicker dialog is shown and after successful selection of an image the image is uploaded. On completion of the upload the command returns the storage location of the image so that a new image chat entry can be created.
_selectImageCommandSubscription = sl
.get<ImageManager>()
.selectAndUploadImageCommand
.listen((imageLocation) async {
if (imageLocation == null) return;
// this calls the execute method of the command
sl.get<EventManager>().createChatEntryCommand(new ChatEntry(
event: widget.event,
isImage: true,
content: imageLocation.downloadUrl,
));
});
_selectImageIsExecutingSubscription = sl
.get<ImageManager>()
.selectAndUploadImageCommand
.isExecuting
.listen((busy) {
if (busy) {
MySpinner.show(context);
} else {
MySpinner.hide();
}
});
_selectImageErrorSubscription = sl
.get<ImageManager>()
.selectAndUploadImageCommand
.thrownExceptions
.listen((ex) => showMessageDialog(context, 'Upload problem',
"We cannot upload your selected image at the moment. Please check your internet connection"));
to
selectImageListener = RxCommandListener(
command: sl.get<ImageManager>().selectAndUploadImageCommand,
onValue: (imageLocation) async {
if (imageLocation == null) return;
sl.get<EventManager>().createChatEntryCommand(new ChatEntry(
event: widget.event,
isImage: true,
content: imageLocation.downloadUrl,
));
},
onIsBusy: () => MySpinner.show(context),
onNotBusy: MySpinner.hide,
onError: (ex) => showMessageDialog(context, 'Upload problem',
"We cannot upload your selected image at the moment. Please check your internet connection"));
As a rule of thumb I would only use an RxCommandListener if I want to listen to more than one Stream.
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 Stream to control if the command can be executet
factory MockCommand({Stream<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.results, 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);
});