A lightweight, yet powerful way to bind your application state with your business logic.
A lightweight, yet powerful way to bind your application state with your business logic.
As other state management pattern, binder aims to separate the application state from the business logic that updates it:
We can see the whole application state as the agglomeration of a multitude of tiny states. Each state being independent from each other. A view can be interested in some particular states and has to use a logic component to update them.
In the
pubspec.yamlof your flutter project, add the following dependency:
dependencies: binder:
In your library add the following import:
import 'package:binder/binder.dart';
Any state has to be declared through a
StateRefwith its initial value:
final counterRef = StateRef(0);
Note: A state should be immutable, so that the only way to update it, is through methods provided by this package.
Any logic component has to be declared through a
LogicRefwith a function that will be used to create it:
final counterViewLogicRef = LogicRef((scope) => CounterViewLogic(scope));
The
scopeargument can then be used by the logic to mutate the state and access other logic components.
Note: You can declare
StateRefand
LogicRefobjects as public global variables if you want them to be accessible from other parts of your app.
If we want our
CounterViewLogicto be able to increment our counter state, we might write something like this:
/// A business logic component can apply the [Logic] mixin to have access to /// useful methods, such as `write` and `read`. class CounterViewLogic with Logic { const CounterViewLogic(this.scope);/// This is the object which is able to interact with other components. @override final Scope scope;
/// We can use the [write] method to mutate the state referenced by a /// [StateRef] and [read] to obtain its current state. void increment() => write(counterRef, read(counterRef) + 1); }
In order to bind all of this together in a Flutter app, we have to use a dedicated widget called
BinderScope. This widget is responsible for holding a part of the application state and for providing the logic components. You will typically create this widget above the
MaterialAppwidget:
BinderScope( child: MaterialApp( home: CounterView(), ), );
In any widget under the
BinderScope, you can call extension methods on
BuildContextto bind the view to the application state and to the business logic components:
class CounterView extends StatelessWidget { const CounterView({Key key}) : super(key: key);@override Widget build(BuildContext context) { /// We call the [watch] extension method on a [StateRef] to rebuild the /// widget when the underlaying state changes. final counter = context.watch(counterRef);
return Scaffold( appBar: AppBar(title: const Text('Binder example')), body: Center( child: Column( mainAxisAlignment: MainAxisAlignment.center, children: <widget>[ const Text('You have pushed the button this many times:'), Text('$counter', style: Theme.of(context).textTheme.headline4), ], ), ), floatingActionButton: FloatingActionButton( /// We call the [use] extension method to get a business logic component /// and call the appropriate method. onPressed: () => context.use(counterViewLogicRef).increment(), tooltip: 'Increment', child: const Icon(Icons.add), ), );
} }
This is all you need to know for a basic usage.
Note: The whole code for the above snippets is available in the example file.
A state can be of a simple type as an
intor a
Stringbut it can also be more complex, such as the following:
class User { const User(this.firstName, this.lastName, this.score);final String firstName; final String lastName; final int score; }
Some views of an application are only interested in some parts of the global state. In these cases, it can be more efficient to select only the part of the state that is useful for these views.
For example, if we have an app bar title which is only responsible for displaying the full name of a
User, and we don't want it to rebuild every time the score changes, we will use the
selectmethod of the
StateRefto watch only a sub part of the state:
class AppBarTitle extends StatelessWidget { const AppBarTitle({Key key}) : super(key: key);@override Widget build(BuildContext context) { final fullName = context.watch( userRef.select((user) => '${user.firstName} ${user.lastName}'), ); return Text(fullName); } }
If you want to rebuild only a part of your widget tree and don't want to create a new widget, you can use the
Consumerwidget. This widget can take a watchable (a
StateRefor even a selected state of a
StateRef).
class MyAppBar extends StatelessWidget { const MyAppBar({Key key}) : super(key: key);@override Widget build(BuildContext context) { return AppBar( title: Consumer( watchable: userRef.select((user) => '${user.firstName} ${user.lastName}'), builder: (context, String fullName, child) => Text(fullName), ), ); } }
If you want to trigger an asynchronous data load of a logic, from the widget side,
LogicLoaderis the widget you need!
To use it, you have to implement the
Loadableinterface in the logic which needs to load data. Then you'll have to override the
loadmethod and fetch the data inside it.
final usersRef = StateRef(const []); final loadingRef = StateRef(false);final usersLogicRef = LogicRef((scope) => UsersLogic(scope));
class UsersLogic with Logic implements Loadable { const UsersLogic(this.scope);
@override final Scope scope;
UsersRepository get _usersRepository => use(usersRepositoryRef);
@override Future load() async { write(loadingRef, true); final users = await _usersRepository.fetchAll(); write(usersRef, users); write(loadingRef, false); } }
From the widget side, you'll have to use the
LogicLoaderand provide it the logic references you want to load:
class Home extends StatelessWidget { const Home({ Key key, }) : super(key: key);@override Widget build(BuildContext context) { return LogicLoader( refs: [usersLogicRef], child: const UsersView(), ); } }
You can watch the state in a subtree to display a progress indicator when the data is fetching:
class UsersView extends StatelessWidget { const UsersView({ Key key, }) : super(key: key);@override Widget build(BuildContext context) { final loading = context.watch(loadingRef); if (loading) { return const CircularProgressIndicator(); }
// Display the users in a list when have been fetched. final users = context.watch(usersRef); return ListView(...);
} }
Alternatively, you can use the
builderparameter to achieve the same goal:
class Home extends StatelessWidget { const Home({ Key key, }) : super(key: key);@override Widget build(BuildContext context) { return LogicLoader( refs: [usersLogicRef], builder: (context, loading, child) { if (loading) { return const CircularProgressIndicator(); }
// Display the users in a list when have been fetched. final users = context.watch(usersRef); return ListView(); }, );
} }
It can be useful to be able to override the initial state of
StateRefor the factory of
LogicRefin some conditions: - When we want a subtree to have its own state/logic under the same reference. - For mocking values in tests.
Let's say we want to create an app where a user can create counters and see the sum of all counters:
We could do this by having a global state being a list of integers, and a business logic component for adding counters and increment them:
final countersRef = StateRef(const []);final countersLogic = LogicRef((scope) => CountersLogic(scope));
class CountersLogic with Logic { const CountersLogic(this.scope);
@override final Scope scope;
void addCounter() { write(countersRef, read(countersRef).toList()..add(0)); }
void increment(int index) { final counters = read(countersRef).toList(); counters[index]++; write(countersRef, counters); } }
We can then use the
selectextension method in a widget to watch the sum of this list:
final sum = context.watch(countersRef.select( (counters) => counters.fold(0, (a, b) => a + b), ));
Now, for creating the counter view, we can have an
indexparameter in the constructor of this view. This has some drawbacks: - If a child widget needs to access this index, we would need to pass the
indexfor every widget down the tree, up to our child. - We cannot use the
constkeyword anymore.
A better approach would be to create a
BinderScopeabove each counter widget. We would then configure this
BinderScopeto override the state of a
StateReffor its descendants, with a different initial value.
Any
StateRefor
LogicRefcan be overriden in a
BinderScope. When looking for the current state, a descendant will get the state of the first reference overriden in a
BinderScopeuntil the root
BinderScope. This can be written like this:
final indexRef = StateRef(0);class HomeView extends StatelessWidget { @override Widget build(BuildContext context) { final countersCount = context.watch(countersRef.select((counters) => counters.length));
return Scaffold( ... child: GridView( ... children: [ for (int i = 0; i < countersCount; i++) BinderScope( overrides: [indexRef.overrideWith(i)], child: const CounterView(), ), ], ), ... );
} }
The
BinderScopeconstructor has an
overridesparameter which can be supplied from an
overrideWithmethod on
StateRefand
LogicRefinstances.
Note: The whole code for the above snippets is available in the example file.
Let's say you have an api client in your app:
final apiClientRef = LogicRef((scope) => ApiClient());
If you want to provide a mock instead, while testing, you can do:
testWidgets('Test your view by mocking the api client', (tester) async { final mockApiClient = MockApiClient();// Build our app and trigger a frame. await tester.pumpWidget( BinderScope( overrides: [apiClientRef.overrideWith((scope) => mockApiClient)], child: const MyApp(), ), );
expect(...); });
Whenever the
apiClientRefis used in your app, the
MockApiClientinstance will be used instead of the real one.
You may encounter a situation where different widgets are interested in a derived state which is computed from different sates. In this situation it can be helpful to have a way to define this derived state globally, so that you don't have to copy/paste this logic across your widgets. Binder comes with a
Computedclass to help you with that use case.
Let's say you have a list of products referenced by
productsRef, each product has a price, and you can filter these products according to a price range (referenced by
minPriceRefand
maxPriceRef).
You could then define the following
Computedinstance: ```dart final filteredProductsRef = Computed((watch) { final products = watch(productsRef); final minPrice = watch(minPriceRef); final maxPrice = watch(maxPriceRef);
return products .where((p) => p.price >= minPrice && p.price <= maxPrice) .toList(); }); ```
Like
StateRefyou can watch a
Computedin the build method of a widget:
@override Widget build(BuildContext context) { final filteredProducts = context.watch(filteredProductsRef); ... // Do something with `filteredProducts`. }
Note: The whole code for the above snippets is available in the example file.
You may want to observe when the state changed and do some action accordingly (for example, logging state changes). To do so, you'll need to implement the
StateObserverinterface (or use a
DelegatingStateObserver) and provide an instance to the
observersparameter of the
BinderScopeconstructor.
bool onStateUpdated(StateRef ref, T oldState, T newState, Object action) { logs.add( '[${ref.key.name}#$action] changed from $oldState to $newState', );// Indicates whether this observer handled the changes. // If true, then other observers are not called. return true; } ... BinderScope( observers: [DelegatingStateObserver(onStateUpdated)], child: const SubTree(), );
Binder comes with a built-in way to move in the timeline of the state changes. To be able to undo/redo a state change, you must add a
MementoScopein your tree. The
MementoScopewill be able to observe all changes made below it:
return MementoScope( child: Builder(builder: (context) { return MaterialApp( home: const MyHomePage(), ); }), );
Then, in a business logic, stored below the
MementoScope, you will be able to call
undo/
redomethods.
Note: You will get an AssertionError at runtime if you don't provide a
MementoScopeabove the business logic calling
undo/
redo.
In some situation, you'll want to do some action before the
BinderScopehosting a business logic component, is disposed. To have the chance to do this, your logic will need to implement the
Disposableinterface.
class MyLogic with Logic implements Disposable { void dispose(){ // Do some stuff before this logic go away. } }
If you want to navigate to another screen or show a dialog when a state change, you can use the
StateListenerwidget.
For example, in an authentication view, you may want to show an alert dialog when the authentication failed. To do it, in the logic component you could set a state indicating whether the authentication succeeded or not, and have a
StateListenerin your view do respond to these state changes:
return StateListener( watchable: authenticationResultRef, onStateChanged: (context, AuthenticationResult state) { if (state is AuthenticationFailure) { showDialog( context: context, builder: (context) { return AlertDialog( title: const Text('Error'), content: const Text('Authentication failed'), actions: [ TextButton( onPressed: () => Navigator.of(context).pop(), child: const Text('Ok'), ), ], ); }, ); } else { Navigator.of(context).pushReplacementNamed(route_names.home); } }, child: child, );
In the above snippet, each time the state referenced by
authenticationResultRefchanges, the
onStateChangedcallback is fired. In this callback we simply verify the type of the state to determine whether we have to show an alert dialog or not.
Binder wants to simplify the debugging of your app. By using the DartDev tools, you will be able to inspect the current states hosted by any
BinderScope.
You can find code snippets for vscode at snippets.
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