get_it_mixin

A set of mixins that allow widgets to watch data registered with GetIt. Widgets that watch data will rebuild automatically whenever that data changes.

Supported data types that can be watched are ChangeNotifier, ValueNotifier, Stream and Future.

ChangeNotifier based example:

// Create a ChangeNotifier based model
class UserModel extends ChangeNotifier {
  get name = _name;
  String _name = '';
  set name(String value){
    _name = value;
    notifyListeners();
  }
  ...
}

// Register it 
getIt.registerSingleton<UserModel>(UserModel());

// Watch it
class UserNameText extends StatelessWidget with GetItMixin {
  @override
  Widget build(BuildContext context) {
    final userName = watchOnly((UserModel m) => m.name);
    return Text(userName);
  }
}

Reading Data

Reading data is already quite easy with GetIt, but it gets even easier with the mixin. Just add a GetItMixin to a StatelessWidget and call get<T>:

class MyWidget extends StatelessWidget with GetItMixin {
  void _handleSubmitPressed() {
    final email = get<Model>().emailAddress;
    ...
  }
}

You can do the same thing on StatefulWidget using a GetItStatefulWidgetMixin and GetItStateMixin:

class MyWidget extends StatefulWidget with GetItStatefulWidgetMixin {
  _MyWidgetState createState() => _MyWidgetState();
}
class _MyWidgetState extends State<MyWidget> with GetItStateMixin {
  void _handleSubmitPressed() {
    final email = get<Model>().emailAddress;
    ...
  }
}

NOTE: The GetItMixin API is generally the same regardless of whether you use Stateless or Stateful widgets.

Watching Data

Where GetItMixin really shines is data-binding. It comes with a set of watch methods to rebuild a widget when data changes.

Imagine you had a very simple shared model, with multiple fields, one of them being country:

class Model {
    final country = ValueNotifier<String>('Canada');
    ...
}
getIt.registerSingleton<Model>(Model());

You could tell your view to rebuild any time country changes with a simple call to watchX:

class MyWidget extends StatelessWidget with GetItStatefulWidgetMixin {
  @override
  Widget build(BuildContext context) {
    String country = watchX((Model x) => x.country);
    ...
  }
}

There are various watch methods, for common types of data sources, including ChangeNotifier, ValueNotifier, Stream and Future:

APIDescription
.watchBind to a ValueListenable value
.watchXBind to the results of a select method on a ValueListenable value
.watchOnlyBind to a basic Listenable (like ChangeNotifier)
.watchXOnlyBind to the results of a select method on a Listenable
.watchStreamSubscribe to a Stream
.watchFutureBind to a Future

Just call watch_ to listen to the data type you need, and GetItMixin will take care of cancelling bindings and subscriptions when the widget is destroyed.

The primary benefit to the watch methods is that they eliminate the need for ValueListenableBuilders, StreamBuilder etc. Each binding consumes only one line and there is no nesting.

Here we watch three ValueListenable which would normally be three builders, 12+ lines of code and several levels of indentation. With GetItMixin, it's three lines:

class MyWidget extends StatelessWidget with GetItMixin {
  @override
  Widget build(BuildContext context) {
    bool loggedIn = watchXOnly((UserModel x) => x.isLoggedIn);
    String userName = watchXOnly((UserModel x) => x.user.name);
    bool darkMode = watchXOnly((SettingsModel x) => x.darkMode);
    ...
  }
}

This can be used to eliminate StreamBuilder and FutureBuilder from your UI as well:

class MyWidget extends StatelessWidget with GetItMixin {
  @override
  Widget build(BuildContext context) {
    final currentUser = watchStream((UserModel x) => x.userNameUpdates, 'NoUser');
    final ready = watchFuture((AppModel x) => x.initializationReady, false).data;
    bool appIsLoading = ready == false || currentUser.hasData == false;
    
    if(appIsLoading) return CircularProgressIndicator();
    return Text(currentUser.data);    
  }
}

Side Effects / Event Handlers

Instead of rebuilding, you might instead want to show a toast notification or dialog when a Stream emits a value or a ValueListenable changes.

To run an action when data changes you can use the register methods:

APIDescription
.registerHandlerAdd an event handler for a ValueListenable
.registerStreamHandlerAdd an event handler for a Stream
.registerFutureHandlerAdd an event handler for a Future

The first param in the register methods is a select delegate that can be used to watch a specific field. The second param is the action which will be triggered when that field changes:

class MyWidget extends StatelessWidget with GetItMixin {
  @override
  Widget build(BuildContext context) {
    registerHandler(
        (Model x) => x.name,
        (context, value, cancel) => showNameDialog(context, value));
    ...
  }
}

In the example above you see that the handler function receives the value that is returned from the select delegate ((Model x) => x.name), as well as a cancel function that the handler can call to cancel registration at any time.

As with watch calls, all registerHandler calls are cleaned up when the Widget is destroyed.

Rules

There are some important rules to follow in order to avoid bugs with the watch methods:

  • watch methods must be called within build()
    • It is good practice to define them at the top of your build method
  • must be called on every build, in the same order (no conditional watching). This is similar to flutter_hooks.
  • do not use them inside of a builder as it will break the mixins ability to rebuild

isReady

A common use case is to toggle a loading state when side effects are in-progress. To check whether any registered actions have completed you can use allReady() and isReady<T>(). These methods return the current state of any registered async operations and a rebuild is triggered when they change.

class MyWidget extends StatelessWidget with GetItMixin {
  @override
  Widget build(BuildContext context) {
    allReady(onReady: (context)
      => Navigator.of(context).pushReplacement(MainPageRoute()));
    return CircularProgressIndicator();
  }
}

Check out the GetIt docs for more information on the isReady and allReady functionality: https://pub.dev/packages/get_it

Pushing a new GetIt Scope

With pushScope() you can push a scope when a Widget/State is mounted, and automatically pop when the Widget/State is destroyed. You can pass an optional init or dispose function.

  void pushScope({void Function(GetIt getIt) init, void Function() dispose});

This can be very useful for injecting mock services when views are opened so you can easily test them. Of course you can also push and pop scope directly from your tests as well!

Find out more!

For more background on the history of GetItMixin you can check out the README_EXTENDED.

For a more complete explanation of the API watch the presentation: GetIt in action By Thomas Burkhart.

Libraries

get_it_mixin