WidgetDriver #

A Flutter presentation layer framework, which will clean up your
widget code and make your widgets testable without a need for thousands of mock objects.
Let's go driving! 🚙💨

Features #

In Flutter everything is a Widget. And that is great!
But maybe you should not put all of your code directly in the widgets.

Doing that will:

• give you a headache when you try to write unit tests
• make it tougher to reuse your code
• clutter the declarative UI part of Flutter

WidgetDriver to the rescue! WidgetDriver gives you a MVVM style presentation layer framework.
It effectively guides you into moving all of the business logic parts of your code out from widgets and into something called WidgetDrivers.

Core features of WidgetDriver #

• Clear separation of concern.
  Business logic goes in the Driver and only the view logic goes in the Widget
• Better testability of your Widgets.
  When you use WidgetDriver then you can test your Widgets in isolation!
  You do not need to mock hundreds of child dependencies!

Testing benefits #

We have all been there. We write widgetTests and over time we end up with 90% of our testing code being about creating mock objects. Why is that?

Well since widgets tend to contain other child widgets and these widget contains yet more widgets. All these widgets have their own set of dependencies which needs to be resolved when they get created.

So when you want to write a simple widgetTest for a widget, then you end up needing to provide mock data for all the dependencies of all your children. 😱

WidgetDriver to the rescue again! WidgetDriver uses a special TestDriver when you are running tests. These TestDrivers provide predefined default values to all child widgets so that you do not need to provide any dependencies! 🥳

This mean you can finally test your widgets in isolation!
Just focus on the current widget under test, and forget about all other child widgets and their dependencies!

Installation #

Update your pubspec.yaml with this:

dependencies:
  widget_driver: <latest_version>

...

dev_dependencies:
  build_runner: <latest_version>
  widget_driver_generator: <latest_version>

Usage #

Let's get started and create our first WidgetDriver!

1: counter_widget_driver.dart #

part 'counter_widget_driver.g.dart';

@GenerateTestDriver()
class CounterWidgetDriver extends WidgetDriver {
  final CounterService _counterService;
  final Localization _localization;
  StreamSubscription? _subscription;

  CounterWidgetDriver(
    BuildContext context, {
    CounterService? counterService,
  })  : _counterService = counterService ?? GetIt.I.get<CounterService>(),
        _localization = context.read<Localization>(),
        super(context) {

    _subscription = _counterService.valueStream.listen((_) {
      notifyWidget();
    });

  }

  @TestDriverDefaultValue('The title of the counter')
  String get counterTitle => _localization.counterTitle;

  @TestDriverDefaultValue(1)
  int get value => _counterService.value;

  @TestDriverDefaultValue()
  void increment() {
    _counterService.increment();
  }

  @TestDriverDefaultValue()
  void decrement() {
    _counterService.decrement();
  }

  @override
  void dispose() {
    _subscription?.cancel();
    super.dispose();
  }
}

Okay wait, what was all that?! That looks very complicated!

Well, not really. Let's dive into what happens:

1. First we just define a driver which extend the WidgetDriver.
   The "part '...'" definition and the @GenerateTestDriver() annotation above it is needed later for the code generation to work.

   part 'counter_widget_driver.g.dart';
   
   @GenerateTestDriver()
   class CounterWidgetDriver extends WidgetDriver {
   

   And thats all you need to conform to the WidgetDriver interface. The rest of the code in a WidgetDriver depends on your use case.

2. Next, we define the dependencies which the driver needs. In our case we need access to some service which can keep track of the count and we need some localizations.
   In the constructor of the driver we have the option to resolve these dependencies either from the BuildContext (for example using something like the Provider package), or we can load them using a DI package such as get_it. (There are some caveats, please refer to Working with changing dependencies injected into the BuildContext)

   final CounterService _counterService;
   final Locator _locator;
   StreamSubscription? _subscription;
   
   CounterWidgetDriver(
     BuildContext context, {
     CounterService? counterService,
   })  : _counterService = counterService ?? GetIt.I.get<CounterService>(),
       _locator = context.read,
       super(context) {
     ...
   }
   

3. Now we need to add some logic where the Driver listens to changes from the CounterService and when a changes happens, then it will notify the widget that it needs to update. You do this by calling the notifyWidget() method in the driver. As soon as your Driver has new data to display, then you want to call the notifyWidget().

   _subscription = _counterService.valueStream.listen((_) {
     notifyWidget();
   });
   

4. Finally, we define the public API of the driver. Your widget will use these to grab data for its views.

   The annotations are used by the TestDriver to provide default values during testing.

   For each public property and method you add the @TestDriverDefaultValue({default value}). As a parameter to this annotation you provide the default value which will be used in testing when other widgets create this widget.

   If you have a method or property which returns a future, then you can use the @TestDriverDefaultFutureValue({default value}) instead. It will take the default value and return it as a future.

   @TestDriverDefaultValue(1)
   int get value => _counterService.value;
   
   @TestDriverDefaultValue()
   void increment() {
     _counterService.increment();
   }
   
   // For methods which return futures, use this annotations:
   @TestDriverDefaultFutureValue(123)
   Future<int> getTheNextIncrement() {
     return _counterService.getNextIncrement();
   }
   

   As mentioned, these annotations are used by the TestDriver, and you have to provide them! The values provided there are only used in testing. Never in any production code. Because in production you use the real business logic.

   But in your widgetTests these values are used when your widget is rendered as a child-widget. This helps you to test widgets in isolation, without caring about which dependencies all your child widgets need.

   For a more in-depth documentation about TestDrivers read here.

Thats it! Wohoo! 🥳
Now we can continue to the next step!

2: Code generation #

Now it is time to generate some code so that we get our TestDriver and WidgetDriverProvider set up for us.
If you prefer to do this the old fashioned way without code generation, then that is also possible.
Read more about that here

At the root of your flutter project, run this command:

flutter pub run build_runner build

When the build runner completes then we are ready to start building our widget

3: counter_widget.dart #

class CounterWidget extends DrivableWidget<CounterWidgetDriver> {
  @override
  Widget build(BuildContext context) {
    return Column(children: [
      const SizedBox(height: 20),
      Row(
        mainAxisAlignment: MainAxisAlignment.center,
        children: [
          Text(driver.counterTitle),
          const SizedBox(width: 5),
          Text('${driver.value}'),
        ],
      ),
      ElevatedButton(
        onPressed: driver.increment,
        child: const Text('increment'),
      ),
      ElevatedButton(
        onPressed: driver.decrement,
        child: const Text('decrement'),
      ),
    ]);
  }

  @override
  WidgetDriverProvider<CounterWidgetDriver> get driverProvider => $CounterWidgetDriverProvider();
}

And voila, we are done!
Now check out that widget code! Isn't that clean.
Just focused on UI. No StreamBuilders or BuildContext being watched.
No business logic in your view code! And no dependency being created or resolved!

Let's go over what happens:

1. First we name our widget CounterWidget and then we need to make it drivable! We do this by making it extend DrivableWidget. Pass in the name of your Driver as a generic type to the DrivableWidget.

   class CounterWidget extends DrivableWidget<CounterWidgetDriver> {
   

2. Now your IDE will complain and say that you are Missing concrete implementation ...
   To fix this, tap the Create 2 missing overrides-button and voila! You get some placeholder code which looks like this:

   @override
   Widget build(BuildContext context) {
     ...
   }
   
   @override
   // TODO: implement driverProvider
   WidgetDriverProvider<CounterWidgetDriver> get driverProvider => throw UnimplementedError();
   

   The build method is nothing new. There you just put your widget creation code!
   And the driverProvider is the thing which knows how to create your driver. Now what do you put here?
   No worries, we generated a DriverProvider for you 🤩 Just replace that line with this:

   @override
   WidgetDriverProvider<CounterWidgetDriver> get driverProvider => $CounterWidgetDriverProvider();
   

   For each WidgetDriver you get a generated WidgetDriverProvider. They are all prefixed with $ to indicate that they are generated. So just grab the provider which belongs to your driver and create an instance of it here.

3. And that is all you need to do!

   Now your widget will automatically have access to a property called driver. This driver is the very same WidgetDriver which we defined earlier. So now you can access all the data from it and assign it to your widgets. For example you can say this:

   Text(driver.counterTitle),
   

4: Handling updates to data in WidgetDriver #

But what if my data changes? How do I update the widget? Do I need a builder? Or a context.watch?

NO! The WidgetDriver frameworks handles this for you!
The driver decides when the widget needs to update. So inside your driver code, whenever something important changed and you want to update the widget, then you just call the notifyWidget().

This will automatically make sure that your widget get reloaded and it can consume the latest values from your driver.

If you do want to pass data from the widget to the driver #

Say we have a ListView which contains multiple coffees. When the user clicks on one of the items, we want to redirect him to a details page. So how do we pass that coffee object through our widget to the driver to properly use it. Easy...

1. First, we annotate the variable in the driver with the @driverProvidableProperty annotation. This tells the generator to allow this variable to be passed through from the widget.

     import 'package:widget_driver/widget_driver.dart';
   
     import '../../../../models/coffee.dart';
   
     part 'coffee_detail_page_driver.g.dart';
   
     @GenerateTestDriver()
     class CoffeeDetailPageDriver extends WidgetDriver {
       final int index;
       final Coffee _coffee;
   
       CoffeeDetailPageDriver(
         BuildContext context, 
         @driverProvidableProperty this.index, {
         @driverProvidableProperty required Coffee coffee,
       })  : _coffee = coffee,
             super(context);
   
       @TestDriverDefaultValue(TestCoffee.testCoffeeName)
       String get coffeeName {
         return '$index. ${_coffee.name}';
       }
   
       @TestDriverDefaultValue(TestCoffee.testCoffeeDescription)
       String get coffeeDescription {
         return _coffee.description;
       }
   
       @TestDriverDefaultValue(TestCoffee.testCoffeeImageUrl)
       String get coffeeImageUrl {
         return _coffee.imageUrl;
       }
     }
   

2. Then we just run the generator like we did before...

3. We should get a compiler warning in the generated file. To resolve that we just have to add the generated mixin _$DriverProvidedPropertiesMixin to our driver like this:

   @GenerateTestDriver()
     class CoffeeDetailPageDriver extends WidgetDriver with _$DriverProvidedPropertiesMixin {
       ...
     }
   

4. This requires us to override updateDriverProvidedProperties(...) which gets called whenever the corresponding widgets gets asked to re-render by its parent (same as didUpdateWidget). That way we can respond to new values to our provided properties given to us by the widget. (Technical Note: This is because the Driver does not get rebuilt when the widget gets rebuilt. And a call to notifyWidget() is not necessary, this function gets called before the widget shows the new data.)

   @GenerateTestDriver()
     class CoffeeDetailPageDriver extends WidgetDriver with _$DriverProvidedPropertiesMixin {
       int index;
       Coffee _coffee;
   
       CoffeeDetailPageDriver(
         BuildContext context, 
         @driverProvidableProperty this.index, {
         @driverProvidableProperty required Coffee coffee,
       })  : _coffee = coffee,
             super(context);
   
       ...
   
       @override
       void updateDriverProvidedProperties(
         int newIndex,
         Coffee newCoffee,
       ) {
         index = newIndex;
         _coffee = newCoffee;
   
         // And whatever else you want to do on state change.
       }
     }
   

5. After that we just need to hand the variable over to the DriverProvider and that's it. 🥳

    class CoffeeDetailPage extends DrivableWidget<CoffeeDetailPageDriver> {
      final int index;
      final Coffee coffee;
   
      CoffeeDetailPage({Key? key, required this.index, required this.coffee}) : super(key: key);
   
      @override
      Widget build(BuildContext context) {
        ...
      }
   
      @override
      WidgetDriverProvider<CoffeeDetailPageDriver> get driverProvider => $CoffeeDetailPageDriverProvider(
        index: index,
        coffee: coffee,
      );
    }
   

If your WidgetDriver exposes classes that require a lot of overrides #

Some classes have a lot of fields and functions that have to be overridden to construct them. In this case, adding a proper TestDriverDefaultValue can be burdening. To make it easier, we added a class called EmptyDefault. By extending this and implementing the complex class you want the test driver to mock, you can create a empty test class that you can pass to the Test Driver.

1. Create the testDriver class by extending EmptyDefault and implementing the class you want to pass to the Widget Driver

    class _TestDriverMyComplexService extends EmptyDefault implements MyComplexService {
      const _TestDriverReadyToPairConfirmationService();
    }
   

2. Construct the newly created class and pass it as TestDriverDefaultValue

    class MyWidgetDriver extends WidgetDriver {
      ...
       
      @TestDriverDefaultValue(_TestDriverMyComplexService())
      MyComplexService get value => myComplexService;
       
      ...
    }
   

Working with changing dependencies injected into the BuildContext #

The Driver is bound to the lifecycle of the widget's state object, this means it lifes as long as the state of a stateful widget. Because like with StatefulWidget, we do not want to rebuild the Driver on every UI change, that would increase the build time of a DriveableWidget. That's also why we need the mixin and the generated method for @driverProvidableProperties annotated properties. This way we do not need to rebuild the driver with every UI change. (Side Note: Under the hood DriveableWidget is a StatefulWidget) However we want to resolve our dependencies in our Driver's constructor. (Using e.g. Provider) So how do those get updated? We tied the recreation of the driver to the didChangeDependencies state function. Should you watch, listen or subscribe to updates to your dependencies, we will rebuild the driver for you. Thus allowing you to re-resolve your services and create new listeners etc. As the Provider package is one of the most used packages in that category, here is an example:

Example with Provider

We want to read a ThemeDataServiceService from the context, which could change between light and dark Theme at runtime. If we were to resolve it like this:

class SomeDriver extends WidgetDriver {
  final ThemeDataServiceService _themeDataService;

  SomeDriver(
    BuildContext context, {
    ThemeDataService? themeDataService,
  })  : _themeDataService = themeDataService ?? context.read<ThemeDataService>(),
        super(context);
}

The driver would not get an update should the ThemeDataService be changed. The Provider package however offers us the option to watch the provided value.

class SomeDriver extends WidgetDriver {
  final ThemeDataService _themeDataService;

  SomeDriver(
    BuildContext context, {
    ThemeDataService? themeDataService,
  })  : _themeDataService = themeDataService ?? context.watch<ThemeDataService>(),
        super(context);
}

This registers the widget to get updated once the ThemeDataService changes and the WidgetDriver Framework takes care of rebuilding the driver.

Should you not want the driver to be rebuilt you can also use a Locator.

class SomeDriver extends WidgetDriver {
  final Locator _locator

  SomeDriver(
    BuildContext context,
    )  : _locator = context.read,
        super(context);

  bool get isDarkMode => _locator<ThemeDataService>().isDarkMode;
}

This way you resolve the ThemeDataService once you need it, making the recreation obsolete. (Note: Saving the BuildContext for that purpose is NOT a good practice)

Demo #

Here is a demo of how the final product looks like

Guidelines #

WidgetDriver gives you two big benefits.

• Move business logic out of the widget. Clean up the UI code.
• Test widgets in isolation without the of mocks mocks mocks.

But you only gain these benefits if you follow these guidelines.

Guideline One - Architecture 🏗️ #

Move out Business Logic out from the Widgets!
That logic should now instead go inside the WidgetDriver.

It is NOT the job of a Widget to create and combine dependencies!
The Driver is the place where you want to create/resolve dependencies and combine them.
The Driver has access to the BuildContext which was used to create your widget, so you can resolve any needed dependency directly out from that context if you want.

The widget is ONLY interested in creating UI and reacting to interactions from the user.

WidgetDriver gives you all the tools you need to stop putting business logic in the widget.

Guideline Two - Testing 🔬 #

Thanks to the TestDrivers which get auto-created for you when running widget tests, you do not have to mock your dependencies in the tests.

When you run tests, then the real driver is not created. Instead the testDriver gets created and it just provides hard coded default values.

The only thing you need to do when testing a widget is to mock the driver for that specific widget. You just need to mock the driver used by the widget which is currently under test and that is it! No other mocks are needed.

But you only get this benefit if you move the business logic out of the widget.

When you keep business logic in your widgets, then the parent widgets will need to provide mock values for all those child widgets and you end up in mock-mock-mock-world

So please follow guideline two -> Put your business logic in the Driver, not in the Widget!

More about testing and the benefits are described here

Examples #

Please see this example app for inspiration on how to use WidgetDrivers in your app.
The app also contains examples on how you can test your DrivableWidgets and their Drivers.

WidgetDriver and State Management #

WidgetDriver is NOT a state management framework/library.
It is a presentation layer framework. It gives structure and organization to the code which drives your widgets.

But the application state should not be stored in the Driver! It is not the owner of state. Think of Drivers as ViewModels. They take in state and transform it and then provide a tailor made version of it to the widget. E.g. the driver might get in a Date object which it then transforms into a string representation and gives this string to the widget.

So the Drivers will need to access state somehow. Here is a list of recommended state management approaches. BUT just make sure to use these state management systems in the Driver.

For e.g. if you choose to use Provider, then don't use your Provider in the widgets. So don't use context.watch<T>(). Instead you then want to use the Providers in your Driver. See the example app for inspiration on how to do this.

Documentation #

• WidgetDrivers
• DrivableWidgets
• Testing
• Code generation
• Using WidgetDrivers without code generation
• Contribution guide