class_switch lets you switch over all the sub-classes of a class instance or all possible combinations of sub-classes for multiple base class instances. Using the class_switch_generator library and annotations from class_switch you can generate functions and mixins to do customizable type safe switching. It pairs wonderfully with bloc helping you get rid of event and state handling boilerplate. See the "Example With Bloc" section below for more.

The class_switch library specifically contains the annotations used by the class_switch_generator library to generate code to switch over classes.

How To Use

1. Add class_switch as a normal dependency.
2. Add class_switch_generator as a dev dependency.
3. Annotate classes with @ClassSwitch.
4. Ensure the sub classes of the annotated class / classes provided in the annotation parameter are in the same file as the annotation.
5. Include part 'YOUR_FILE_NAME.g.dart'; in the file containing the annotated class.
6. Run pub run build_runner watcher.
7. You can now switch over the annotated classes by using the generated $switchXXYY functions.

Benefits of Class Switch

Some benefits of using class_switch are:

• An API as close as possible to being able to switch(){} over all sub classes of an annotated class.
• Compile time guarantees (when pub run build_runner watcher is running) that all possible sub classes are covered by a class switch.
• Autocompleteable class switch statements with all the cases ready to be filled in.
• Switcher mixin classes which provide a great autocomplete experience: add a new sub-type, then on any classes implementing the Mixin you can autocomplete the missing functions for the new sub-type.
• The ability to switch over multiple different base classes, resulting in switchers which have case statements for every possible combination of sub-types. Super useful when used with the Bloc library!
• Highly customizable code generation via annotation options with multiple different configurable DSLs to match your usage and make the generated code as readable as possible.

Important Caveats:

• When annotating a base class all of its sub-classes must be in the same file as or included via the part statement in the file with the annotation , otherwise class_switcher will not find sub-classes outside this and the generated code will throw runtime errors if provided with these unknown sub-classes.

Example Usages

ClassSwitch will generate for a class named BaseClass annotated with @ClassSwitch() (when using the default mode DSL_MODE.CLASS_WRAPPER):

Global $switch Functions

A global $switchBaseClass function which takes an instance of BaseClass and returns a callable class which can then be provided with case functions for every direct sub-class of BaseClass to perform the switch:

   @ClassSwitch()
   abstract class BaseClass {}
   class A extends BaseClass {}
   class B extends BaseClass {}

   // The above will generate a function you can use like so:
   var x = $switchBaseClass(A())(
     (a) => 1, //
     (b) => 2);
   assert(x == 1);
   
   // Get autocomplete to help you by first typing `.call` or `.cases` when
   // writing your switch!
   x = $switchBaseClass(A()).cases(
     (a) => 1, //
     (b) => 2);
   assert(x == 1);

Extension Methods

An extension method on the annotated class called .$switch which when no additional classes are provided via the classes parameter will switch using the instance. When other classes are provided you then will need to provide all instances at once.

   @ClassSwitch()
   abstract class BaseClass {}
   class A extends BaseClass {}
   class B extends BaseClass {}

   // The above will an extension method you can use like so:
   BaseClass anUnknownSubType = A();
   var x = anUnknownSubType.$switch(
     (a) => 1, //
     (b) => 2);
   assert(x == 1);

Switcher Mixin Classes

An abstract Switcher Mixin class which has:

• Abstract sub-class methods for each possible sub-class found in the same file as the annotated class.
• A $switch method which takes an instance of the annotated class and calls the appropriate sub-class method given the type of the instance.

   @ClassSwitch()
   abstract class BaseClass {}
   class A extends BaseClass {}
   class B extends BaseClass {}

   // The above will generate a mixin you can use like so:
   class MySwitcher extends _$BaseClassSwitcher<int>{
     @override
     int a(A a) => 1;

     @override
     int b(B b) => 2;
   };

   assert(MySwitcher().$switch(A()) == 1);

Switcher Mixin Classes With Defaults

• An abstract SwitcherWithDefault Mixin class which has:
  • An abstract default method allowing you to set a default for all types where you have not overridden the sub-class method.
  • sub-class methods for each possible sub-class found in the same file as the annotated class, which will return the result of the default method unless overridden.

  @ClassSwitch()
  abstract class BaseClass {}
  class A extends BaseClass {}
  class B extends BaseClass {}
  
  // The above will generate a mixin you can use like so:
  class MySwitcher extends _$BaseClassSwitcherWithDefault<int>{
    @override
    int defaultValue() => 1;
  
    @override
    int b(B b) => 2;
  };
  
  assert(MySwitcher().$switch(A()) == 1);
  

Switching over Multiple Base Classes

The ability for all the above features to specify multiple different base classes to switch over. This is amazing for working with bloc!

Example With Bloc

The example below shows how class_switch can be used with the Bloc library. However the multi base class switch works just as well without Bloc in any similar situation.

   abstract class BlocState {}
   class StateA extends BlocState {}
   class StateB extends BlocState {}

   abstract class BlocEvent {}
   class EventA extends BlocEvent {}
   class EventB extends BlocEvent {}

   // This will generate a mixin you can use with Bloc like so:
   @ClassSwitch(classes:[BlocState, BlocEvent])
   class MyBloc extends Bloc<BlocEvent, BlocState> with _$MyBlocSwitcher<BlocState> {

     @override
     Stream<BlocState> mapEventToState(
       TodoEvent event,
     ) async* {
       yield this.$switch(this.state, event);
     }

     @override
     stateAEventA(StateA stateA, EventA eventA) => stateA;

     @override
     stateAEventB(StateA stateA, EventB eventB) => stateA;

     @override
     stateBEventA(StateB stateB, EventA eventA) => stateB;

     @override
     stateBEventA(StateB stateB, EventB eventB) => stateB;
   }

   // Or used as a function directly:
   var r = $switchMyBloc(StateA(), EventA()).cases(
     (State stateA, EventA eventA) => 'a a',
     (State stateA, EventB eventB) => 'a b',
     (State stateB, EventA eventA) => 'b a',
     (State stateB, EventB eventB) => 'b b',
   );
   assert(r == 'a a');

Other provided DSLs and generation customization

See DSL_MODE and ClassSwitchOptions for further information on the different DSL's class_switch_generator can make and how to customize the generated code.