The Cake pattern is a dependency injection pattern for Scala that uses traits to define components and their dependencies. It allows defining components and their implementations separately. An object extends the necessary component traits to resolve dependencies and provides the application object graph. The graph is constructed at compile time, ensuring type safety. However, the pattern results in boilerplate code and does not provide strong lifecycle management for the components.

1. Cake pattern
1
Alex Fomin
@AlexSane

2. 2

3. DI IoC
3

4. DI != IoC
4

5. DI <: IoC
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6. Cake Pattern <: DI
6

7. Example
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8. class ShoppingRobot(listProvider: ShopListProvider, location: GeoLocation) {
def goShopping() = {
val list = listProvider.get
val shop = location.findNearestShop
shop.map(buy(_, list))
}
def buy(shop:Shop, list:List[Item]) = ???
}
class ShopListProvider {
def get:List[Item] = ???
}
class GeoLocation {
def findNearestShop:Maybe[Shop] = ???
}
8
Dependencies

9. object Application extends App {
val shoplistProvider = new ShopListProvider();
val location = new GeoLocation();
val robot = new ShoppingRobot(shopListProvider, location);
robot.goShopping
}
9
InjectionConstructor

10. Cake pattern
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11. trait ShopListProviderComponent {
val shopListProvider: ShopListProvider
trait ShopListProvider {
def get: List[Item]
}
}
trait GeoLocationComponent {
val geoLocation: GeoLocation
trait GeoLocation {
def findNearestShop: Maybe[Shop]
}
}
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trait ShoppingRobotComponent {
val robot: ShoppingRobot
trait ShoppingRobot {
def goShopping: Maybe[Purchase]
}
}

12. trait ShopListProviderComponentImpl extends ShopListProviderComponent {
val shopListProvider: ShopListProvider = new ShopListProviderImpl
class ShopListProviderImpl extends ShopListProvider {
override def get: List[Item] = ???
}
}
trait GeoLocationComponentImpl extends GeoLocationComponent {
val geoLocation: GeoLocation = new GeoLocationImpl
class GeoLocationImpl extends GeoLocation {
override def findNearestShop: Maybe[Shop] = ???
}
}
12

13. trait ShoppingRobotComponentImpl extends ShoppingRobotComponent {
this: ShoppingRobotComponentImpl
with ShopListProviderComponent
with GeoLocationComponent
=>
val robot: ShoppingRobot = new DefaultRobot
class DefaultRobot extends ShoppingRobot {
override def goShopping: Maybe[Purchase] = {
val list = listProvider.get
val shop = location.findNearestShop
shop.map(buy(_, list))
}
}
}
13
Dependency

14. object Registry
extends ShoppingRobotComponentImpl
with GeoLocationComponentImpl
with ShopListProviderComponentImpl
object Application extends App {
val robot = Registry.robot
robot.goShopping
}
14
Injection

15. object FailedRegistry
extends ShoppingRobotComponentImpl
with GeoLocationComponentImpl
error: illegal inheritance;
self-type FailedRegistry.type does not conform to ShoppingRobotComponentImpl's selftype
ShoppingRobotComponentImpl with ShopListProviderComponent with GeoLocationComponent
object FailedRegistry extends ShoppingRobotComponentImpl with
GeoLocationComponentImpl
15

16. object TestRegistry
extends ShoppingRobotComponentImpl
with FakeGeoLocationComponentImpl
with FakeShopListProviderComponentImpl
class Registry2(shop: Shop)
extends ShoppingRobotComponentImpl
with ShopListProviderComponentImpl
with GeoLocationComponent {
val geoLocation: GeoLocation = new GeoLocation {
override def findNearestShop: Maybe[Shop] = Some(shop)
}
}
16

17. Pros and Cons
+ Pure Scala
+ Compile time
validation
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− Boilerplate
overhead
− Weak lifecycle
control

18. Q&A
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