Implement the View

The View is a Spine projection that persists the data emitted by the Reaction. The Generator queries the View to obtain the information it needs to produce Java code.

Class declaration 

internal class WhenFieldView : View<FieldRef, WhenField, WhenField.Builder>()

The three type parameters are:

Subscriber method 

@Subscribe
fun on(e: WhenFieldDiscovered) = alter {
    subject = e.subject
    errorMessage = e.errorMessage
    bound = e.bound
    type = e.type
}

The @Subscribe annotation registers on as the event handler. The alter { } DSL block provides access to the state builder; any assignments inside the block are applied atomically when the block exits. One @Subscribe method is required for each event type the View handles.

Views only accumulate data. Validation and business logic belong in the Reaction; by the time an event reaches the View, the Reaction has already confirmed that the option application is correct.

For the full source, see WhenOption.kt in the Spine Time repository.

What’s next 

• Implement the Generator
• Back to Custom Validation