UseCases.md

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---
title: Clean Architecture: Kotlin Use Cases
---

Use Cases

Use Cases can be both asynchronous and synchronous.

The primary difference is that a synchronous Use Case will return it's result, and an asynchronous Use Case will call a callback with it's result.

It is possible to generalise the boundary interfaces of these two types of Use Cases.

Asynchronous Example

Asynchronous use cases provide greater control over rendering to the UI. However they add complexities to testing.

An example generalisation can be found here, with a derivative here for a specific use case.

package com.acmeindustries.widget.usecase

interface ViewWidgets {
    fun execute(request: Request, presenter: Presenter)
    
    data class Request(...)
    interface Presenter {
        fun onSuccess()
        fun onError()
    }
}

Synchronous Example

Synchronous Use Cases provide a simpler interface for testing, but can make representing failure paths and control over the UI harder to maintain.

package com.acmeindustries.widget.usecase

interface ViewWidgetPerFooBarReport {
    fun execute(request: Request): Response
  
    data class Request(public val fromDate: String, public val endDate: String)
    data class Response(...)
}

package com.acmeindustries.widget

import com.acmeindustries.widget.usecase.ViewWidgetPerFooBarReport
import com.acmeindustries.widget.usecase.ViewWidgetPerFooBarReport.*
import com.acmeindustries.widget.domain.Widget

class WidgetPerFooBarReport(val widgetGateway: WidgetGateway) : ViewWidgetPerFooBarReport {
    fun execute(request: Request): Response {
        val widgets = widgetGateway.all()
        //secret sauce here
        return Response(...) //return response populated with data
    }
}

interface WidgetGateway {
    fun all(): List<Widget>
}