1. Introduction

One of the many design patterns we developers use in our projects today is the facade pattern, a structural design pattern that provides a simplified interface to a complex system.

In this tutorial, we’ll explore the facade pattern a little more deeply. We’ll start with a definition of the pattern. Then, we will learn what problems it solves and how we can implement it in Kotlin.

2. The Facade Pattern at a Glance

2.1. Definition

As mentioned earlier, the facade pattern is a design pattern that simplifies the interface to a complex system of classes, objects, and subsystems. It acts as a mediator between the client code and the system’s internal components, hiding the complexity of the system and presenting a simplified interface to the outside world. What’s more, by encapsulating a group of related subsystems in a single class, the facade pattern provides a unified and simplified interface to their functionality.

2.2. Problem It Solves

The facade pattern is particularly useful in situations where there are multiple subsystems that need to be managed. By providing a unified interface to these subsystems, the facade pattern can help reduce the complexity of the overall system and make it easier to maintain over time.

To demonstrate, we’re going to implement this pattern in a Database application.

3. Implementation: Database Application

We’ll create a simple system that performs basic functions, such as connecting to the database, performing some business logic, sending a notification, and lastly, disconnecting from the database. By using a facade to provide a simplified interface to our system, we’ll be able to shield the client from the complexity of the entire system.

Finally, let’s get to the coding part. Our application consists of several classes that work together to perform a specific task.

First, let’s define the complex system classes. We’ll have a class for handling database operations, a class for performing business logic, and a class for sending out notifications:

class DatabaseHandler {
    fun connect() {
    }

    fun disconnect() {
    }

    fun executeQuery(query: String) {
    }
}
class BusinessLogicHandler {
    fun performTask() {
    }
}
class NotificationHandler {
    fun sendNotification() {
    }
}

Next, let’s create the facade class that provides a simplified interface for the clients:

class SystemFacade {
    private val databaseHandler = DatabaseHandler()
    private val businessLogicHandler = BusinessLogicHandler()
    private val notificationHandler = NotificationHandler()

    fun performSystemTask() {
        databaseHandler.connect()
        businessLogicHandler.performTask()
        notificationHandler.sendNotification()
        databaseHandler.disconnect()
    }
}

4. Testing

Now that we’ve defined our facade class, let’s create some unit tests to make sure the class works as expected. To do this, we’re going to use the Mockito library for mocking subsystem objects:

class SystemFacadeTest {
    private val databaseHandler = mock(DatabaseHandler::class.java)
    private val businessLogicHandler = mock(BusinessLogicHandler::class.java)
    private val notificationHandler = mock(NotificationHandler::class.java)

    @Test
    fun `test performSystemTask executes necessary operations in correct order`() {
        `when`(databaseHandler.connect()).then {}
        `when`(businessLogicHandler.performTask()).then {}
        `when`(notificationHandler.sendNotification()).then {}

        val systemFacade = SystemFacade(databaseHandler, businessLogicHandler, notificationHandler)
        systemFacade.performSystemTask()

        inOrder(databaseHandler, businessLogicHandler, notificationHandler).apply {
            verify(databaseHandler).connect()
            verify(businessLogicHandler).performTask()
            verify(notificationHandler).sendNotification()
            verify(databaseHandler).disconnect()
        }
    }
}

In summary, this code tests our implementation of the facade pattern in Kotlin. The performSystemTask() method serves as the facade, and it coordinates the actions of the three subsystems — DatabaseHandler, BusinessLogicHandler, and NotificationHandler — to perform the system’s task efficiently and effectively. Also, we test the order in which the methods are called as dictated by the performSystemTask() method.

5. Common Use Cases for the Facade Pattern

Although we’ve elaborately specified the main use of this pattern, it’s still worth mentioning various scenarios where this pattern comes in handy.

5.1. Simplify Interaction With Third-Party Libraries

First, the facade pattern can help simplify interaction with third-party libraries. If we are using a third-party library that has a complex API, we can create a facade that provides a simpler and more intuitive interface to the library. This can help to reduce the learning curve for developers who are new to the library, and also make the code more readable and maintainable.

For instance, take a look at a facade for a third-party logging library:

class LoggerFacade {
    private val logger = Logger.getLogger("MyApp")

    fun logInfo(message: String) {
        logger.info(message)
    }

    fun logWarning(message: String) {
        logger.warning(message)
    }

    fun logSevere(message: String) {
        logger.severe(message)
    }
}

5.2. Hide the Complexity of a Subsystem

For a complex subsystem that consists of multiple components, we can create a facade that hides the complexity of the subsystem and provides a simpler interface to the rest of the system. This can help to reduce coupling between the subsystem and the rest of the system while also making the subsystem easier to test and maintain.

A perfect example of this can be seen in our SystemFacade class in the implementation section above. Notice that this class creates objects of each subsystem and calls functions from these subsystems, thereby hiding the complexity of the whole system from the user.

5.3. Provide a Unified Interface to Multiple Subsystems

Lastly, this pattern is suitable for providing a unified interface to multiple subsystems. For large systems consisting of multiple subsystems, we can create a facade that provides a unified interface to all of them. This can help to reduce the complexity of the system and make it easier to understand and maintain.

Also, let’s have a look at a facade class for multiple subsystems:

class SystemFacade {
    private val subsystemA = SubsystemAFacade()
    private val subsystemB = SubsystemBFacade()

    fun doSomething() {
        subsystemA.doSomething()
        subsystemB.doSomething()
    }
}

6. Drawbacks of the Facade Pattern

While the facade pattern simplifies the interface to a complex subsystem, it can also have some drawbacks. One potential issue is that if the subsystem changes frequently, the facade may also need to be updated frequently. This can lead to a lot of maintenance overhead.

Another potential drawback is that the facade can hide important details about the subsystem from the client, making it difficult for the client to fully understand and utilize the subsystem. For example, let’s consider another facade implementation:

class ComplexSystem {
    fun doSomething() {
    }
}
class Facade {
    private val complexSystem = ComplexSystem()

    fun doSomething() {
        complexSystem.doSomething()
    }
}

In this example, the facade hides the fact that ComplexSystem is actually doing the work when the client makes a call to the Facade.doSomething() method. If the client needs to modify or extend the behavior of ComplexSystem, they may not know to what extent it affects the rest of the system.

7. Conclusion

In this article, we discussed the importance of the facade pattern in software development and gave several examples of how to implement the pattern in Kotlin. We first created a simple database application that performs basic operations and showed how we can test it. Finally, we highlighted some drawbacks of this pattern.

As always, the code samples and relevant test cases for this article can be found over on GitHub.

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