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Common Design Patterns and App Architectures for Android

Discover how to make your Android code cleaner and easier to understand with these common design patterns for Android apps. “Future You” will appreciate it!

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Version

  • Kotlin 1.4, Android 4.4, Android Studio 4.2

Update Note: Aaqib Hussain updated this tutorial. Matt Luedke wrote the original.

Beyond your clients and employer, there’s one more important person you should keep happy in your career as a developer: Future You! The artist’s conception of Future You is no guarantee that such cool shirts will be available to developers in the near future. :]

At some point down the road, Future You will inherit the code you write and likely have several questions about why you coded things the way you did. You could leave tons of confusing comments in your code, but a better approach is to adopt common Design Patterns and App Architectures.

If you plan to leave your current job, you also want to make things easy for whoever takes over your role. You don’t want them praising the spaghetti code you left behind.

This article will introduce the most common Design Patterns and App Architectures you can use while developing apps. Design Patterns are reusable solutions to common software problems. App Architectures provide solutions to an app’s data flow or extensibility issues.

This isn’t an exhaustive list of Design Patterns and App Architectures or an academic paper. Instead, this article serves as a ‘hands-on reference’ and a starting point for further learning.

Note: This article assumes you’re familiar with the basics of Android development. If you’re new to Kotlin or the Android platform, take a look at Beginning Android Development Series and Kotlin For Android: An Introduction before you start.

Getting Started

“Is there anywhere in this project where I’ll have to reuse the same piece of code?” – Future You

Future You should minimize time spent doing detective work, looking for intricate project dependencies. So you should create a project that’s as reusable, readable and recognizable as possible. These goals span from a single object all the way to the entire project and lead to patterns that fall into the following categories:

  • Creational patterns: How you create objects.
  • Structural patterns: How you compose objects.
  • Behavioral patterns: How you coordinate object interactions.

Design patterns usually deal with objects. They present a solution to a reoccurring problem that an object shows and help eradicate design-specific problems. In other words, they represent challenges, other developers already faced and prevent you from reinventing the wheel by showing you proven ways to solve those problems.

You may use one or several of these patterns already without having “A Capitalized Fancy Name” for it. However, Future You will appreciate that you didn’t leave design decisions to intuition alone.

In the sections that follow, you’ll cover these patterns from each category and see how they apply to Android:

Creational Patterns

  • Builder
  • Dependency Injection
  • Singleton
  • Factory

Structural Patterns

  • Adapter
  • Facade
  • Decorator
  • Composite

Behavioral Patterns

  • Command
  • Observer
  • Strategy
  • State
Note: This article isn’t like a traditional raywenderlich.com tutorial because it doesn’t have an accompanying sample project you can follow along with. Instead, treat it as an article to get you up to speed with the different patterns you’ll see in our other Android tutorials and a way to improve your code.

Creational Patterns

“When I need a particularly complex object, how do I get an instance of it?” – Future You

Future You hopes the answer isn’t “Just copy and paste the same code every time you need an instance of this object“. Instead, Creational patterns make object instantiation straightforward and repeatable.

Builder

At a certain restaurant, you create your own sandwich: you choose the bread, ingredients and condiments you’d like on your sandwich from a checklist on a slip of paper. Even though the checklist instructs you to build your own sandwich, you only fill out the form and hand it over the counter. You don’t build the sandwich, just customize and consume it. :]

Similarly, the Builder pattern simplifies the creation of objects, like slicing bread and stacking pickles, from its representation, a yummy sandwich. Thus, the same construction process can create objects of the same class with different properties.

In Android, an example of the Builder pattern is AlertDialog.Builder:

AlertDialog.Builder(this)
  .setTitle("Sandwich Dialog")
  .setMessage("Please use the spicy mustard.")
  .setNegativeButton("No thanks") { dialogInterface, i ->
    // "No thanks" action
  }
  .setPositiveButton("OK") { dialogInterface, i ->
    // "OK" action
  }
  .show()

This builder proceeds step-by-step and lets you specify only the parts of AlertDialog that you need to specify. Take a look at the AlertDialog.Builder documentation. You’ll see there are quite a few commands to choose from when building your alert.

The code block above produces the following alert:

Android Message Alert Dialog

A different set of choices would result in a completely different sandwich– er, alert. :]

Dependency Injection

Dependency injection is like moving into a furnished apartment. Everything you need is already there. You don’t have to wait for furniture delivery or follow pages of IKEA instructions to put together a Borgsjö bookshelf.

In software terms, dependency injection has you provide any required objects to instantiate a new object. This new object doesn’t need to construct or customize the objects themselves.

In Android, you might find you need to access the same complex objects from various points in your app, such as a network client, image loader or SharedPreferences for local storage. You can inject these objects into your activities and fragments and access them right away.

Currently, there are three main libraries for dependency injection: Dagger ‘2’, Dagger Hilt, and Koin. Let’s take a look at an example with Dagger. In it you annotate a class with @Module, and populate it with @Provides methods like:

@Module
class AppModule(private val app: Application) {
  @Provides
  @Singleton
  fun provideApplication(): Application = app

  @Provides
  @Singleton
  fun provideSharedPreferences(app: Application): SharedPreferences {
    return app.getSharedPreferences("prefs", Context.MODE_PRIVATE)
  }
}

The module above creates and configures all required objects. As an additional best practice in larger apps, you could create multiple modules separated by function.

Then, you make a Component interface to list your modules and the classes you’ll inject:

@Singleton
@Component(modules = [AppModule::class])
interface AppComponent {
  fun inject(activity: MainActivity)
  // ...
}

The component ties together where the dependencies are coming from, the modules, and where they’re going to, the injection points.

Finally, you use the @Inject annotation to request the dependency wherever you need it, along with lateinit to initialize a non-nullable property after you create the containing object:

@Inject lateinit var sharedPreferences: SharedPreferences

As an example, you could use this in your MainActivity and then use local storage, without the Activity needing to know how the SharedPreferences object came to be.

Admittedly, this is a simplified overview, but you can read the Dagger documentation for more implementation details. You can also click the links above in the mentioned libraries for in-depth tutorials for each topic.

This pattern may seem complicated and magical at first, but it can help simplify your activities and fragments.

Android injecting dependencies image

Singleton

The Singleton pattern specifies that only a single instance of a class should exist with a global access point. This pattern works well when modeling real-world objects with only one instance. For example, if you have an object that makes network or database connections, having more than one instance of the project may cause problems and mix data. That’s why in some scenarios you want to restrict the creation of more than one instance.

The Kotlin object keyword declares a singleton without needing to specify a static instance like in other languages:

object ExampleSingleton {
  fun exampleMethod() {
    // ...
  }
}

When you need to access members of the singleton object, you make a call like this:

ExampleSingleton.exampleMethod()

Behind the scenes, an INSTANCE static field backs the Kotlin object. So, if you need to use a Kotlin object from Java code, you modify the call like this:

ExampleSingleton.INSTANCE.exampleMethod();

By using object, you’ll know you’re using the same instance of that class throughout your app.

The Singleton is probably the most straightforward pattern to understand initially but can be dangerously easy to overuse and abuse. Since it’s accessible from multiple objects, the singleton can undergo unexpected side effects that are difficult to track down, which is exactly what Future You doesn’t want to deal with. It’s important to understand the pattern, but other design patterns may be safer and easier to maintain.

Factory

As the name suggests, Factory takes care of all the object creational logic. In this pattern, a factory class controls which object to instantiate. Factory pattern comes in handy when dealing with many common objects. You can use it where you might not want to specify a concrete class.

Take a look at the code below for a better understanding:

// 1
interface HostingPackageInterface {
  fun getServices(): List<String>
}

// 2
enum class HostingPackageType {
  STANDARD,
  PREMIUM
}

// 3
class StandardHostingPackage : HostingPackageInterface {
  override fun getServices(): List<String> {
    return ...
  }
}

// 4
class PremiumHostingPackage : HostingPackageInterface {
  override fun getServices(): List<String> {
    return ...
  }
}

// 5
object HostingPackageFactory {
  // 6
  fun getHostingFrom(type: HostingPackageType): HostingPackageInterface {
    return when (type) {
      HostingPackageType.STANDARD -> {
        StandardHostingPackage()
      }
      HostingPackageType.PREMIUM -> {
        PremiumHostingPackage()
      }
    }
  }
}

Here’s a walk through the code:

  1. This is a basic interface for all the hosting plans.
  2. This enum specifies all the hosting package types.
  3. StandardHostingPackage conforms to the interface and implements the required method to list all the services.
  4. PremiumHostingPackage conforms to the interface and implements the required method to list all the services.
  5. HostingPackageFactory is a singleton class with a helper method.
  6. getHostingFrom inside HostingPackageFactory is responsible for creating all the objects.

You can use it like this:

val standardPackage = HostingPackageFactory.getHostingFrom(HostingPackageType.STANDARD)

It helps to keep all object creation in one class. If used inappropriately, a Factory class can get bloated due to excessive objects. Testing can also become difficult as the factory class itself is responsible for all the objects.

Structural Patterns

“So, when I open this class, how will I remember what’s it doing and how it’s put together?” – Future You

Future You will undoubtedly appreciate the Structural Patterns you used to organize the guts of your classes and objects into familiar arrangements that perform typical tasks. Adapter and Facade are two commonly-seen patterns in Android.

Adapter

A famous scene in the movie Apollo 13 features a team of engineers tasked with fitting a square peg into a round hole. This, metaphorically, is the role of an adapter. In software terms, this pattern lets two incompatible classes work together by converting a class’s interface into the interface the client expects.

Consider your app’s business logic. It might be a Product or a User or Tribble. It’s the square peg. Meanwhile, a RecyclerView is the same basic object across all Android apps. It’s the round hole.

In this situation, you can use a subclass of RecyclerView.Adapter and implement the required methods to make everything work:

class TribbleAdapter(private val tribbles: List<Tribble>) : RecyclerView.Adapter<TribbleViewHolder>() {
  override fun onCreateViewHolder(viewGroup: ViewGroup, i: Int): TribbleViewHolder {
    val inflater = LayoutInflater.from(viewGroup.context)
    val view = inflater.inflate(R.layout.row_tribble, viewGroup, false)
    return TribbleViewHolder(view)
  }

  override fun onBindViewHolder(viewHolder: TribbleViewHolder, itemIndex: Int) {
    viewHolder.bind(tribbles[itemIndex])
  }

  override fun getItemCount() = tribbles.size
}

RecyclerView doesn’t know what a Tribble is, as it’s never seen a single episode of Star Trek, not even the new movies. :] Instead, it’s the adapter’s job to handle the data and send the bind command to the correct ViewHolder.

Facade

The Facade pattern provides a higher-level interface that makes a set of other interfaces easier to use. The following diagram illustrates this idea in more detail:

block diagram of a facade block pointing to three boxes below it with names: Dependency A, Dependency B, and Dependency C

If your Activity needs a list of books, it should be able to ask a single object for that list without understanding the inner workings of your local storage, cache and API client. Beyond keeping your Activities and Fragments code clean and concise, this lets Future You make any required changes to the API implementation without impacting the Activity.

Square’s Retrofit is an open-source Android library that helps you implement the Facade pattern. You create an interface to provide API data to client classes like so:

interface BooksApi {
  @GET("books")
  fun listBooks(): Call<List<Book>>
}

The client needs to call listBooks() to receive a list of Book objects in the callback. It’s nice and clean. For all it knows, you could have an army of Tribbles assembling the list and sending it back via transporter beam. :]

This lets you make all types of customizations underneath without affecting the client. For example, you can specify a customized JSON deserializer that the Activity has no clue about:

val retrofit = Retrofit.Builder()
  .baseUrl("http://www.myexampleurl.com")
  .addConverterFactory(GsonConverterFactory.create())
  .build()

val api = retrofit.create<BooksApi>(BooksApi::class.java)

Notice the use of GsonConverterFactory, working behind the scenes as a JSON deserializer. With Retrofit, you can further customize operations with Interceptor and OkHttpClient to control caching and logging behavior without the client knowing what’s going on.

The less each object knows about what’s going on behind the scenes, the easier it’ll be for Future You to manage changes in the app.

Image of Android with gears inside its stomach

Decorator

The Decorator pattern dynamically attaches additional responsibilities to an object to extended its functionality at runtime. Take a look at the example below:

//1
interface Salad {
  fun getIngredient(): String
}

//2
class PlainSalad : Salad {
  override fun getIngredient(): String {
    return "Arugula & Lettuce"
  }
}

//3
open class SaladDecorator(protected var salad: Salad) : Salad {
  override fun getIngredient(): String {
    return salad.getIngredient()
  }
}

//4
class Cucumber(salad: Salad) : SaladDecorator(salad) {
  override fun getIngredient(): String {
    return salad.getIngredient() + ", Cucumber"
  }
}

//5
class Carrot(salad: Salad) : SaladDecorator(salad) {
  override fun getIngredient(): String {
    return salad.getIngredient() + ", Carrot"
  }
}

Here’s what the above code defines:

  1. A Salad interface helps with knowing the ingredients.
  2. Every salad needs a base. This base is Arugula & Lettuce thus, PlainSalad.
  3. A SaladDecorator helps add more toppings to the PlainSalad.
  4. Cumcumber inherts from SaladDecorator.
  5. Carrot inherts from SaladDecorator.

By using the SaladDecorator class, you can extend your salad easily without having to change PlainSalad. You can also remove or add any salad decorator on runtime. Here’s how you use it:

val cucumberSalad = Cucumber(Carrot(PlainSalad()))
print(cucumberSalad.getIngredient()) // Arugula & Lettuce, Carrot, Cucumber
val carrotSalad = Carrot(PlainSalad())
print(carrotSalad.getIngredient()) // Arugula & Lettuce, Carrot

Composite

You use the Composite pattern when you want to represent a tree-like structure consisting of uniform objects. A Composite pattern can have two types of objects: composite and leaf. A composite object can have further objects, whereas a leaf object is the last object.

Take a look at the following code to understand it better:

//1
interface Entity {
  fun getEntityName(): String
}

//2
class Team(private val name: String) : Entity {
  override fun getEntityName(): String {
    return name
  }
}

//3
class Raywenderlich(private val name: String) : Entity {
  private val teamList = arrayListOf<Entity>()

  override fun getEntityName(): String {
    return name + ", " + teamList.map { it.getEntityName() }.joinToString(", ")
  }

  fun addTeamMember(member: Entity) {
    teamList.add(member)
  }
}

In the code above you have:

  1. Component, an interface Entity in Composite pattern.
  2. A Team class implements an Entity. It’s a Leaf.
  3. Raywenderlich also implements an Entity interface. It’s a Composite.

Logically and technically the organization, in this case Raywenderlich, adds an Entity to the Team. Here’s how you use it:

val composite = Raywenderlich("Ray")
val ericTeamComposite = Raywenderlich("Eric")
val aaqib = Team("Aaqib")
val vijay = Team("Vijay")
ericTeamComposite.addTeamMember(aaqib)
ericTeamComposite.addTeamMember(vijay)
composite.addTeamMember(ericTeamComposite)
print(composite.getEntityName()) // Ray, Eric, Aaqib, Vijay

Behavioral Patterns

“So… how do I tell which class is responsible for what?” – Future You

Behavioral Patterns let you assign responsibility for different app functions. Future You can use them to navigate the project’s structure and architecture.

These patterns can vary in scope, from the relationship between two objects to your app’s entire architecture. Often, developers use several behavioral patterns together in the same app.

Command

When you order some Saag Paneer at an Indian restaurant, you don’t know which cook will prepare your dish. You just give your order to the waiter, who posts the order in the kitchen for the next available cook.

Similarly, the Command pattern lets you issue requests without knowing the receiver. You encapsulate a request as an object and send it off. Deciding how to complete the request is an entirely separate mechanism.

Greenrobot’s EventBus is a popular Android framework that supports this pattern in the following manner:

Flow chart that starts with a square block of a publisher, then points to the right to an Event Bus shown in a circle, the arrow connecting both has an Event square and a Post() method. The Event bus, has two arrows coming out and pointing to two subscribers,each labeled with a box, and the arrows also have events on them. The subscriber boxes trigger an OnEvent() method

An Event is a command-style object that’s triggered by user input, server data or pretty much anything else in your app. You can create specific subclasses which carry data as well:

class MySpecificEvent { /* Additional fields if needed */ }

After defining your event, you obtain an instance of EventBus and register an object as a subscriber. For example, if you register an Activity you’ll have:

override fun onStart() {
  super.onStart()
  EventBus.getDefault().register(this)
}

override fun onStop() {
  super.onStop()
  EventBus.getDefault().unregister(this)
}

Now that the object is a subscriber, tell it what type of event to subscribe to and what it should do when it receives one:

@Subscribe(threadMode = ThreadMode.MAIN)
fun onEvent(event: MySpecificEvent?) {
  /* Do something */
}

Finally, create and post one of those events based on your criteria:

EventBus.getDefault().post(MySpecificEvent())

Since so much of this pattern works its magic at run-time, Future You might have a little trouble tracing this pattern unless you have good test coverage. Still, a well-designed flow of commands balances out the readability and should be easy to follow later.

Observer

The Observer pattern defines a one-to-many dependency between objects. When one object changes state, its dependents get a notification and updates automatically.

This pattern is versatile. You can use it for operations of indeterminate time, such as API calls. You can also use it to respond to user input.

It was originally popularized by the RxAndroid framework, also known as Reactive Android. This library lets you implement this pattern throughout your app:

apiService.getData(someData)
  .subscribeOn(Schedulers.io())
  .observeOn(AndroidSchedulers.mainThread())
  .subscribe (/* an Observer */)

In short, you define Observable objects that will emit values. These values can emit all at once, as a continuous stream or at any rate and duration.

Subscriber objects will listen for these values and react to them as they arrive. For example, you can open a subscription when you make an API call, listen to the server’s response and react accordingly.

More recently Android also introduced a native way to implement this pattern through LiveData. You can learn more about this topic here.

Strategy

You use a Strategy pattern when you have multiple objects of the same nature with different functionalities. For a better understanding, take a look at the following code:

// 1
interface TransportTypeStrategy {
  fun travelMode(): String
}

// 2
class Car : TransportTypeStrategy {
  override fun travelMode(): String {
    return "Road"
  }
}

class Ship : TransportTypeStrategy {
  override fun travelMode(): String {
    return "Sea"
  }
}

class Aeroplane : TransportTypeStrategy {
  override fun travelMode(): String {
    return "Air"
  }
}

// 3
class TravellingClient(var strategy: TransportTypeStrategy) {
  fun update(strategy: TransportTypeStrategy) {
    this.strategy = strategy
  }

  fun howToTravel(): String {
    return "Travel by ${strategy.travelMode()}"
  }
}

Here’s a code breakdown:

  1. A TransportTypeStrategy interface has a common type for other strategies so it can be interchanged at runtime.
  2. All the concrete classes conform to TransportTypeStrategy.
  3. TravellingClient composes strategy and uses its functionalities inside the functions exposed to the client side.

Here’s how you use it:

val travelClient = TravellingClient(Aeroplane())
print(travelClient.howToTravel()) // Travel by Air
// Change the Strategy to Ship
travelClient.update(Ship())
print(travelClient.howToTravel()) // Travel by Sea

State

In the State pattern, the state of an object alters its behavior accordingly when the internal state of the object changes. Take a look at the following snippets:

// 1
interface PrinterState {
  fun print()
}

// 2
class Ready : PrinterState {
  override fun print() {
    print("Printed Successfully.")
  }
}

// 3
class NoInk : PrinterState {
  override fun print() {
    print("Printer doesn't have ink.")
  }
}

// 4
class Printer() {
  private val noInk = NoInk()
  private val ready = Ready()
  private var state: PrinterState
  private var ink = 2

  init {
    state = ready
  }

  private fun setPrinterState(state: PrinterState) {
    this.state = state
  }

  // 5
  fun startPrinting() {
    ink--
    if (ink >= 0) {
      setPrinterState(ready)
    } else {
      setPrinterState(noInk)
    }
    state.print()
  }

  // 6
  fun installInk() {
    ink = 2
    print("Ink installed.")
  }
}

Here’s a code breakdown:

  1. PrinterState defines the states of a printer.
  2. Ready is a concrete class implementing PrinterState to define a ready state of the printer.
  3. NoInk is a concrete class implementing PrinterState to define that the printer has no ink.
  4. Printer handler does all the printing.
  5. startPrinting starts printing.
  6. installInk installs ink.

Here’s how you use it:

val printing = Printer()
printing.startPrinting() // Printed Successfully.
printing.startPrinting() // Printed Successfully.
printing.startPrinting() // Printer doesn't have ink.
printing.installInk() // Ink installed.
printing.startPrinting() // Printed Successfully.    

So, you create an object of the class Printer to print. The Printer class handles all the states of the printer internally. It’s either on a Ready state or in a NoInk state.

App Architectures

“Could there ever be a requirement where I’ll have to modify or implement new features in this project?” – Future You

App architectures play a vital part in structuring a loosely coupled codebase. You can use it anywhere, irrespective of the platform. App architectures help you write easily testable, extensible and decoupled code.

In simple terms, Architecture refers to the overall organization of your code in things like:

  1. Responsibilities for each class
  2. Folder organization
  3. Structure of the code: network calls, responses, errors.

Types of App Architectures

The App Architectures used to create solid and maintainable codebases are many, but in this article you will learn the most popular ones:

  • Model View Controller
  • Model View ViewModel
  • Clean Architecture

Model View Controller

Model View Controller, or MVC, refers to one of the most popular architectural patterns and the one from which many others derive. It’s particularly easy to set your project up this way on Android. Its name refers to the three ways to classify the classes in your code:

  • Model: Your data classes. If you have User or Product objects, these model the real world.
  • In Kotlin we use Data classes, while in Java they were called POJOs (Plain Old Java Object).

  • View: Your visual classes. Everything the user sees falls under this category. In Android’s case it is mainly XML, although you can also make views in code and with the introduction of compose you will have other types of views.
  • Controller: The glue between the two. It updates the view, takes user input and makes changes to the model.

Dividing your code between these three categories will go a long way toward making it decoupled and reusable.

Eventually, a client will ask Future You to add a new screen to the app that uses its existing data. Following the MVC paradigm means Future You can easily re-use the same models and only change the views.

Or perhaps the client will ask Future You to move that fancy widget from the home screen to the detail screen. Separating your view logic makes this an easy task.

Additionally, moving as much layout and resource logic as possible into Android XML keeps your View layer clean and tidy. Nice!

You may have to do some drawing in Kotlin from time to time. In that case, it’ll help to separate the drawing operations from your activity and fragment classes.

Over time, you’ll find making architectural decisions easier under MVC and Future You can more easily solve issues as they arise.

Model View ViewModel

This unfortunately-quite-confusingly-named presentation architectural pattern is similar to the MVC pattern. The Model and View components are the same.

The ViewModel object is the glue between the model and view layers but operates differently than the Controller component. Instead, it exposes commands for the view and binds the view to the model. When the model updates, the corresponding views update via the data binding.

Similarly, as the user interacts with the view, the bindings work in the opposite direction to automatically update the model.

The MVVM pattern is trending upwards in popularity but is still a fairly recent addition to the pattern library. Google is now recommending this pattern as part of its Architecture Components library. Future You would love to keep your eye on this one! :]

Clean Architecture

Clean Architecture is not in itself an architecture but a concept. It describes the overall app architecture: how the various layers of an app, business objects, use cases, presenters, data storage and UI, communicate with one another. MVC and MVVM exist within Clean Architecture at the outer presentation and UI layers.

You can find the original Clean Architecture definition in this article. There are many examples of Clean Architecture for Android available, including Architecting Android…The evolution.

Where to Go From Here?

While it feels great to keep abreast of the latest flashy APIs, keeping your apps updated can quickly lead to redesign-fatigue. Investing in software design patterns and app architectures early on will improve your return on development time. You’ll start to notice you get more done with less effort.

Check out Design Pattern by Tutorials to cover all the Design patterns in detail with code samples and their applications. If you want to have a deep understanding of App Architectures, check out Advanced Android App Architecture.

You can also read timeless classics such as Design Patterns by the “Gang of Four.” Compared to Material Design or Android Wear, this book might seem ancient, but it documents many useful design patterns that predate Android itself!

I hope this article serves as a starting point for your research into common design patterns for Android! If you have any questions or comments, please join the forum discussion below. Future You would be happy if you did. :]

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