Kotlin – All about

Singleton

How to create an object once and use it everywhere?

It’s a singleton class. A singleton class is a class that is defined in such a way that only one instance of the class can be created and used everywhere.

Java

// Create a Singleton class

public class SingletonService {
    // Static instance
    private static final SingletonService instance = new SingletonService();
    public static SingletonService getInstance() {
        return instance;
    }

    // Private constructor
    private SingletonService() {
      ... do something
    }

    // Access a property everywhere
    private ServiceConnection serviceConnection = null;
    public void setServiceConnection(ServiceConnection serviceConnection) {
        this.serviceConnection = serviceConnection;
    }

    public ServiceConnection getServiceConnection() {
        return serviceConnection;
    }
}

// Usage

public class Somewhere {
...

  // Retrieve one instance
  SingletonService service = SingletonService.getInstance();

  // Update property value
  service.setServiceConnection(xxxxxx);
...

}

////////////////////////////////////////////////////////////////
////////////////////////////////////////////////////////////////

public class MainActivity extends ReactActivity {

...

  @Override
  protected void onDestroy() {
    super.onDestroy();
    
    // Retrieve one instance
    SingletonService service = SingletonService.getInstance();

    // Retrieve a property and use it
    ServiceConnection serviceConnection =  service.getServiceConnection();
    if (serviceConnection != null) {
      unbindService(serviceConnection);
    }
  }
...

Kotlin

// Create a Singleton class

object SingletonService { // 1. Use object keyword for singleton class
    
    init { // 2. Object doesn't have constructor, use init instead
      ... do something
    }

    // 3. Access a property everywhere
    var serviceConnection: ServiceConnection? = null
        get() = field
        set(value) {
            field = value
        }
}

// Usage in Kotlin

class Somewhere {
...

  // Retrieve a property and SET its value
  SingletonService.serviceConnection = xxxxxx

...

  // Retrieve a property and use it
  SingletonService.serviceConnection?.let {
    unbindService(
      it
    )
  }

...
}

// Use Kotlin singleton class in Java
public class MainActivity extends ReactActivity {

...

  @Override
  protected void onDestroy() {
    super.onDestroy();
    SingletonService service = SingletonService.INSTANCE;
    ServiceConnection serviceConnection =  service.getServiceConnection();
    if (serviceConnection != null) {
      unbindService(serviceConnection);
    }
  }
...

How to access companion object from Java?

Kotlin

class AudioPlayerService {
    companion object {
        var serviceConnection: ServiceConnection? = null
    }
}

Usage in Kotlin

// Usage

// Retrieve property
AudioPlayerService.serviceConnection?.let {
  // Check if serviceConnection not null, then do something with servicecConnection using "it"
  ...
}

// Update value to property
AudioPlayerService.serviceConnection = xxxxxxx

Usage in Java

• Add @kotlin.jvm.JvmField into the companion object

class AudioPlayerService {
    companion object {
        @kotlin.jvm.JvmField // Add this!!
        var serviceConnection: ServiceConnection? = null
    }
}

• Then, you can access companion object from Java like this

    ServiceConnection serviceConnection =  AudioPlayerService.serviceConnection;

    // Check if serviceConnection not null, then do something with servicecConnection
    if (serviceConnection != null) {
      unbindService(serviceConnection);
    }