Testable and decoupled preferences interface for Android apps

Preferences.kt

/**
 * Data store for application preferences.
 */
interface AppPreferences {
  /**
   * Get the preference value for the given key.
   * @param T The value type.
   */
  operator fun <T : Any> get(key: String): T?

  /**
   * Set a preference value for the given key.
   * @param value the new value.
   */
  operator fun <T : Any> set(key: String, value: T)

  /**
   * Check whether this store contains a preference entry for the given key or not.
   */
  fun containsKey(key: String): Boolean

  /**
   * Observe the preference value for the given key.
   * @return an [Observable] that emits the preference value as it changes.
   */
  fun <T> observe(key: String): Observable<T>

  /**
   * All stored key/value pairs.
   */
  val all: Map<String, *>


}


class SharedAppPreferences(context: Context) : AppPreferences, SharedPreferences.OnSharedPreferenceChangeListener {

  override fun <T : Any> get(key: String): T? = throwIfClosed {
    vlog(TAG, "get(key: $key)")
    sharedPrefs!!.all[key] as? T
  }

  override fun <T : Any> set(key: String, value: T) = throwIfClosed {
    vlog(TAG, "set(key: $key, value: $value)")
    sharedPrefs.edit {
      when (value) {
        is Int     -> putInt(key, value)
        is Long    -> putLong(key, value)
        is Float   -> putFloat(key, value)
        is Boolean -> putBoolean(key, value)
        is String  -> putString(key, value)
        else       -> error("type not supported: ${value.javaClass}")
      }
    }
  }

  override fun containsKey(key: String): Boolean = throwIfClosed { sharedPrefs.contains(key) }

  override fun <T> observe(key: String): Observable<T> = throwIfClosed {
    vlog(TAG, "observe(key: $key)")
    changeSubjects.getOrPut(key) { BehaviorSubject.create() }.map { it as T }
  }

  override val all: Map<String, *>
    get() = throwIfClosed {
      vlog(TAG, "all: get()")
      sharedPrefs.all
    }

  override val isClosed: Boolean
    get() = closed.get()

  override fun close() {
    vlog(TAG, "close()")
    sharedPrefs?.unregisterOnSharedPreferenceChangeListener(this)
    changeSubjects.run {
      values.forEach { it.onComplete() }
      clear()
    }
    closed.getAndSet(true)
  }

  override fun onSharedPreferenceChanged(prefs: SharedPreferences, key: String) {
    vlog(TAG, "onSharedPreferenceChanged(key: $key)")
    changeSubjects.getOrPut(key) { BehaviorSubject.create() }.onNext(prefs.all[key]!!)
  }


  private val closed = AtomicBoolean(false)
  private val changeSubjects = mutableMapOf<String, BehaviorSubject<Any>>()
  private val sharedPrefs = context.getSharedPreferences(FILE_NAME, Context.MODE_PRIVATE).apply {
    registerOnSharedPreferenceChangeListener(this@SharedAppPreferences)
  }


  companion object {
    private const val TAG = "SharedAppPreferences"
    private const val FILE_NAME = "mypreferencesfile"
  }

}