Creates LiveData objects that observe a value in SharedPreferences while they have active listeners.

example.kt

// 1. Get a reference to SharedPreferences however you normally would.
val prefs: SharedPreferences

// 2. Use the extension functions to create a LiveData object of whatever type you need and observe the result.
prefs.booleanLiveData("analytics_enabled", false).observe(this, { enabled ->
    if (enabled != null && enabled) {
      sendAnalytics()
    }
})

SharedPreferenceLiveData.kt

import android.arch.lifecycle.LiveData
import android.content.SharedPreferences

abstract class SharedPreferenceLiveData<T>(val sharedPrefs: SharedPreferences,
                                  val key: String,
                                  val defValue: T) : LiveData<T>() {

  private val preferenceChangeListener = SharedPreferences.OnSharedPreferenceChangeListener { sharedPreferences, key ->
    if (key == this.key) {
      value = getValueFromPreferences(key, defValue)
    }
  }

  abstract fun getValueFromPreferences(key: String, defValue: T): T

  override fun onActive() {
    super.onActive()
    value = getValueFromPreferences(key, defValue)
    sharedPrefs.registerOnSharedPreferenceChangeListener(preferenceChangeListener)
  }

  override fun onInactive() {
    sharedPrefs.unregisterOnSharedPreferenceChangeListener(preferenceChangeListener)
    super.onInactive()
  }
}

class SharedPreferenceIntLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Int) :
    SharedPreferenceLiveData<Int>(sharedPrefs, key, defValue) {
  override fun getValueFromPreferences(key: String, defValue: Int): Int = sharedPrefs.getInt(key, defValue)
}

class SharedPreferenceStringLiveData(sharedPrefs: SharedPreferences, key: String, defValue: String) :
    SharedPreferenceLiveData<String>(sharedPrefs, key, defValue) {
  override fun getValueFromPreferences(key: String, defValue: String): String = sharedPrefs.getString(key, defValue)
}

class SharedPreferenceBooleanLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Boolean) :
    SharedPreferenceLiveData<Boolean>(sharedPrefs, key, defValue) {
  override fun getValueFromPreferences(key: String, defValue: Boolean): Boolean = sharedPrefs.getBoolean(key, defValue)
}

class SharedPreferenceFloatLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Float) :
    SharedPreferenceLiveData<Float>(sharedPrefs, key, defValue) {
  override fun getValueFromPreferences(key: String, defValue: Float): Float = sharedPrefs.getFloat(key, defValue)
}

class SharedPreferenceLongLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Long) :
    SharedPreferenceLiveData<Long>(sharedPrefs, key, defValue) {
  override fun getValueFromPreferences(key: String, defValue: Long): Long = sharedPrefs.getLong(key, defValue)
}

class SharedPreferenceStringSetLiveData(sharedPrefs: SharedPreferences, key: String, defValue: Set<String>) :
    SharedPreferenceLiveData<Set<String>>(sharedPrefs, key, defValue) {
  override fun getValueFromPreferences(key: String, defValue: Set<String>): Set<String> = sharedPrefs.getStringSet(key, defValue)
}

fun SharedPreferences.intLiveData(key: String, defValue: Int): SharedPreferenceLiveData<Int> {
  return SharedPreferenceIntLiveData(this, key, defValue)
}
fun SharedPreferences.stringLiveData(key: String, defValue: String): SharedPreferenceLiveData<String> {
  return SharedPreferenceStringLiveData(this, key, defValue)
}
fun SharedPreferences.booleanLiveData(key: String, defValue: Boolean): SharedPreferenceLiveData<Boolean> {
  return SharedPreferenceBooleanLiveData(this, key, defValue)
}
fun SharedPreferences.floatLiveData(key: String, defValue: Float): SharedPreferenceLiveData<Float> {
  return SharedPreferenceFloatLiveData(this, key, defValue)
}
fun SharedPreferences.longLiveData(key: String, defValue: Long): SharedPreferenceLiveData<Long> {
  return SharedPreferenceLongLiveData(this, key, defValue)
}
fun SharedPreferences.stringSetLiveData(key: String, defValue: Set<String>): SharedPreferenceLiveData<Set<String>> {
  return SharedPreferenceStringSetLiveData(this, key, defValue)
}

Yes, with my code a default value of null is ambiguous and prefs.liveData(key, null) won't compile. So you have to write null as String? or null as Set<String>? to let the compiler know what you want. That's the downside of overloading a single method name (and why I expect most people would prefer your original naming convention). 😞