Concatenate LiveData

LiveDataPromise.kt

fun <T, R> LiveData<T>.then(callback: (T) -> LiveData<R>): LiveData<R> {
  return Transformations.switchMap(this, {
    result(it)
  })
}

UsageExample.kt

/**
 * @param owner is needed to observe the final response and call the view
 */
fun promise(owner: LifecycleOwner) {
    func1().then {
      Log.d("PROMISE", "first $it")
      func2()
    }.then {
      Log.d("PROMISE", "second $it")
      func1()
    }.observe(owner, Observer {
      Log.d("PROMISE", "observe $it")
    })
  }

  /**
   * Example functions representing networking requests or disk read threads
   */
  fun func1(): LiveData<String> {
    val result = MutableLiveData<String>()
    Handler().postDelayed({
      result.postValue("foo")
    }, 2000)
    return result
  }

  fun func2(): LiveData<String> {
    val result = MutableLiveData<String>()
    Handler().postDelayed({
      result.postValue("bar")
    }, 2000)
    return result
  }