code: public static <T1, T2> Func2<T1, T2, Observable<Void>> toAsync(Action2<? super T1, ? super T2> action) {
          return toAsync(action, Schedulers.computation());
      }
docs: Convert a synchronous action call into an asynchronous function call through an Observable.
  <p>
  <img width="640" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/toAsync.an.png" alt="">
  
  @param <T1> the first parameter type
  @param <T2> the second parameter type
  @param action the action to convert
  @return a function that returns an Observable that executes the {@code action} and emits {@code null}
  @see <a href="https://github.com/ReactiveX/RxJava/wiki/Async-Operators#wiki-toasync-or-asyncaction-or-asyncfunc">RxJava Wiki: toAsync()</a>
  @see <a href="http://msdn.microsoft.com/en-us/library/hh211875.aspx">MSDN: Observable.ToAsync</a>