HistoryTransformer.java

import android.support.v4.util.Pair;
import rx.Observable;

public class HistoryTransformer<T> implements Observable.Transformer<T, Pair<T,T>> {
    
    @Override
    public Observable<Pair<T,T>> call(Observable<T> observable) {
        //noinspection unchecked
        return observable
                .map(value -> {
                    final Object[] array = new Object[2];
                    array[0] = value;
                    return array;
                })
                .scan((oldArray, newArray) -> {
                    if (oldArray != null) { newArray[1] = oldArray[0]; }
                    return newArray;
                })
                .map(array -> new Pair<>((T)array[0], (T)array[1]));
    }
    
}