src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> distinctUntilChanged(BiPredicate<? super T, ? super T> comparer) {
        ObjectHelper.requireNonNull(comparer, "comparer is null");
        return RxJavaPlugins.onAssembly(new ObservableDistinctUntilChanged<T, T>(this, Functions.<T>identity(), comparer));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> distinctUntilChanged(BiPredicate<? super T, ? super T> comparer) {
        ObjectHelper.requireNonNull(comparer, "comparer is null");
        return RxJavaPlugins.onAssembly(new ObservableDistinctUntilChanged<T, T>(this, Functions.<T>identity(), comparer));
    }
Returns an Observable that emits all items emitted by the source ObservableSource that are distinct from their
immediate predecessors when compared with each other via the provided comparator function.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/distinctUntilChanged.png" alt="">
<p>
Note that the operator always retains the latest item from upstream regardless of the comparison result
and uses it in the next comparison with the next upstream item.
<p>
Note that if element type {@code T} in the flow is mutable, the comparison of the previous and current
item may yield unexpected results if the items are mutated externally. Common cases are mutable
{@code CharSequence}s or {@code List}s where the objects will actually have the same
references when they are modified and {@code distinctUntilChanged} will evaluate subsequent items as same.
To avoid such situation, it is recommended that mutable data is converted to an immutable one,
for example using {@code map(CharSequence::toString)} or {@code map(list -> Collections.unmodifiableList(new ArrayList<>(list)))}.
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code distinctUntilChanged} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param comparer the function that receives the previous item and the current item and is
expected to return true if the two are equal, thus skipping the current value.
@return an Observable that emits those items from the source ObservableSource that are distinct from their
immediate predecessors
@see <a href="http://reactivex.io/documentation/operators/distinct.html">ReactiveX operators documentation: Distinct</a>
@since 2.0