src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> doOnEach(final Consumer<? super Notification<T>> onNotification) {
        ObjectHelper.requireNonNull(onNotification, "onNotification is null");
        return doOnEach(
                Functions.notificationOnNext(onNotification),
                Functions.notificationOnError(onNotification),
                Functions.notificationOnComplete(onNotification),
                Functions.EMPTY_ACTION
            );
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Observable<T> doOnEach(final Consumer<? super Notification<T>> onNotification) {
        ObjectHelper.requireNonNull(onNotification, "onNotification is null");
        return doOnEach(
                Functions.notificationOnNext(onNotification),
                Functions.notificationOnError(onNotification),
                Functions.notificationOnComplete(onNotification),
                Functions.EMPTY_ACTION
            );
    }
Modifies the source ObservableSource so that it invokes an action for each item it emits.
<p>
<img width="640" height="310" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/doOnEach.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code doOnEach} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param onNotification
the action to invoke for each item emitted by the source ObservableSource
@return the source ObservableSource with the side-effecting behavior applied
@see <a href="http://reactivex.io/documentation/operators/do.html">ReactiveX operators documentation: Do</a>