src/main/java/io/reactivex/Observable.java
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<Boolean> all(Predicate<? super T> predicate) {
        ObjectHelper.requireNonNull(predicate, "predicate is null");
        return RxJavaPlugins.onAssembly(new ObservableAllSingle<T>(this, predicate));
    }
@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Single<Boolean> all(Predicate<? super T> predicate) {
        ObjectHelper.requireNonNull(predicate, "predicate is null");
        return RxJavaPlugins.onAssembly(new ObservableAllSingle<T>(this, predicate));
    }
Returns a Single that emits a Boolean that indicates whether all of the items emitted by the source
ObservableSource satisfy a condition.
<p>
<img width="640" height="264" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/all.o.png" alt="">
<dl>
<dt><b>Scheduler:</b></dt>
<dd>{@code all} does not operate by default on a particular {@link Scheduler}.</dd>
</dl>

@param predicate
a function that evaluates an item and returns a Boolean
@return a Single that emits {@code true} if all items emitted by the source ObservableSource satisfy the
predicate; otherwise, {@code false}
@see <a href="http://reactivex.io/documentation/operators/all.html">ReactiveX operators documentation: All</a>