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/**
* Copyright (c) 2016-present, RxJava Contributors.
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
* compliance with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License is
* distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
* the License for the specific language governing permissions and limitations under the License.
*/
package io.reactivex;
import io.reactivex.annotations.*;
import io.reactivex.disposables.Disposable;
/**
* Provides a mechanism for receiving push-based notification of a single value or an error.
*
* When a {@code SingleObserver} is subscribed to a {@link SingleSource} through the {@link SingleSource#subscribe(SingleObserver)} method,
* the {@code SingleSource} calls {@link #onSubscribe(Disposable)} with a {@link Disposable} that allows
* disposing the sequence at any time. A well-behaved
* {@code SingleSource} will call a {@code SingleObserver}'s {@link #onSuccess(Object)} method exactly once or the {@code SingleObserver}'s
* {@link #onError} method exactly once as they are considered mutually exclusive terminal signals.
*
* Calling the {@code SingleObserver}'s method must happen in a serialized fashion, that is, they must not
* be invoked concurrently by multiple threads in an overlapping fashion and the invocation pattern must
* adhere to the following protocol:
*
onSubscribe (onSuccess | onError)?
*
* Subscribing a {@code SingleObserver} to multiple {@code SingleSource}s is not recommended. If such reuse
* happens, it is the duty of the {@code SingleObserver} implementation to be ready to receive multiple calls to
* its methods and ensure proper concurrent behavior of its business logic.
*
* Calling {@link #onSubscribe(Disposable)}, {@link #onSuccess(Object)} or {@link #onError(Throwable)} with a
* {@code null} argument is forbidden.
*
* The implementations of the {@code onXXX} methods should avoid throwing runtime exceptions other than the following cases:
*
* - If the argument is {@code null}, the methods can throw a {@code NullPointerException}.
* Note though that RxJava prevents {@code null}s to enter into the flow and thus there is generally no
* need to check for nulls in flows assembled from standard sources and intermediate operators.
*
* - If there is a fatal error (such as {@code VirtualMachineError}).
*
* @see ReactiveX documentation: Observable
* @param
* the type of item the SingleObserver expects to observe
* @since 2.0
*/
public interface SingleObserver {
/**
* Provides the SingleObserver with the means of cancelling (disposing) the
* connection (channel) with the Single in both
* synchronous (from within {@code onSubscribe(Disposable)} itself) and asynchronous manner.
* @param d the Disposable instance whose {@link Disposable#dispose()} can
* be called anytime to cancel the connection
* @since 2.0
*/
void onSubscribe(@NonNull Disposable d);
/**
* Notifies the SingleObserver with a single item and that the {@link Single} has finished sending
* push-based notifications.
*
* The {@link Single} will not call this method if it calls {@link #onError}.
*
* @param t
* the item emitted by the Single
*/
void onSuccess(@NonNull T t);
/**
* Notifies the SingleObserver that the {@link Single} has experienced an error condition.
*
* If the {@link Single} calls this method, it will not thereafter call {@link #onSuccess}.
*
* @param e
* the exception encountered by the Single
*/
void onError(@NonNull Throwable e);
}