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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.rxjava3.core;
import io.reactivex.rxjava3.annotations.NonNull;
import io.reactivex.rxjava3.disposables.Disposable;
/**
* Provides a mechanism for receiving push-based notifications.
*
* When an {@code Observer} is subscribed to an {@link ObservableSource} through the {@link ObservableSource#subscribe(Observer)} method,
* the {@code ObservableSource} calls {@link #onSubscribe(Disposable)} with a {@link Disposable} that allows
* disposing the sequence at any time, then the
* {@code ObservableSource} may call the Observer's {@link #onNext} method any number of times
* to provide notifications. A well-behaved
* {@code ObservableSource} will call an {@code Observer}'s {@link #onComplete} method exactly once or the {@code Observer}'s
* {@link #onError} method exactly once.
*
* Calling the {@code Observer}'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 onNext* (onError | onComplete)?
*
* Subscribing an {@code Observer} to multiple {@code ObservableSource}s is not recommended. If such reuse
* happens, it is the duty of the {@code Observer} 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 #onNext(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
* (see Rule 2.13 of the Reactive Streams specification):
*
* - 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}).
*
*
* Violating Rule 2.13 results in undefined flow behavior. Generally, the following can happen:
*
* - An upstream operator turns it into an {@link #onError} call.
* - If the flow is synchronous, the {@link ObservableSource#subscribe(Observer)} throws instead of returning normally.
* - If the flow is asynchronous, the exception propagates up to the component ({@link Scheduler} or {@link java.util.concurrent.Executor})
* providing the asynchronous boundary the code is running and either routes the exception to the global
* {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)} handler or the current thread's
* {@link java.lang.Thread.UncaughtExceptionHandler#uncaughtException(Thread, Throwable)} handler.
*
* From the {@code Observable}'s perspective, an {@code Observer} is the end consumer thus it is the {@code Observer}'s
* responsibility to handle the error case and signal it "further down". This means unreliable code in the {@code onXXX}
* methods should be wrapped into `try-catch`es, specifically in {@link #onError(Throwable)} or {@link #onComplete()}, and handled there
* (for example, by logging it or presenting the user with an error dialog). However, if the error would be thrown from
* {@link #onNext(Object)}, Rule 2.13 mandates
* the implementation calls {@link Disposable#dispose()} and signals the exception in a way that is adequate to the target context,
* for example, by calling {@link #onError(Throwable)} on the same {@code Observer} instance.
*
* If, for some reason, the {@code Observer} won't follow Rule 2.13, the {@link Observable#safeSubscribe(Observer)} can wrap it
* with the necessary safeguards and route exceptions thrown from {@code onNext} into {@code onError} and route exceptions thrown
* from {@code onError} and {@code onComplete} into the global error handler via {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)}.
* @see ReactiveX documentation: Observable
* @param
* the type of item the Observer expects to observe
*/
public interface Observer<@NonNull T> {
/**
* Provides the {@link Observer} with the means of cancelling (disposing) the
* connection (channel) with the {@link Observable} in both
* synchronous (from within {@link #onNext(Object)}) and asynchronous manner.
* @param d the {@link Disposable} instance whose {@link Disposable#dispose()} can
* be called anytime to cancel the connection
* @since 2.0
*/
void onSubscribe(@NonNull Disposable d);
/**
* Provides the {@link Observer} with a new item to observe.
*
* The {@link Observable} may call this method 0 or more times.
*
* The {@code Observable} will not call this method again after it calls either {@link #onComplete} or
* {@link #onError}.
*
* @param t
* the item emitted by the Observable
*/
void onNext(@NonNull T t);
/**
* Notifies the {@link Observer} that the {@link Observable} has experienced an error condition.
*
* If the {@code Observable} calls this method, it will not thereafter call {@link #onNext} or
* {@link #onComplete}.
*
* @param e
* the exception encountered by the Observable
*/
void onError(@NonNull Throwable e);
/**
* Notifies the {@link Observer} that the {@link Observable} has finished sending push-based notifications.
*
* The {@code Observable} will not call this method if it calls {@link #onError}.
*/
void onComplete();
}