/*
 * Copyright (c) 2011-2019 Contributors to the Eclipse Foundation
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License 2.0 which is available at
 * http://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
 * which is available at https://www.apache.org/licenses/LICENSE-2.0.
 *
 * SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
 */

package io.vertx.core.streams;

import io.vertx.codegen.annotations.Fluent;
import io.vertx.codegen.annotations.Nullable;
import io.vertx.codegen.annotations.VertxGen;
import io.vertx.core.AsyncResult;
import io.vertx.core.Future;
import io.vertx.core.Handler;
import io.vertx.core.Promise;
import io.vertx.core.streams.impl.PipeImpl;

Represents a stream of items that can be read from.
 Any class that implements this interface can be used by a Pipe to pipe data from it to a WriteStream. 

Streaming mode
The stream is either in flowing or fetch mode.

  Initially the stream is in flowing mode.
  
When the stream is in flowing mode, elements are delivered to the handler.
  
When the stream is in fetch mode, only the number of requested elements will be delivered to the handler.
 The mode can be changed with the pause(), resume() and fetch methods: 

  
Calling resume() sets the flowing mode
  
Calling pause() sets the fetch mode and resets the demand to 0
  
Calling fetch(long) requests a specific amount of elements and adds it to the actual demand

Author:
Tim Fox/**
 * Represents a stream of items that can be read from.
 * <p>
 * Any class that implements this interface can be used by a {@link Pipe} to pipe data from it
 * to a {@link WriteStream}.
 * <p>
 * <h3>Streaming mode</h3>
 * The stream is either in <i>flowing</i> or <i>fetch</i> mode.
 * <ul>
 *   <i>Initially the stream is in <i>flowing</i> mode.</i>
 *   <li>When the stream is in <i>flowing</i> mode, elements are delivered to the {@code handler}.</li>
 *   <li>When the stream is in <i>fetch</i> mode, only the number of requested elements will be delivered to the {@code handler}.</li>
 * </ul>
 * The mode can be changed with the {@link #pause()}, {@link #resume()} and {@link #fetch} methods:
 * <ul>
 *   <li>Calling {@link #resume()} sets the <i>flowing</i> mode</li>
 *   <li>Calling {@link #pause()} sets the <i>fetch</i> mode and resets the demand to {@code 0}</li>
 *   <li>Calling {@link #fetch(long)} requests a specific amount of elements and adds it to the actual demand</li>
 * </ul>
 *
 * @author <a href="http://tfox.org">Tim Fox</a>
 */
@VertxGen(concrete = false)
public interface ReadStream<T> extends StreamBase {

  
Set an exception handler on the read stream.
Params:
handler –  the exception handler
Returns:
a reference to this, so the API can be used fluently/**
   * Set an exception handler on the read stream.
   *
   * @param handler  the exception handler
   * @return a reference to this, so the API can be used fluently
   */
  ReadStream<T> exceptionHandler(Handler<Throwable> handler);

  
Set a data handler. As data is read, the handler will be called with the data.
Returns:
a reference to this, so the API can be used fluently/**
   * Set a data handler. As data is read, the handler will be called with the data.
   *
   * @return a reference to this, so the API can be used fluently
   */
  @Fluent
  ReadStream<T> handler(@Nullable Handler<T> handler);

  
Pause the ReadStream, it sets the buffer in fetch mode and clears the actual demand. 
 While it's paused, no data will be sent to the data handler. 
Returns:
a reference to this, so the API can be used fluently/**
   * Pause the {@code ReadStream}, it sets the buffer in {@code fetch} mode and clears the actual demand.
   * <p>
   * While it's paused, no data will be sent to the data {@code handler}.
   *
   * @return a reference to this, so the API can be used fluently
   */
  @Fluent
  ReadStream<T> pause();

  
Resume reading, and sets the buffer in flowing mode. 
 If the ReadStream has been paused, reading will recommence on it. 
Returns:
a reference to this, so the API can be used fluently/**
   * Resume reading, and sets the buffer in {@code flowing} mode.
   * <p/>
   * If the {@code ReadStream} has been paused, reading will recommence on it.
   *
   * @return a reference to this, so the API can be used fluently
   */
  @Fluent
  ReadStream<T> resume();

  
Fetch the specified amount of elements. If the ReadStream has been paused, reading will recommence with the specified amount of items, otherwise the specified amount will be added to the current stream demand. 
Returns:
a reference to this, so the API can be used fluently/**
   * Fetch the specified {@code amount} of elements. If the {@code ReadStream} has been paused, reading will
   * recommence with the specified {@code amount} of items, otherwise the specified {@code amount} will
   * be added to the current stream demand.
   *
   * @return a reference to this, so the API can be used fluently
   */
  @Fluent
  ReadStream<T> fetch(long amount);

  
Set an end handler. Once the stream has ended, and there is no more data to be read, this handler will be called.
Returns:
a reference to this, so the API can be used fluently/**
   * Set an end handler. Once the stream has ended, and there is no more data to be read, this handler will be called.
   *
   * @return a reference to this, so the API can be used fluently
   */
  @Fluent
  ReadStream<T> endHandler(@Nullable Handler<Void> endHandler);

  
Pause this stream and return a Pipe to transfer the elements of this stream to a destination WriteStream. 
 The stream will be resumed when the pipe will be wired to a WriteStream. 
Returns:
a pipe/**
   * Pause this stream and return a {@link Pipe} to transfer the elements of this stream to a destination {@link WriteStream}.
   * <p/>
   * The stream will be resumed when the pipe will be wired to a {@code WriteStream}.
   *
   * @return a pipe
   */
  default Pipe<T> pipe() {
    pause();
    return new PipeImpl<>(this);
  }

  
Same as pipeTo(WriteStream, Handler) but returns a Future of the asynchronous result /**
   * Same as {@link #pipeTo(WriteStream, Handler)} but returns a {@code Future} of the asynchronous result
   */
  default Future<Void> pipeTo(WriteStream<T> dst) {
    Promise<Void> promise = Promise.promise();
    new PipeImpl<>(this).to(dst, promise);
    return promise.future();
  }

  
Pipe this ReadStream to the WriteStream. 

Elements emitted by this stream will be written to the write stream until this stream ends or fails.
 Once this stream has ended or failed, the write stream will be ended and the handler will be called with the result. 
Params:
dst – the destination write stream/**
   * Pipe this {@code ReadStream} to the {@code WriteStream}.
   * <p>
   * Elements emitted by this stream will be written to the write stream until this stream ends or fails.
   * <p>
   * Once this stream has ended or failed, the write stream will be ended and the {@code handler} will be
   * called with the result.
   *
   * @param dst the destination write stream
   */
  default void pipeTo(WriteStream<T> dst, Handler<AsyncResult<Void>> handler) {
    new PipeImpl<>(this).to(dst, handler);
  }
}