text: //
  // Copyright 2019 Google LLC
  //
  // 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.
  //
  
  #import <Foundation/Foundation.h>
  
  NS_ASSUME_NONNULL_BEGIN
  
  /**
   *  The simple double-ended queue that provides synchronized append and remove operations.
   *
   *  Elements can be appended to the blocking queue and retrieved from either end. The retrieval can
   *  be synchronized in the way that it waits until there is an element. This is primarily for
   *  producer-consumer use cases.
   *
   *  After the queue is closed, no more objects can be appended but one can still fetch objects
   *  until it is empty, in such case, the timeout will not affect and return @c nil immediately.
   */
  @interface EDOBlockingQueue<ObjectType> : NSObject
  
  /** Whether the queue has any objects. */
  @property(readonly, getter=isEmpty) BOOL empty;
  
  /** The number of objects in the queue. */
  @property(readonly) NSUInteger count;
  
  /** Appends the @c object to the end of the queue.
   *
   *  @return YES if the object is appended; NO, if the queue is closed already and the message
   *          will not be enqueued.
   */
  - (BOOL)appendObject:(ObjectType)object;
  
  /**
   *  Fetches an object from the head of the queue.
   *
   *  @note This will block the current thread until an object has been added to the queue.
   *
   *  @param timeout The timeout to wait until the element is available.
   *  @return The object in the queue, or @c nil if timing out.
   */
  - (nullable ObjectType)firstObjectWithTimeout:(dispatch_time_t)timeout;
  
  /**
   *  Fetches an object from the tail of the queue.
   *
   *  @note This will block the current thread until an object has been added to the queue.
   *
   *  @param timeout The timeout to wait until the element is available.
   *  @return The object in the queue, or @c nil if timing out.
   */
  - (nullable ObjectType)lastObjectWithTimeout:(dispatch_time_t)timeout;
  
  /**
   *  Closes the queue so no more messages can be appended.
   *
   *  @note The closed queue can still fetch objects but will not wait if the queue is empty.
   *  @return YES if the queue is just closed; NO if the queue is already closed.
   */
  - (BOOL)close;
  
  @end
  
  NS_ASSUME_NONNULL_END
  