src/objective-c/RxLibrary/GRXWriter.h

/*
 *
 * Copyright 2015, Google Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

#import <Foundation/Foundation.h>

#import "GRXWriteable.h"

typedef NS_ENUM(NSInteger, GRXWriterState) {

  // The writer has not yet been given a writeable to which it can push its
  // values. To have an writer transition to the Started state, send it a
  // startWithWriteable: message.
  //
  // An writer's state cannot be manually set to this value.
  GRXWriterStateNotStarted,

  // The writer might push values to the writeable at any moment.
  GRXWriterStateStarted,

  // The writer is temporarily paused, and won't send any more values to the
  // writeable unless its state is set back to Started. The writer might still
  // transition to the Finished state at any moment, and is allowed to send
  // didFinishWithError: to its writeable.
  //
  // Not all implementations of writer have to support pausing, and thus
  // trying to set an writer's state to this value might have no effect.
  GRXWriterStatePaused,

  // The writer has released its writeable and won't interact with it anymore.
  //
  // One seldomly wants to set an writer's state to this value, as its
  // writeable isn't notified with a didFinishWithError: message. Instead, sending
  // finishWithError: to the writer will make it notify the writeable and then
  // transition to this state.
  GRXWriterStateFinished
};

// An object that conforms to this protocol can produce, on demand, a sequence
// of values. The sequence may be produced asynchronously, and it may consist of
// any number of elements, including none or an infinite number.
//
// GRXWriter is the active dual of NSEnumerator. The difference between them
// is thus whether the object plays an active or passive role during usage: A
// user of NSEnumerator pulls values off it, and passes the values to a writeable.
// A user of GRXWriter, though, just gives it a writeable, and the
// GRXWriter instance pushes values to the writeable. This makes this protocol
// suitable to represent a sequence of future values, as well as collections
// with internal iteration.
//
// An instance of GRXWriter can start producing values after a writeable is
// passed to it. It can also be commanded to finish the sequence immediately
// (with an optional error). Finally, it can be asked to pause, but the
// conforming instance is not required to oblige.
//
// Unless otherwise indicated by a conforming class, no messages should be sent
// concurrently to a GRXWriter. I.e., conforming classes aren't required to
// be thread-safe.
@protocol GRXWriter <NSObject>

// This property can be used to query the current state of the writer, which
// determines how it might currently use its writeable. Some state transitions can
// be triggered by setting this property to the corresponding value, and that's
// useful for advanced use cases like pausing an writer. For more details,
// see the documentation of the enum.
@property(nonatomic) GRXWriterState state;

// Start sending messages to the writeable. Messages may be sent before the method
// returns, or they may be sent later in the future. See GRXWriteable.h for the
// different messages a writeable can receive.
//
// If this writer draws its values from an external source (e.g. from the
// filesystem or from a server), calling this method will commonly trigger side
// effects (like network connections).
//
// This method might only be called on writers in the NotStarted state.
- (void)startWithWriteable:(id<GRXWriteable>)writeable;

// Send didFinishWithError:errorOrNil immediately to the writeable, and don't send
// any more messages to it.
//
// This method might only be called on writers in the Started or Paused
// state.
//
// TODO(jcanizales): Consider adding some guarantee about the immediacy of that
// stopping. I know I've relied on it in part of the code that uses this, but
// can't remember the details in the presence of concurrency.
- (void)finishWithError:(NSError *)errorOrNil;
@end

// A "proxy" class that simply forwards values, completion, and errors from its
// input writer to its writeable.
// It is useful as a superclass for pipes that act as a transformation of their
// input writer, and for classes that represent objects with input and
// output sequences of values, like an RPC.
@interface GRXWriter : NSObject<GRXWriter>
- (instancetype)initWithWriter:(id<GRXWriter>)writer NS_DESIGNATED_INITIALIZER;
@end