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 *
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 *
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 * notice, this list of conditions and the following disclaimer.
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 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
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 * this software without specific prior written permission.
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 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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#import <Foundation/Foundation.h>

#import "GRXWriteable.h"

// States of a writer.
typedef NS_ENUM(NSInteger, GRXWriterState) {

  // The writer has not yet been given a writeable to which it can push its values. To have a writer
  // transition to the Started state, send it a startWithWriteable: message.
  //
  // A 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 writesFinishedWithError: to its writeable.
  GRXWriterStatePaused,

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

// An GRXWriter object 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, and resumed later. All GRXWriter objects support pausing and early termination.
//
// Thread-safety:
//
// State transitions take immediate effect if the object is used from a single thread. Subclasses
// might offer stronger guarantees.
//
// Unless otherwise indicated by a conforming subclass, no messages should be sent concurrently to a
// GRXWriter. I.e., conforming classes aren't required to be thread-safe.
@interface 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 further down.
@property(nonatomic) GRXWriterState state;

// Transition to the Started state, and start sending messages to the writeable (a reference to it
// is retained). Messages to the writeable 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 writesFinishedWithError:errorOrNil to the writeable. Then release the reference to it and
// transition to the Finished state.
//
// This method might only be called on writers in the Started or Paused state.
- (void)finishWithError:(NSError *)errorOrNil;
@end