src/objective-c/RxLibrary/GRXBufferedPipe.m

/*
 *
 * Copyright 2015, Google Inc.
 * All rights reserved.
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 * 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.
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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
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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#import "GRXBufferedPipe.h"

@implementation GRXBufferedPipe {
  id<GRXWriteable> _writeable;
  NSMutableArray *_queue;
  BOOL _inputIsFinished;
  NSError *_errorOrNil;
  dispatch_queue_t _writeQueue;
}

@synthesize state = _state;

+ (instancetype)pipe {
  return [[self alloc] init];
}

- (instancetype)init {
  if (self = [super init]) {
    _queue = [NSMutableArray array];
    _state = GRXWriterStateNotStarted;
    _writeQueue = dispatch_queue_create(NULL, DISPATCH_QUEUE_SERIAL);
  }
  return self;
}

- (id)popValue {
  id value = _queue[0];
  [_queue removeObjectAtIndex:0];
  return value;
}

- (void)writeBufferUntilPausedOrStopped {
  dispatch_async(_writeQueue, ^(void) {
    while (_queue.count > 0) {
      BOOL started;
      @synchronized (self) {
        started = (_state == GRXWriterStateStarted);
      }
      if (started) {
        [_writeable writeValue:[self popValue]];
      } else {
        break;
      }
    }
    if (_inputIsFinished && _queue.count == 0) {
      // Our writer finished normally while we were paused or not-started-yet.
      [self finishWithError:_errorOrNil];
    }
  });
}

#pragma mark GRXWriteable implementation

// Returns whether events can be simply propagated to the other end of the pipe.
- (BOOL)shouldFastForward {
  BOOL started;
  @synchronized (self) {
    started = (_state == GRXWriterStateStarted);
  }
  return _state == started && _queue.count == 0;
}

- (void)writeValue:(id)value {
  if ([value respondsToSelector:@selector(copy)]) {
    value = [value copy];
  }
  dispatch_async(_writeQueue, ^(void) {
    if (self.shouldFastForward) {
      // Skip the queue.
      [_writeable writeValue:value];
    } else {
      // Even if we're paused and with enqueued values, we can't excert back-pressure to our writer.
      // So just buffer the new value.
      // We need a copy, so that it doesn't mutate before it's written at the other end of the pipe.
      [_queue addObject:value];
    }
  });
}

- (void)writesFinishedWithError:(NSError *)errorOrNil {
  _inputIsFinished = YES;
  _errorOrNil = errorOrNil;
  if (errorOrNil || self.shouldFastForward) {
    // No need to write pending values.
    [self finishWithError:_errorOrNil];
  }
}

#pragma mark GRXWriter implementation

- (void)setState:(GRXWriterState)newState {
  // Manual transitions are only allowed from the started or paused states.
  if (_state == GRXWriterStateNotStarted || _state == GRXWriterStateFinished) {
    return;
  }

  switch (newState) {
    case GRXWriterStateFinished:
      _state = newState;
      _queue = nil;
      // Per GRXWriter's contract, setting the state to Finished manually means one doesn't wish the
      // writeable to be messaged anymore.
      _writeable = nil;
      return;
    case GRXWriterStatePaused:
      _state = newState;
      return;
    case GRXWriterStateStarted:
      if (_state == GRXWriterStatePaused) {
        _state = newState;
        [self writeBufferUntilPausedOrStopped];
      }
      return;
    case GRXWriterStateNotStarted:
      return;
  }
}

- (void)startWithWriteable:(id<GRXWriteable>)writeable {
  _state = GRXWriterStateStarted;
  _writeable = writeable;
  [self writeBufferUntilPausedOrStopped];
}

- (void)finishWithError:(NSError *)errorOrNil {
  id<GRXWriteable> writeable = _writeable;
  self.state = GRXWriterStateFinished;
  [writeable writesFinishedWithError:errorOrNil];
}

@end