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diff --git a/src/objective-c/RxLibrary/GRXWriter.h b/src/objective-c/RxLibrary/GRXWriter.h
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+/*
+ *
+ * Copyright 2014, 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