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diff --git a/src/objective-c/RxLibrary/GRXConcurrentWriteable.m b/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
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+++ b/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
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+/*
+ *
+ * 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 "GRXConcurrentWriteable.h"
+
+#import <RxLibrary/GRXWriteable.h>
+
+@interface GRXConcurrentWriteable ()
+// This is atomic so that cancellation can nillify it from any thread.
+@property(atomic, strong) id<GRXWriteable> writeable;
+@end
+
+@implementation GRXConcurrentWriteable {
+  dispatch_queue_t _writeableQueue;
+  // This ensures that writesFinishedWithError: is only sent once to the writeable.
+  dispatch_once_t _alreadyFinished;
+}
+
+- (instancetype)init {
+  return [self initWithWriteable:nil];
+}
+
+// Designated initializer
+- (instancetype)initWithWriteable:(id<GRXWriteable>)writeable {
+  if (self = [super init]) {
+    _writeableQueue = dispatch_get_main_queue();
+    _writeable = writeable;
+  }
+  return self;
+}
+
+- (void)enqueueValue:(id)value completionHandler:(void (^)())handler {
+  dispatch_async(_writeableQueue, ^{
+    // We're racing a possible cancellation performed by another thread. To turn all already-
+    // enqueued messages into noops, cancellation nillifies the writeable property. If we get it
+    // before it's nil, we won the race.
+    id<GRXWriteable> writeable = self.writeable;
+    if (writeable) {
+      [writeable writeValue:value];
+      handler();
+    }
+  });
+}
+
+- (void)enqueueSuccessfulCompletion {
+  dispatch_async(_writeableQueue, ^{
+    dispatch_once(&_alreadyFinished, ^{
+      // Cancellation is now impossible. None of the other three blocks can run concurrently with
+      // this one.
+      [self.writeable writesFinishedWithError:nil];
+      // Skip any possible message to the wrapped writeable enqueued after this one.
+      self.writeable = nil;
+    });
+  });
+}
+
+- (void)cancelWithError:(NSError *)error {
+  NSAssert(error, @"For a successful completion, use enqueueSuccessfulCompletion.");
+  dispatch_once(&_alreadyFinished, ^{
+    // Skip any of the still-enqueued messages to the wrapped writeable. We use the atomic setter to
+    // nillify writeable because we might be running concurrently with the blocks in
+    // _writeableQueue, and assignment with ARC isn't atomic.
+    id<GRXWriteable> writeable = self.writeable;
+    self.writeable = nil;
+
+    dispatch_async(_writeableQueue, ^{
+      [writeable writesFinishedWithError:error];
+    });
+  });
+}
+
+- (void)cancelSilently {
+  dispatch_once(&_alreadyFinished, ^{
+    // Skip any of the still-enqueued messages to the wrapped writeable. We use the atomic setter to
+    // nillify writeable because we might be running concurrently with the blocks in
+    // _writeableQueue, and assignment with ARC isn't atomic.
+    self.writeable = nil;
+  });
+}
+@end