2 files changed, 88 insertions, 6 deletions

diff --git a/src/objective-c/RxLibrary/GRXWriteable.m b/src/objective-c/RxLibrary/GRXWriteable.m
index 2729d62b72..028ba9b551 100644
--- a/src/objective-c/RxLibrary/GRXWriteable.m
+++ b/src/objective-c/RxLibrary/GRXWriteable.m
@@ -1,6 +1,6 @@
 /*
  *
- * Copyright 2015, Google Inc.
+ * Copyright 2015-2016, Google Inc.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -42,11 +42,42 @@
   if (!handler) {
     return [[self alloc] init];
   }
-  return [[self alloc] initWithValueHandler:^(id value) {
-    handler(value, nil);
-  } completionHandler:^(NSError *errorOrNil) {
-    if (errorOrNil) {
-      handler(nil, errorOrNil);
+  // We nilify this variable when the block is invoked, so that handler is only invoked once even if
+  // the writer tries to write multiple values.
+  __block GRXEventHandler eventHandler = ^(BOOL done, id value, NSError *error) {
+    // Nillify eventHandler before invoking handler, in case the latter causes the former to be
+    // executed recursively. Because blocks can be deallocated even during execution, we have to
+    // first retain handler locally to guarantee it's valid.
+    // TODO(jcanizales): Just turn this craziness into a simple subclass of GRXWriteable.
+    GRXSingleHandler singleHandler = handler;
+    eventHandler = nil;
+
+    if (value) {
+      singleHandler(value, nil);
+    } else if (error) {
+      singleHandler(nil, error);
+    } else {
+      NSDictionary *userInfo = @{
+        NSLocalizedDescriptionKey: @"The writer finished without producing any value."
+      };
+      // Even though RxLibrary is independent of gRPC, the domain and code here are, for the moment,
+      // set to the values of kGRPCErrorDomain and GRPCErrorCodeInternal. This way, the error formed
+      // is the one user of gRPC would expect if the server failed to produce a response.
+      //
+      // TODO(jcanizales): Figure out a way to keep errors of RxLibrary generic without making users
+      // of gRPC take care of two different error domains and error code enums. A possibility is to
+      // add error handling to GRXWriters or GRXWriteables, and use them to translate errors between
+      // the two domains.
+      static NSString *kGRPCErrorDomain = @"io.grpc";
+      static NSUInteger kGRPCErrorCodeInternal = 13;
+      singleHandler(nil, [NSError errorWithDomain:kGRPCErrorDomain
+                                             code:kGRPCErrorCodeInternal
+                                         userInfo:userInfo]);
+    }
+  };
+  return [self writeableWithEventHandler:^(BOOL done, id value, NSError *error) {
+    if (eventHandler) {
+      eventHandler(done, value, error);
     }
   }];
 }
diff --git a/src/objective-c/tests/RxLibraryUnitTests.m b/src/objective-c/tests/RxLibraryUnitTests.m
index d342662814..ae9465f58c 100644
--- a/src/objective-c/tests/RxLibraryUnitTests.m
+++ b/src/objective-c/tests/RxLibraryUnitTests.m
@@ -64,6 +64,8 @@
 }
 @end
 
+// TODO(jcanizales): Split into one file per tested class.
+
 @interface RxLibraryUnitTests : XCTestCase
 @end
 
@@ -79,6 +81,7 @@
   // If:
   id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
   [writeable writeValue:anyValue];
+  [writeable writesFinishedWithError:nil];
 
   // Then:
   XCTAssertEqual(handler.timesCalled, 1);
@@ -101,6 +104,54 @@
   XCTAssertEqualObjects(handler.errorOrNil, anyError);
 }
 
+- (void)testWriteableSingleHandlerIsCalledOnlyOnce_ValueThenError {
+  // Given:
+  CapturingSingleValueHandler *handler = [CapturingSingleValueHandler handler];
+  id anyValue = @7;
+  NSError *anyError = [NSError errorWithDomain:@"domain" code:7 userInfo:nil];
+
+  // If:
+  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
+  [writeable writeValue:anyValue];
+  [writeable writesFinishedWithError:anyError];
+
+  // Then:
+  XCTAssertEqual(handler.timesCalled, 1);
+  XCTAssertEqualObjects(handler.value, anyValue);
+  XCTAssertEqualObjects(handler.errorOrNil, nil);
+}
+
+- (void)testWriteableSingleHandlerIsCalledOnlyOnce_ValueThenValue {
+  // Given:
+  CapturingSingleValueHandler *handler = [CapturingSingleValueHandler handler];
+  id anyValue = @7;
+
+  // If:
+  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
+  [writeable writeValue:anyValue];
+  [writeable writeValue:anyValue];
+  [writeable writesFinishedWithError:nil];
+
+  // Then:
+  XCTAssertEqual(handler.timesCalled, 1);
+  XCTAssertEqualObjects(handler.value, anyValue);
+  XCTAssertEqualObjects(handler.errorOrNil, nil);
+}
+
+- (void)testWriteableSingleHandlerFailsOnEmptyWriter {
+  // Given:
+  CapturingSingleValueHandler *handler = [CapturingSingleValueHandler handler];
+
+  // If:
+  id<GRXWriteable> writeable = [GRXWriteable writeableWithSingleHandler:handler.block];
+  [writeable writesFinishedWithError:nil];
+
+  // Then:
+  XCTAssertEqual(handler.timesCalled, 1);
+  XCTAssertEqualObjects(handler.value, nil);
+  XCTAssertNotNil(handler.errorOrNil);
+}
+
 #pragma mark BufferedPipe
 
 - (void)testBufferedPipePropagatesValue {