Add basic KVO test.

Add a simple KVO test just to verify that KVO continues to function for protobufs.

objectivec/Tests/GPBMessageTests.m

@@ -41,6 +41,52 @@
 #import "google/protobuf/Unittest.pbobjc.h"
 #import "google/protobuf/UnittestObjc.pbobjc.h"
 #import "google/protobuf/UnittestObjcOptions.pbobjc.h"
+#import "google/protobuf/UnittestImport.pbobjc.h"
+
+// Helper class to test KVO.
+@interface GPBKVOTestObserver : NSObject {
+  id observee_;
+  NSString *keyPath_;
+}
+
+@property (nonatomic) BOOL didObserve;
+- (id)initWithObservee:(id)observee keyPath:(NSString *)keyPath;
+@end
+
+@implementation GPBKVOTestObserver
+
+@synthesize didObserve;
+
+- (id)initWithObservee:(id)observee keyPath:(NSString *)keyPath {
+  if (self = [super init]) {
+    observee_ = [observee retain];
+    keyPath_ = [keyPath copy];
+    [observee_ addObserver:self forKeyPath:keyPath_ options:0 context:NULL];
+  }
+  return self;
+}
+
+- (void)dealloc {
+  [observee_ removeObserver:self forKeyPath:keyPath_];
+  [observee_ release];
+  [keyPath_ release];
+  [super dealloc];
+}
+
+- (void)observeValueForKeyPath:(NSString *)keyPath
+                      ofObject:(id)object
+                        change:(NSDictionary *)change
+                       context:(void *)context
+{
+#pragma unused(object)
+#pragma unused(change)
+#pragma unused(context)
+  if ([keyPath isEqualToString:keyPath_]) {
+    self.didObserve = YES;
+  }
+}
+
+@end
 
 @interface MessageTests : GPBTestCase
 @end
@@ -431,6 +477,55 @@ - (void)testKVC_SetValue_ForKey {
   [self assertAllFieldsKVCMatch:message];
 }
 
+- (void)testKVOBasic {
+  TestAllTypes *message = [TestAllTypes message];
+  GPBKVOTestObserver *observer =
+      [[[GPBKVOTestObserver alloc] initWithObservee:message
+                                            keyPath:@"optionalString"]
+       autorelease];
+  XCTAssertFalse(observer.didObserve);
+  message.defaultString = @"Hello";
+  XCTAssertFalse(observer.didObserve);
+  message.optionalString = @"Hello";
+  XCTAssertTrue(observer.didObserve);
+}
+
+- (void)testKVOAutocreate {
+  TestAllTypes *message = [TestAllTypes message];
+  GPBKVOTestObserver *autocreateObserver =
+      [[[GPBKVOTestObserver alloc] initWithObservee:message
+                                            keyPath:@"optionalImportMessage"]
+       autorelease];
+ GPBKVOTestObserver *innerFieldObserver =
+     [[[GPBKVOTestObserver alloc] initWithObservee:message
+                                           keyPath:@"optionalImportMessage.d"]
+      autorelease];
+  XCTAssertFalse(autocreateObserver.didObserve);
+  XCTAssertFalse(innerFieldObserver.didObserve);
+
+  int a = message.optionalImportMessage.d;
+  XCTAssertEqual(a, 0);
+
+  // Autocreation of fields is not observed by KVO when getting values.
+  XCTAssertFalse(autocreateObserver.didObserve);
+  XCTAssertFalse(innerFieldObserver.didObserve);
+
+  message.optionalImportMessage.d = 2;
+
+  // Autocreation of fields is not observed by KVO.
+  // This is undefined behavior. The library makes no guarantees with regards
+  // to KVO firing if an autocreation occurs as part of a setter.
+  // This test exists just to be aware if the behavior changes.
+  XCTAssertFalse(autocreateObserver.didObserve);
+
+  // Values set inside of an autocreated field are observed.
+  XCTAssertTrue(innerFieldObserver.didObserve);
+
+  // Explicit setting of a message field is observed.
+  message.optionalImportMessage = [ImportMessage message];
+  XCTAssertTrue(autocreateObserver.didObserve);
+}
+
 - (void)testDescription {
   // No real test, just exercise code
   TestAllTypes *message = [TestAllTypes message];