Emit notification when message is read.

We observe this in Signal to remove notifications for messages that were
read remotely.

// FREEBIE

src/Devices/OWSReadReceiptObserver.h

@@ -6,9 +6,9 @@ NS_ASSUME_NONNULL_BEGIN
 
 @interface OWSReadReceiptObserver : NSObject
 
-- (instancetype)initWithMessagesManager:(TSMessagesManager *)messagesManager;
+- (instancetype)initWithMessagesManager:(TSMessagesManager *)messagesManager NS_DESIGNATED_INITIALIZER;
 - (void)startObserving;
 
 @end
 
-NS_ASSUME_NONNULL_END
+NS_ASSUME_NONNULL_END

src/Devices/OWSReadReceiptObserver.m

@@ -19,6 +19,16 @@ NS_ASSUME_NONNULL_BEGIN
 
 @implementation OWSReadReceiptObserver
 
+- (void)dealloc
+{
+    [[NSNotificationCenter defaultCenter] removeObserver:self];
+}
+
+- (instancetype)init
+{
+    return [self initWithMessagesManager:[TSMessagesManager sharedManager]];
+}
+
 - (instancetype)initWithMessagesManager:(TSMessagesManager *)messagesManager
 {
     self = [super init];

src/Devices/OWSReadReceiptsProcessor.h

@@ -6,6 +6,8 @@ NS_ASSUME_NONNULL_BEGIN
 @class OWSReadReceipt;
 @class TSIncomingMessage;
 
+extern NSString *const OWSReadReceiptsProcessorMarkedMessageAsReadNotification;
+
 @interface OWSReadReceiptsProcessor : NSObject
 
 - (instancetype)initWithReadReceiptProtos:(NSArray<OWSSignalServiceProtosSyncMessageRead *> *)readReceiptProtos;
@@ -17,4 +19,4 @@ NS_ASSUME_NONNULL_BEGIN
 
 @end
 
-NS_ASSUME_NONNULL_END
+NS_ASSUME_NONNULL_END

src/Devices/OWSReadReceiptsProcessor.m

@@ -8,6 +8,9 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+NSString *const OWSReadReceiptsProcessorMarkedMessageAsReadNotification =
+    @"OWSReadReceiptsProcessorMarkedMessageAsReadNotification";
+
 @interface OWSReadReceiptsProcessor ()
 
 @property (nonatomic, readonly) NSArray<OWSReadReceipt *> *readReceipts;
@@ -74,6 +77,9 @@ NS_ASSUME_NONNULL_BEGIN
             [message markAsReadFromReadReceipt];
             // If it was previously saved, no need to keep it around any longer.
             [readReceipt remove];
+            [[NSNotificationCenter defaultCenter]
+                postNotificationName:OWSReadReceiptsProcessorMarkedMessageAsReadNotification
+                              object:message];
         } else {
             DDLogDebug(@"%@ Received read receipt for an unkown message. Saving it for later.", self.tag);
             [readReceipt save];