Do not present local notifications if a read receipt arrives immediately after.

// FREEBIE

Signal/src/environment/NotificationsManager.m

@@ -136,7 +136,7 @@
         }
         notification.alertBody = alertBodyString;
 
-        [[PushManager sharedManager] presentNotification:notification];
+        [[PushManager sharedManager] presentNotification:notification checkForCancel:NO];
     } else {
         if ([Environment.preferences soundInForeground]) {
             AudioServicesPlayAlertSound(_newMessageSound);
@@ -193,7 +193,7 @@
                 break;
         }
 
-        [[PushManager sharedManager] presentNotification:notification];
+        [[PushManager sharedManager] presentNotification:notification checkForCancel:YES];
     } else {
         if ([Environment.preferences soundInForeground]) {
             AudioServicesPlayAlertSound(_newMessageSound);

Signal/src/network/PushManager.h

@@ -66,7 +66,10 @@ typedef void (^pushTokensSuccessBlock)(NSString *pushToken, NSString *voipToken)
 
 - (TOCFuture *)registerPushKitNotificationFuture;
 - (BOOL)supportsVOIPPush;
-- (void)presentNotification:(UILocalNotification *)notification;
+// If checkForCancel is set, the notification will be delayed for
+// a moment.  If a relevant cancel notification is received in that window,
+// the notification will not be displayed.
+- (void)presentNotification:(UILocalNotification *)notification checkForCancel:(BOOL)checkForCancel;
 - (void)cancelNotificationsWithThreadId:(NSString *)threadId;
 
 #pragma mark Push Notifications Delegate Methods

Signal/src/network/PushManager.m

@@ -23,6 +23,7 @@
 @property TOCFutureSource *registerWithServerFutureSource;
 @property UIAlertView *missingPermissionsAlertView;
 @property (nonatomic, retain) NSMutableArray *currentNotifications;
+@property (nonatomic, retain) NSMutableArray *pendingNotifications;
 @property (nonatomic) UIBackgroundTaskIdentifier callBackgroundTask;
 @property (nonatomic, readonly) OWSMessageSender *messageSender;
 @property (nonatomic, readonly) OWSMessageFetcherJob *messageFetcherJob;
@@ -75,7 +76,8 @@
                                                     cancelButtonTitle:NSLocalizedString(@"OK", @"")
                                                     otherButtonTitles:nil, nil];
     _callBackgroundTask = UIBackgroundTaskInvalid;
-    _currentNotifications = [NSMutableArray array];
+    _currentNotifications = [NSMutableArray new];
+    _pendingNotifications = [NSMutableArray new];
 
     OWSSingletonAssert();
 
@@ -163,7 +165,7 @@
                     failedSendNotif.alertBody =
                         [NSString stringWithFormat:NSLocalizedString(@"NOTIFICATION_SEND_FAILED", nil), [thread name]];
                     failedSendNotif.userInfo = @{ Signal_Thread_UserInfo_Key : thread.uniqueId };
-                    [self presentNotification:failedSendNotif];
+                    [self presentNotification:failedSendNotif checkForCancel:NO];
                     completionHandler();
                 }];
         }
@@ -429,19 +431,61 @@ NSString *const PushManagerUserInfoKeysCallBackSignalRecipientId = @"PushManager
     return NO;
 }
 
-- (void)presentNotification:(UILocalNotification *)notification {
-    [[UIApplication sharedApplication] scheduleLocalNotification:notification];
-    [self.currentNotifications addObject:notification];
+- (void)presentNotification:(UILocalNotification *)notification checkForCancel:(BOOL)checkForCancel
+{
+    OWSAssert([NSThread isMainThread]);
+
+    NSString *threadId = notification.userInfo[Signal_Thread_UserInfo_Key];
+    if (checkForCancel && threadId != nil) {
+        [_pendingNotifications addObject:notification];
+
+        // The longer we wait, the more obsolete notifications we can suppress -
+        // but the more lag we introduce to notification delivery.
+        const CGFloat kDelaySeconds = 0.3f;
+        dispatch_after(
+            dispatch_time(DISPATCH_TIME_NOW, (int64_t)(NSEC_PER_SEC * kDelaySeconds)), dispatch_get_main_queue(), ^{
+                if (![_pendingNotifications containsObject:notification]) {
+                    DDLogVerbose(@"%@ notification was cancelled before it was presented: %@, %@",
+                                 self.tag,
+                                 notification.alertBody,
+                                 threadId);
+                } else {
+                    [_pendingNotifications removeObject:notification];
+                    [[UIApplication sharedApplication] scheduleLocalNotification:notification];
+                    [self.currentNotifications addObject:notification];
+                }
+            });
+    } else {
+        [[UIApplication sharedApplication] scheduleLocalNotification:notification];
+        [self.currentNotifications addObject:notification];
+    }
 }
 
 - (void)cancelNotificationsWithThreadId:(NSString *)threadId {
+    OWSAssert([NSThread isMainThread]);
+
+    // Cull matching pending notifications.
     NSMutableArray *toDelete = [NSMutableArray array];
-    [self.currentNotifications enumerateObjectsUsingBlock:^(UILocalNotification *notif, NSUInteger idx, BOOL *stop) {
-      if ([notif.userInfo[Signal_Thread_UserInfo_Key] isEqualToString:threadId]) {
-          [[UIApplication sharedApplication] cancelLocalNotification:notif];
-          [toDelete addObject:notif];
-      }
+    [self.pendingNotifications enumerateObjectsUsingBlock:^(
+        UILocalNotification *notification, NSUInteger idx, BOOL *stop) {
+        NSString *notificationThreadId = notification.userInfo[Signal_Thread_UserInfo_Key];
+        OWSAssert(notificationThreadId != nil);
+        if ([notificationThreadId isEqualToString:threadId]) {
+            DDLogError(@"%@ cancelling delayed notification: %@, %@", self.tag, notification.alertBody, threadId);
+            [toDelete addObject:notification];
+        }
     }];
+    [self.pendingNotifications removeObjectsInArray:toDelete];
+
+    // Cull matching active notifications.
+    [toDelete removeAllObjects];
+    [self.currentNotifications
+        enumerateObjectsUsingBlock:^(UILocalNotification *notification, NSUInteger idx, BOOL *stop) {
+            if ([notification.userInfo[Signal_Thread_UserInfo_Key] isEqualToString:threadId]) {
+                [[UIApplication sharedApplication] cancelLocalNotification:notification];
+                [toDelete addObject:notification];
+            }
+        }];
     [self.currentNotifications removeObjectsInArray:toDelete];
 }