Imitate Android's behavior of delaying local notifications based on incoming sync messages.

Signal/src/network/PushManager.m

@@ -9,6 +9,7 @@
 #import "Signal-Swift.h"
 #import "ThreadUtil.h"
 #import <SignalServiceKit/NSDate+OWS.h>
+#import <SignalServiceKit/OWSDevice.h>
 #import <SignalServiceKit/OWSMessageReceiver.h>
 #import <SignalServiceKit/OWSMessageSender.h>
 #import <SignalServiceKit/OWSReadReceiptManager.h>
@@ -440,10 +441,17 @@ NSString *const PushManagerUserInfoKeysCallBackSignalRecipientId = @"PushManager
     dispatch_async(dispatch_get_main_queue(), ^{
         NSString *threadId = notification.userInfo[Signal_Thread_UserInfo_Key];
         if (checkForCancel && threadId != nil) {
-            // The longer we wait, the more obsolete notifications we can suppress -
-            // but the more lag we introduce to notification delivery.
-            const CGFloat kDelaySeconds = 0.5f;
-            notification.fireDate = [NSDate dateWithTimeIntervalSinceNow:kDelaySeconds];
+            if ([[OWSDeviceManager sharedManager] hasReceivedSyncMessageInLastSeconds:60.f]) {
+                // "If you’ve heard from desktop in last minute, wait 5 seconds."
+                //
+                // This provides a window in which we can cancel notifications
+                // already viewed on desktop before they are presented here.
+                const CGFloat kDelaySeconds = 5.f;
+                notification.fireDate = [NSDate dateWithTimeIntervalSinceNow:kDelaySeconds];
+            } else {
+                notification.fireDate = [NSDate new];
+            }
+
             notification.timeZone = [NSTimeZone localTimeZone];
         }
 

SignalServiceKit/src/Devices/OWSDevice.h

@@ -18,6 +18,9 @@ extern uint32_t const OWSDevicePrimaryDeviceId;
 - (BOOL)mayHaveLinkedDevices:(YapDatabaseConnection *)dbConnection;
 - (void)setMayHaveLinkedDevices:(BOOL)value dbConnection:(YapDatabaseConnection *)dbConnection;
 
+- (BOOL)hasReceivedSyncMessageInLastSeconds:(NSTimeInterval)intervalSeconds;
+- (void)setHasReceivedSyncMessage;
+
 @end
 
 #pragma mark -

SignalServiceKit/src/Devices/OWSDevice.m

@@ -19,6 +19,7 @@ NSString *const kTSStorageManager_MayHaveLinkedDevices = @"kTSStorageManager_May
 @interface OWSDeviceManager ()
 
 @property (atomic, nullable) NSNumber *mayHaveLinkedDevicesCached;
+@property (atomic) NSDate *lastReceivedSyncMessage;
 
 @end
 
@@ -73,6 +74,16 @@ NSString *const kTSStorageManager_MayHaveLinkedDevices = @"kTSStorageManager_May
     }
 }
 
+- (BOOL)hasReceivedSyncMessageInLastSeconds:(NSTimeInterval)intervalSeconds
+{
+    return (self.lastReceivedSyncMessage && fabs(self.lastReceivedSyncMessage.timeIntervalSinceNow) < intervalSeconds);
+}
+
+- (void)setHasReceivedSyncMessage
+{
+    self.lastReceivedSyncMessage = [NSDate new];
+}
+
 @end
 
 #pragma mark -

SignalServiceKit/src/Messages/OWSMessageManager.m

@@ -11,6 +11,7 @@
 #import "OWSAttachmentsProcessor.h"
 #import "OWSBlockingManager.h"
 #import "OWSCallMessageHandler.h"
+#import "OWSDevice.h"
 #import "OWSDisappearingConfigurationUpdateInfoMessage.h"
 #import "OWSDisappearingMessagesConfiguration.h"
 #import "OWSDisappearingMessagesJob.h"
@@ -267,6 +268,8 @@ NS_ASSUME_NONNULL_BEGIN
 
         if (content.hasSyncMessage) {
             [self handleIncomingEnvelope:envelope withSyncMessage:content.syncMessage transaction:transaction];
+
+            [[OWSDeviceManager sharedManager] setHasReceivedSyncMessage];
         } else if (content.hasDataMessage) {
             [self handleIncomingEnvelope:envelope withDataMessage:content.dataMessage transaction:transaction];
         } else if (content.hasCallMessage) {