Perform contact intersection ~every 6 hours even if no contacts changed

// FREEBIE

Signal/src/ViewControllers/SignalsViewController.m

@@ -283,8 +283,10 @@ NSString *const SignalsViewControllerSegueShowIncomingCall = @"ShowIncomingCallS
     MessageComposeTableViewController *viewController = [MessageComposeTableViewController new];
 
     [self.contactsManager requestSystemContactsOnceWithCompletion:^(NSError *_Nullable error) {
+        if (error) {
             DDLogError(@"%@ Error when requesting contacts: %@", self.tag, error);
-        // Even if there was an error fetching contacts we proceed to the next screen.
+        }
+        // Even if there is an error fetching contacts we proceed to the next screen.
         // As the compose view will present the proper thing depending on contact access.
         //
         // We just want to make sure contact access is *complete* before showing the compose

Signal/src/contact/SystemContactsFetcher.swift

@@ -15,6 +15,7 @@ class SystemContactsFetcher: NSObject {
 
     private let TAG = "[SystemContactsFetcher]"
     var lastContactUpdateHash: Int?
+    var lastDelegateNotificationDate: Date?
 
     public weak var delegate: SystemContactsFetcherDelegate?
 
@@ -109,7 +110,7 @@ class SystemContactsFetcher: NSObject {
 
         systemContactsHaveBeenRequestedAtLeastOnce = true
 
-        DispatchQueue.global().async {
+        DispatchQueue.default.async {
             var systemContacts = [CNContact]()
             do {
                 let contactFetchRequest = CNContactFetchRequest(keysToFetch: self.allowedContactKeys)
@@ -126,20 +127,46 @@ class SystemContactsFetcher: NSObject {
             }
 
             let contacts = systemContacts.map { Contact(systemContact: $0) }
-
             let contactsHash  = HashableArray(contacts).hashValue
-            guard (self.lastContactUpdateHash != contactsHash) else {
-                Logger.debug("\(self.TAG) System contacts unchanged. hash:\(contactsHash)")
+
             DispatchQueue.main.async {
-                    completion?(nil)
+                var shouldNotifyDelegate = false
+
+                if self.lastContactUpdateHash != contactsHash {
+                    Logger.info("\(self.TAG) contact hash changed. new contactsHash: \(contactsHash)")
+                    shouldNotifyDelegate = true
+                } else {
+
+                    // If nothing has changed, only notify delegate (to perform contact intersection) every N hours
+                    if let lastDelegateNotificationDate = self.lastDelegateNotificationDate {
+                        let kDebounceInterval = TimeInterval(12 * 60 * 60)
+
+                        let expiresAtDate = Date(timeInterval: kDebounceInterval, since:lastDelegateNotificationDate)
+                        if  Date() > expiresAtDate {
+                            Logger.info("\(self.TAG) debounce interval expired at: \(expiresAtDate)")
+                            shouldNotifyDelegate = true
+                        } else {
+                            Logger.info("\(self.TAG) ignoring since debounce interval hasn't expired")
+                        }
+                    } else {
+                        Logger.info("\(self.TAG) first contact fetch. contactsHash: \(contactsHash)")
+                        shouldNotifyDelegate = true
+                    }
                 }
+
+                guard shouldNotifyDelegate else {
+                    Logger.info("\(self.TAG) no reason to notify delegate.")
+
+                    completion?(nil)
+
                     return
                 }
-            self.lastContactUpdateHash = contactsHash
 
                 Logger.debug("\(self.TAG) Notifying delegate that system contacts did change. hash:\(contactsHash)")
 
-            DispatchQueue.main.async {
+                self.lastDelegateNotificationDate = Date()
+                self.lastContactUpdateHash = contactsHash
+
                 self.delegate?.systemContactsFetcher(self, updatedContacts: contacts)
                 completion?(nil)
             }