return unseen identity rather than bool

This turns out to be more versitile for the client app

// FREEBIE

src/Storage/AxolotlStore/TSStorageManager+IdentityKeyStore.h

@@ -42,10 +42,11 @@ extern NSString *const TSStorageManagerTrustedKeysCollection;
 
 /**
  * @param   recipientId unique stable identifier for the recipient, e.g. e164 phone number
- * @returns YES if the recipient's id has not been marked as seen (and it's not a TOFU situation)
+ * @returns nil if the recipient's current id has been seen, or if it's the users first key
- *          NO if the recipient's current id has been seen, or if it's the users first key
+ *          else returns the unseen identity
+ *
  */
-- (BOOL)hasUnseenIdentityChangeForRecipientId:(NSString *)recipientId;
+- (nullable OWSRecipientIdentity *)unseenIdentityChangeForRecipientId:(NSString *)recipientId;
 
 - (void)generateNewIdentityKey;
 - (nullable NSData *)identityKeyForRecipientId:(NSString *)recipientId;

src/Storage/AxolotlStore/TSStorageManager+IdentityKeyStore.m

@@ -199,6 +199,31 @@ const NSTimeInterval kIdentityKeyStoreNonBlockingSecondsThreshold = 5.0;
     }
 }
 
+- (nullable OWSRecipientIdentity *)unseenIdentityChangeForRecipientId:(NSString *)recipientId
+{
+    OWSAssert(recipientId != nil);
+
+    @synchronized([[self class] sharedIdentityKeyLock])
+    {
+        OWSRecipientIdentity *currentIdentity = [OWSRecipientIdentity fetchObjectWithUniqueID:recipientId];
+        if (currentIdentity == nil) {
+            // No preexisting key, Trust On First Use
+            return nil;
+        }
+
+        if (currentIdentity.isFirstKnownKey) {
+            return nil;
+        }
+
+        if (currentIdentity.wasSeen) {
+            return nil;
+        }
+
+        // identity not yet seen
+        return currentIdentity;
+    }
+}
+
 - (BOOL)isTrustedKey:(NSData *)identityKey forSendingToIdentity:(nullable OWSRecipientIdentity *)recipientIdentity
 {
     OWSAssert(identityKey != nil);
@@ -278,24 +303,6 @@ const NSTimeInterval kIdentityKeyStoreNonBlockingSecondsThreshold = 5.0;
     }
 }
 
-
-- (BOOL)hasUnseenIdentityChangeForRecipientId:(NSString *)recipientId
-{
-    OWSAssert(recipientId != nil);
-
-    OWSRecipientIdentity *recipientIdentity = [OWSRecipientIdentity fetchObjectWithUniqueID:recipientId];
-
-    if (!recipientIdentity) {
-        return NO;
-    }
-
-    if (recipientIdentity.isFirstKnownKey) {
-        return NO;
-    }
-
-    return !recipientIdentity.wasSeen;
-}
-
 @end
 
 NS_ASSUME_NONNULL_END