Delete last message updates thread preview (signal ios#833) (#3)

* fix thread preview when deleting last message

because we are doing internal accounting of what our "last" message id
is, when we delete the last message, other things get out of whack.
Caching is hard, don't do it! We can efficiently get the last object
using our existing Yap DB view.

//FREEBIE

* remove lastMessageId in favor of lastMessage for cleaner code

//FREEBIE

src/Contacts/TSThread.m

@@ -19,8 +19,10 @@
 @property (nonatomic, retain) NSDate *creationDate;
 @property (nonatomic, copy) NSDate *archivalDate;
 @property (nonatomic, retain) NSDate *lastMessageDate;
-@property (nonatomic, copy) NSString *latestMessageId;
 @property (nonatomic, copy) NSString *messageDraft;
+
+- (TSInteraction *) lastInteraction;
+
 @end
 
 @implementation TSThread
@@ -34,7 +36,6 @@
 
     if (self) {
         _archivalDate    = nil;
-        _latestMessageId = nil;
         _lastMessageDate = nil;
         _creationDate    = [NSDate date];
         _messageDraft    = nil;
@@ -62,15 +63,12 @@
 #pragma mark Read Status
 
 - (BOOL)hasUnreadMessages {
-    __block TSInteraction *interaction;
-    __block BOOL hasUnread = NO;
-    [[TSStorageManager sharedManager]
-            .dbConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
-      interaction = [TSInteraction fetchObjectWithUniqueID:self.latestMessageId transaction:transaction];
-      if ([interaction isKindOfClass:[TSIncomingMessage class]]) {
-          hasUnread = ![(TSIncomingMessage *)interaction wasRead];
-      }
-    }];
+    TSInteraction *interaction = self.lastInteraction;
+    BOOL hasUnread = NO;
+
+    if ([interaction isKindOfClass:[TSIncomingMessage class]]) {
+        hasUnread = ![(TSIncomingMessage *)interaction wasRead];
+    }
 
     return hasUnread;
 }
@@ -93,6 +91,14 @@
 
 #pragma mark Last Interactions
 
+- (TSInteraction *) lastInteraction {
+    __block TSInteraction *last;
+    [TSStorageManager.sharedManager.dbConnection readWithBlock:^(YapDatabaseReadTransaction *transaction){
+        last = [[transaction ext:TSMessageDatabaseViewExtensionName] lastObjectInGroup:self.uniqueId];
+    }];
+    return (TSInteraction *)last;
+}
+
 - (NSDate *)lastMessageDate {
     if (_lastMessageDate) {
         return _lastMessageDate;
@@ -102,12 +108,11 @@
 }
 
 - (NSString *)lastMessageLabel {
-    __block TSInteraction *interaction;
-    [[TSStorageManager sharedManager]
-            .dbConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
-      interaction = [TSInteraction fetchObjectWithUniqueID:self.latestMessageId transaction:transaction];
-    }];
-    return interaction.description;
+    if (self.lastInteraction == nil) {
+        return @"";
+    } else {
+        return self.lastInteraction.description;
+    }
 }
 
 - (void)updateWithLastMessage:(TSInteraction *)lastMessage transaction:(YapDatabaseReadWriteTransaction *)transaction {
@@ -119,7 +124,6 @@
     }
 
     if (!_lastMessageDate || [lastMessageDate timeIntervalSinceDate:self.lastMessageDate] > 0) {
-        _latestMessageId = lastMessage.uniqueId;
         _lastMessageDate = lastMessageDate;
 
         [self saveWithTransaction:transaction];