Use a single ui connection to share cache.

Primarly two things:
1. using an extension for the first time per connection is slow
2. fetching the same objects (e.g. if you leave/re-enter the same
conversation)

// FREEBIE

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -80,6 +80,7 @@
 #import <SignalServiceKit/OWSMessageManager.h>
 #import <SignalServiceKit/OWSMessageSender.h>
 #import <SignalServiceKit/OWSMessageUtils.h>
+#import <SignalServiceKit/OWSPrimaryStorage.h>
 #import <SignalServiceKit/OWSReadReceiptManager.h>
 #import <SignalServiceKit/OWSVerificationStateChangeMessage.h>
 #import <SignalServiceKit/SignalRecipient.h>
@@ -312,13 +313,17 @@ typedef enum : NSUInteger {
                                                  name:UIContentSizeCategoryDidChangeNotification
                                                object:nil];
     [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(yapDatabaseModified:)
-                                                 name:YapDatabaseModifiedNotification
+                                             selector:@selector(uiDatabaseDidUpdateExternally:)
+                                                 name:OWSUIDatabaseConnectionDidUpdateExternallyNotification
                                                object:OWSPrimaryStorage.sharedManager.dbNotificationObject];
     [[NSNotificationCenter defaultCenter] addObserver:self
-                                             selector:@selector(yapDatabaseModifiedExternally:)
-                                                 name:YapDatabaseModifiedExternallyNotification
-                                               object:nil];
+                                             selector:@selector(uiDatabaseWillUpdate:)
+                                                 name:OWSUIDatabaseConnectionWillUpdateNotification
+                                               object:OWSPrimaryStorage.sharedManager.dbNotificationObject];
+    [[NSNotificationCenter defaultCenter] addObserver:self
+                                             selector:@selector(uiDatabaseDidUpdate:)
+                                                 name:OWSUIDatabaseConnectionDidUpdateNotification
+                                               object:OWSPrimaryStorage.sharedManager.dbNotificationObject];
     [[NSNotificationCenter defaultCenter] addObserver:self
                                              selector:@selector(applicationWillEnterForeground:)
                                                  name:OWSApplicationWillEnterForegroundNotification
@@ -3114,14 +3119,7 @@ typedef enum : NSUInteger {
 
 - (YapDatabaseConnection *)uiDatabaseConnection
 {
-    NSAssert([NSThread isMainThread], @"Must access uiDatabaseConnection on main thread!");
-    if (!_uiDatabaseConnection) {
-        _uiDatabaseConnection = [self.primaryStorage newDatabaseConnection];
-        // Increase object cache limit. Default is 250.
-        _uiDatabaseConnection.objectCacheLimit = 500;
-        [_uiDatabaseConnection beginLongLivedReadTransaction];
-    }
-    return _uiDatabaseConnection;
+    return OWSPrimaryStorage.sharedManager.uiDatabaseConnection;
 }
 
 - (YapDatabaseConnection *)editingDatabaseConnection
@@ -3132,7 +3130,7 @@ typedef enum : NSUInteger {
     return _editingDatabaseConnection;
 }
 
-- (void)yapDatabaseModifiedExternally:(NSNotification *)notification
+- (void)uiDatabaseDidUpdateExternally:(NSNotification *)notification
 {
     OWSAssertIsOnMainThread();
 
@@ -3149,20 +3147,8 @@ typedef enum : NSUInteger {
     }
 }
 
-- (void)yapDatabaseModified:(NSNotification *)notification
+- (void)uiDatabaseWillUpdate:(NSNotification *)notification
 {
-    OWSAssertIsOnMainThread();
-
-    // Currently, we update thread and message state every time
-    // the database is modified.  That doesn't seem optimal, but
-    // in practice it's efficient enough.
-
-    if (!self.shouldObserveDBModifications) {
-        return;
-    }
-
-    DDLogVerbose(@"%@ %s", self.logTag, __PRETTY_FUNCTION__);
-
     // HACK to work around radar #28167779
     // "UICollectionView performBatchUpdates can trigger a crash if the collection view is flagged for layout"
     // more: https://github.com/PSPDFKit-labs/radar.apple.com/tree/master/28167779%20-%20CollectionViewBatchingIssue
@@ -3173,11 +3159,21 @@ typedef enum : NSUInteger {
     //       view items before they are updated.
     [self.collectionView layoutIfNeeded];
     // ENDHACK to work around radar #28167779
+}
+
+- (void)uiDatabaseDidUpdate:(NSNotification *)notification
+{
+    OWSAssertIsOnMainThread();
 
-    // We need to `beginLongLivedReadTransaction` before we update our
-    // models in order to jump to the most recent commit.
-    NSArray *notifications = [self.uiDatabaseConnection beginLongLivedReadTransaction];
+    if (!self.shouldObserveDBModifications) {
+        return;
+    }
+    
+    DDLogVerbose(@"%@ %s", self.logTag, __PRETTY_FUNCTION__);
 
+    NSArray *notifications = notification.userInfo[OWSUIDatabaseConnectionNotificationsKey];
+    OWSAssert([notifications isKindOfClass:[NSArray class]]);
+    
     [self updateBackButtonUnreadCount];
     [self updateNavigationBarSubtitleLabel];
 
@@ -4660,10 +4656,6 @@ typedef enum : NSUInteger {
     if (self.messageMappings != nil) {
         // Before we begin observing database modifications, make sure
         // our mapping and table state is up-to-date.
-        //
-        // We need to `beginLongLivedReadTransaction` before we update our
-        // mapping in order to jump to the most recent commit.
-        [self.uiDatabaseConnection beginLongLivedReadTransaction];
         [self extendRangeToIncludeUnobservedItems];
         [self.uiDatabaseConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
             [self.messageMappings updateWithTransaction:transaction];

SignalServiceKit/src/Storage/OWSPrimaryStorage.h

@@ -6,14 +6,22 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+extern NSString *const OWSUIDatabaseConnectionWillUpdateNotification;
+extern NSString *const OWSUIDatabaseConnectionDidUpdateNotification;
+extern NSString *const OWSUIDatabaseConnectionWillUpdateExternallyNotification;
+extern NSString *const OWSUIDatabaseConnectionDidUpdateExternallyNotification;
+extern NSString *const OWSUIDatabaseConnectionNotificationsKey;
+
 @interface OWSPrimaryStorage : OWSStorage
 
 - (instancetype)init NS_UNAVAILABLE;
 
 + (instancetype)sharedManager NS_SWIFT_NAME(shared());
 
-- (YapDatabaseConnection *)dbReadConnection;
-- (YapDatabaseConnection *)dbReadWriteConnection;
+@property (nonatomic, readonly) YapDatabaseConnection *uiDatabaseConnection;
+@property (nonatomic, readonly) YapDatabaseConnection *dbReadConnection;
+@property (nonatomic, readonly) YapDatabaseConnection *dbReadWriteConnection;
+
 + (YapDatabaseConnection *)dbReadConnection;
 + (YapDatabaseConnection *)dbReadWriteConnection;
 

SignalServiceKit/src/Storage/OWSPrimaryStorage.m

@@ -19,6 +19,12 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+NSString *const OWSUIDatabaseConnectionWillUpdateNotification = @"OWSUIDatabaseConnectionWillUpdateNotification";
+NSString *const OWSUIDatabaseConnectionDidUpdateNotification = @"OWSUIDatabaseConnectionDidUpdateNotification";
+NSString *const OWSUIDatabaseConnectionWillUpdateExternallyNotification = @"OWSUIDatabaseConnectionWillUpdateExternallyNotification";
+NSString *const OWSUIDatabaseConnectionDidUpdateExternallyNotification = @"OWSUIDatabaseConnectionDidUpdateExternallyNotification";
+
+NSString *const OWSUIDatabaseConnectionNotificationsKey = @"OWSUIDatabaseConnectionNotificationsKey";
 NSString *const OWSPrimaryStorageExceptionName_CouldNotCreateDatabaseDirectory
     = @"TSStorageManagerExceptionName_CouldNotCreateDatabaseDirectory";
 
@@ -86,9 +92,6 @@ void VerifyRegistrationsForPrimaryStorage(OWSStorage *storage)
 
 @interface OWSPrimaryStorage ()
 
-@property (nonatomic, readonly, nullable) YapDatabaseConnection *dbReadConnection;
-@property (nonatomic, readonly, nullable) YapDatabaseConnection *dbReadWriteConnection;
-
 @property (atomic) BOOL areAsyncRegistrationsComplete;
 @property (atomic) BOOL areSyncRegistrationsComplete;
 
@@ -98,6 +101,8 @@ void VerifyRegistrationsForPrimaryStorage(OWSStorage *storage)
 
 @implementation OWSPrimaryStorage
 
+@synthesize uiDatabaseConnection = _uiDatabaseConnection;
+
 + (instancetype)sharedManager
 {
     static OWSPrimaryStorage *sharedManager = nil;
@@ -119,8 +124,22 @@ void VerifyRegistrationsForPrimaryStorage(OWSStorage *storage)
     if (self) {
         [self loadDatabase];
 
-        _dbReadConnection = self.newDatabaseConnection;
-        _dbReadWriteConnection = self.newDatabaseConnection;
+        _dbReadConnection = [self newDatabaseConnection];
+        _dbReadWriteConnection = [self newDatabaseConnection];
+        _uiDatabaseConnection = [self newDatabaseConnection];
+        
+        // Increase object cache limit. Default is 250.
+        _uiDatabaseConnection.objectCacheLimit = 500;
+        [_uiDatabaseConnection beginLongLivedReadTransaction];
+        [[NSNotificationCenter defaultCenter] addObserver:self
+                                                 selector:@selector(yapDatabaseModified:)
+                                                     name:YapDatabaseModifiedNotification
+                                                   object:self.dbNotificationObject];
+
+        [[NSNotificationCenter defaultCenter] addObserver:self
+                                                 selector:@selector(yapDatabaseModifiedExternally:)
+                                                     name:YapDatabaseModifiedExternallyNotification
+                                                   object:nil];
 
         OWSSingletonAssert();
     }
@@ -128,6 +147,48 @@ void VerifyRegistrationsForPrimaryStorage(OWSStorage *storage)
     return self;
 }
 
+- (void)yapDatabaseModifiedExternally:(NSNotification *)notification
+{
+    // Notify observers we're about to update the database connection
+    [[NSNotificationCenter defaultCenter] postNotificationName:OWSUIDatabaseConnectionWillUpdateExternallyNotification object:self.dbNotificationObject];
+    
+    // Move uiDatabaseConnection to the latest commit.
+    // Do so atomically, and fetch all the notifications for each commit we jump.
+    NSArray *notifications = [self.uiDatabaseConnection beginLongLivedReadTransaction];
+    
+    // Notify observers that the uiDatabaseConnection was updated
+    NSDictionary *userInfo = @{ OWSUIDatabaseConnectionNotificationsKey: notifications };
+    [[NSNotificationCenter defaultCenter] postNotificationName:OWSUIDatabaseConnectionDidUpdateExternallyNotification
+                                                        object:self.dbNotificationObject
+                                                      userInfo:userInfo];
+}
+
+- (void)yapDatabaseModified:(NSNotification *)notification
+{
+    OWSAssertIsOnMainThread();
+    
+    DDLogVerbose(@"%@ %s", self.logTag, __PRETTY_FUNCTION__);
+    
+    // Notify observers we're about to update the database connection
+    [[NSNotificationCenter defaultCenter] postNotificationName:OWSUIDatabaseConnectionWillUpdateNotification object:self.dbNotificationObject];
+        
+    // Move uiDatabaseConnection to the latest commit.
+    // Do so atomically, and fetch all the notifications for each commit we jump.
+    NSArray *notifications = [self.uiDatabaseConnection beginLongLivedReadTransaction];
+    
+    // Notify observers that the uiDatabaseConnection was updated
+    NSDictionary *userInfo = @{ OWSUIDatabaseConnectionNotificationsKey: notifications };
+    [[NSNotificationCenter defaultCenter] postNotificationName:OWSUIDatabaseConnectionDidUpdateNotification
+                                                        object:self.dbNotificationObject
+                                                      userInfo:userInfo];
+}
+     
+- (YapDatabaseConnection *)uiDatabaseConnection
+{
+    OWSAssertIsOnMainThread();
+    return _uiDatabaseConnection;
+}
+
 - (void)resetStorage
 {
     _dbReadConnection = nil;