Revert "Conversation view always observes view model."

This reverts commit 9d39e829a44f28f324f79e0b74a6c8692678d788.

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -108,6 +108,25 @@ typedef enum : NSUInteger {
 
 #pragma mark -
 
+// We use snapshots to ensure that the view has a consistent
+// representation of view model state which is not updated
+// when the view is not observing view model changes.
+@interface ConversationSnapshot : NSObject
+
+@property (nonatomic) NSArray<id<ConversationViewItem>> *viewItems;
+@property (nonatomic) ThreadDynamicInteractions *dynamicInteractions;
+@property (nonatomic) BOOL canLoadMoreItems;
+
+@end
+
+#pragma mark -
+
+@implementation ConversationSnapshot
+
+@end
+
+#pragma mark -
+
 @interface ConversationViewController () <AttachmentApprovalViewControllerDelegate,
     ContactShareApprovalViewControllerDelegate,
     AVAudioPlayerDelegate,
@@ -141,6 +160,7 @@ typedef enum : NSUInteger {
 
 @property (nonatomic) TSThread *thread;
 @property (nonatomic, readonly) ConversationViewModel *conversationViewModel;
+@property (nonatomic, readonly) ConversationSnapshot *conversationSnapshot;
 
 @property (nonatomic, readonly) OWSAudioActivity *recordVoiceNoteAudioActivity;
 @property (nonatomic, readonly) NSTimeInterval viewControllerCreatedAt;
@@ -189,9 +209,13 @@ typedef enum : NSUInteger {
 
 @property (nonatomic) BOOL isViewCompletelyAppeared;
 @property (nonatomic) BOOL isViewVisible;
+@property (nonatomic) BOOL shouldObserveVMUpdates;
 @property (nonatomic) BOOL viewHasEverAppeared;
 @property (nonatomic) BOOL hasUnreadMessages;
 @property (nonatomic) BOOL isPickingMediaAsDocument;
+@property (nonatomic, nullable) NSNumber *previousLastTimestamp;
+@property (nonatomic, nullable) NSNumber *previousViewItemCount;
+@property (nonatomic, nullable) NSNumber *previousViewTopToContentBottom;
 @property (nonatomic, nullable) NSNumber *viewHorizonTimestamp;
 @property (nonatomic) ContactShareViewHelper *contactShareViewHelper;
 @property (nonatomic) NSTimer *reloadTimer;
@@ -482,6 +506,10 @@ typedef enum : NSUInteger {
     _conversationViewModel =
         [[ConversationViewModel alloc] initWithThread:thread focusMessageIdOnOpen:focusMessageId delegate:self];
 
+    [self updateConversationSnapshot];
+
+    [self updateShouldObserveVMUpdates];
+
     self.reloadTimer = [NSTimer weakScheduledTimerWithTimeInterval:1.f
                                                             target:self
                                                           selector:@selector(reloadTimerDidFire)
@@ -499,9 +527,8 @@ typedef enum : NSUInteger {
 {
     OWSAssertIsOnMainThread();
 
-    if (self.isUserScrolling || !self.isViewCompletelyAppeared || !self.isViewVisible
-        || !CurrentAppContext().isAppForegroundAndActive || !self.viewHasEverAppeared
-        || OWSWindowManager.sharedManager.isPresentingMenuActions) {
+    if (self.isUserScrolling || !self.isViewCompletelyAppeared || !self.isViewVisible || !self.shouldObserveVMUpdates
+        || !self.viewHasEverAppeared) {
         return;
     }
 
@@ -670,6 +697,7 @@ typedef enum : NSUInteger {
 
 - (void)applicationWillResignActive:(NSNotification *)notification
 {
+    [self updateShouldObserveVMUpdates];
     [self cancelVoiceMemo];
     self.isUserScrolling = NO;
     [self saveDraft];
@@ -681,6 +709,7 @@ typedef enum : NSUInteger {
 
 - (void)applicationDidBecomeActive:(NSNotification *)notification
 {
+    [self updateShouldObserveVMUpdates];
     [self startReadTimer];
 }
 
@@ -746,12 +775,12 @@ typedef enum : NSUInteger {
 
 - (NSArray<id<ConversationViewItem>> *)viewItems
 {
-    return self.conversationViewModel.viewItems;
+    return self.conversationSnapshot.viewItems;
 }
 
 - (ThreadDynamicInteractions *)dynamicInteractions
 {
-    return self.conversationViewModel.dynamicInteractions;
+    return self.conversationSnapshot.dynamicInteractions;
 }
 
 - (NSIndexPath *_Nullable)indexPathOfUnreadMessagesIndicator
@@ -840,6 +869,7 @@ typedef enum : NSUInteger {
     // Avoid layout corrupt issues and out-of-date message subtitles.
     self.lastReloadDate = [NSDate new];
     [self.conversationViewModel viewDidResetContentAndLayout];
+    [self tryToUpdateConversationSnapshot];
     [self.collectionView.collectionViewLayout invalidateLayout];
     [self.collectionView reloadData];
 
@@ -1690,7 +1720,7 @@ typedef enum : NSUInteger {
 {
     OWSAssertDebug(self.conversationViewModel);
 
-    self.showLoadMoreHeader = self.conversationViewModel.canLoadMoreItems;
+    self.showLoadMoreHeader = self.conversationSnapshot.canLoadMoreItems;
 }
 
 - (void)setShowLoadMoreHeader:(BOOL)showLoadMoreHeader
@@ -1931,6 +1961,8 @@ typedef enum : NSUInteger {
 - (void)menuActionsDidHide:(MenuActionsViewController *)menuActionsViewController
 {
     [[OWSWindowManager sharedManager] hideMenuActionsWindow];
+
+    [self updateShouldObserveVMUpdates];
 }
 
 - (void)menuActions:(MenuActionsViewController *)menuActionsViewController
@@ -2037,6 +2069,8 @@ typedef enum : NSUInteger {
 
     self.conversationViewModel.mostRecentMenuActionsViewItem = cell.viewItem;
     [[OWSWindowManager sharedManager] showMenuActionsWindow:menuActionsViewController];
+
+    [self updateShouldObserveVMUpdates];
 }
 
 - (NSAttributedString *)attributedContactOrProfileNameForPhoneIdentifier:(NSString *)recipientId
@@ -4083,6 +4117,7 @@ typedef enum : NSUInteger {
 {
     _isViewVisible = isViewVisible;
 
+    [self updateShouldObserveVMUpdates];
     [self updateCellsVisible];
 }
 
@@ -4095,8 +4130,134 @@ typedef enum : NSUInteger {
     }
 }
 
+- (void)updateShouldObserveVMUpdates
+{
+    if (!CurrentAppContext().isAppForegroundAndActive) {
+        self.shouldObserveVMUpdates = NO;
+        return;
+    }
+
+    if (!self.isViewVisible) {
+        self.shouldObserveVMUpdates = NO;
+        return;
+    }
+
+    if (OWSWindowManager.sharedManager.isPresentingMenuActions) {
+        self.shouldObserveVMUpdates = NO;
+        return;
+    }
+
+    self.shouldObserveVMUpdates = YES;
+}
+
+- (void)setShouldObserveVMUpdates:(BOOL)shouldObserveVMUpdates
+{
+    if (_shouldObserveVMUpdates == shouldObserveVMUpdates) {
+        return;
+    }
+
+    _shouldObserveVMUpdates = shouldObserveVMUpdates;
+
+    if (self.shouldObserveVMUpdates) {
+        OWSLogVerbose(@"resume observation of view model.");
+
+        [self updateConversationSnapshot];
+        [self resetContentAndLayout];
+        [self updateBackButtonUnreadCount];
+        [self updateNavigationBarSubtitleLabel];
+        [self updateDisappearingMessagesConfiguration];
+
+        // Detect changes in the mapping's "window" size.
+        if (self.previousViewTopToContentBottom && self.previousViewItemCount
+            && self.previousViewItemCount.unsignedIntegerValue != self.viewItems.count) {
+            CGFloat newContentHeight = self.safeContentHeight;
+            CGPoint newContentOffset
+                = CGPointMake(0, MAX(0, newContentHeight - self.previousViewTopToContentBottom.floatValue));
+            [self.collectionView setContentOffset:newContentOffset animated:NO];
+        }
+
+        // When we resume observing database changes, we want to scroll to show the user
+        // any new items inserted while we were not observing.  We therefore find the
+        // first item at or after the "view horizon".  See the comments below which explain
+        // the "view horizon".
+        id<ConversationViewItem> _Nullable lastViewItem = self.viewItems.lastObject;
+        BOOL hasAddedNewItems = (lastViewItem && self.previousLastTimestamp
+            && lastViewItem.interaction.timestamp > self.previousLastTimestamp.unsignedLongLongValue);
+
+        OWSLogInfo(@"hasAddedNewItems: %d", hasAddedNewItems);
+        if (hasAddedNewItems) {
+            NSIndexPath *_Nullable indexPathToShow = [self firstIndexPathAtViewHorizonTimestamp];
+            if (indexPathToShow) {
+                // The goal is to show _both_ the last item before the "view horizon" and the
+                // first item after the "view horizon".  We can't do "top on first item after"
+                // or "bottom on last item before" or we won't see the other. Unfortunately,
+                // this gets tricky if either is huge.  The largest cells are oversize text,
+                // which should be rare.  Other cells are considerably smaller than a screenful.
+                [self.collectionView scrollToItemAtIndexPath:indexPathToShow
+                                            atScrollPosition:UICollectionViewScrollPositionCenteredVertically
+                                                    animated:NO];
+            }
+        }
+        self.viewHorizonTimestamp = nil;
+        OWSLogVerbose(@"resumed observation of view model.");
+    } else {
+        OWSLogVerbose(@"pausing observation of view model.");
+        // When stopping observation, try to record the timestamp of the "view horizon".
+        // The "view horizon" is where we'll want to focus the users when we resume
+        // observation if any changes have happened while we weren't observing.
+        // Ideally, we'll focus on those changes.  But we can't skip over unread
+        // interactions, so we prioritize those, if any.
+        //
+        // We'll use this later to update the view to reflect any changes made while
+        // we were not observing the database.  See extendRangeToIncludeUnobservedItems
+        // and the logic above.
+        id<ConversationViewItem> _Nullable lastViewItem = self.viewItems.lastObject;
+        if (lastViewItem) {
+            self.previousLastTimestamp = @(lastViewItem.interaction.timestamp);
+            self.previousViewItemCount = @(self.viewItems.count);
+        } else {
+            self.previousLastTimestamp = nil;
+            self.previousViewItemCount = nil;
+        }
+
+        __block TSInteraction *_Nullable firstUnseenInteraction = nil;
+        [self.uiDatabaseConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
+            firstUnseenInteraction =
+            [[TSDatabaseView unseenDatabaseViewExtension:transaction] firstObjectInGroup:self.thread.uniqueId];
+        }];
+        if (firstUnseenInteraction) {
+            // If there are any unread interactions, focus on the first one.
+            self.viewHorizonTimestamp = @(firstUnseenInteraction.timestamp);
+        } else if (lastViewItem) {
+            // Otherwise, focus _just after_ the last interaction.
+            self.viewHorizonTimestamp = @(lastViewItem.interaction.timestamp + 1);
+        } else {
+            self.viewHorizonTimestamp = nil;
+        }
+
+        // Snapshot the scroll state by measuring the "distance from top of view to
+        // bottom of content"; if the mapping's "window" size grows, it will grow
+        // _upward_.
+        OWSAssertDebug([self.collectionView.collectionViewLayout isKindOfClass:[ConversationViewLayout class]]);
+        ConversationViewLayout *conversationViewLayout
+            = (ConversationViewLayout *)self.collectionView.collectionViewLayout;
+        // To avoid laying out the collection view during initial view
+        // presentation, don't trigger layout here (via safeContentHeight)
+        // until layout has been done at least once.
+        if (conversationViewLayout.hasEverHadLayout) {
+            self.previousViewTopToContentBottom = @(self.safeContentHeight - self.collectionView.contentOffset.y);
+        } else {
+            self.previousViewTopToContentBottom = nil;
+        }
+
+        OWSLogVerbose(@"paused observation of view model.");
+    }
+}
+
 - (nullable NSIndexPath *)firstIndexPathAtViewHorizonTimestamp
 {
+    OWSAssertDebug(self.shouldObserveVMUpdates);
+
     if (!self.viewHorizonTimestamp) {
         return nil;
     }
@@ -4443,6 +4604,11 @@ typedef enum : NSUInteger {
 
 #pragma mark - ConversationViewModelDelegate
 
+- (BOOL)isObservingVMUpdates
+{
+    return self.shouldObserveVMUpdates;
+}
+
 - (void)conversationViewModelWillUpdate
 {
     OWSAssertIsOnMainThread();
@@ -4466,6 +4632,11 @@ typedef enum : NSUInteger {
     OWSAssertDebug(conversationUpdate);
     OWSAssertDebug(self.conversationViewModel);
 
+    if (!self.shouldObserveVMUpdates) {
+        return;
+    }
+
+    [self updateConversationSnapshot];
     [self updateBackButtonUnreadCount];
     [self updateNavigationBarSubtitleLabel];
 
@@ -4763,6 +4934,26 @@ typedef enum : NSUInteger {
     [self.inputToolbar updateLayoutWithSafeAreaInsets:safeAreaInsets];
 }
 
+#pragma mark - Conversation Snapshot
+
+- (void)tryToUpdateConversationSnapshot
+{
+    if (!self.isObservingVMUpdates) {
+        return;
+    }
+
+    [self updateConversationSnapshot];
+}
+
+- (void)updateConversationSnapshot
+{
+    ConversationSnapshot *conversationSnapshot = [ConversationSnapshot new];
+    conversationSnapshot.viewItems = self.conversationViewModel.viewItems;
+    conversationSnapshot.dynamicInteractions = self.conversationViewModel.dynamicInteractions;
+    conversationSnapshot.canLoadMoreItems = self.conversationViewModel.canLoadMoreItems;
+    _conversationSnapshot = conversationSnapshot;
+}
+
 @end
 
 NS_ASSUME_NONNULL_END

Signal/src/ViewControllers/ConversationView/ConversationViewModel.h

@@ -76,6 +76,8 @@ typedef NS_ENUM(NSUInteger, ConversationUpdateItemType) {
 
 - (void)conversationViewModelDidDeleteMostRecentMenuActionsViewItem;
 
+- (BOOL)isObservingVMUpdates;
+
 - (ConversationStyle *)conversationStyle;
 
 @end

Signal/src/ViewControllers/ConversationView/ConversationViewModel.m

@@ -542,12 +542,19 @@ static const int kYapDatabaseRangeMaxLength = 25000;
 
     OWSLogVerbose(@"");
 
+    if (!self.delegate.isObservingVMUpdates) {
+        return;
+    }
+
     // External database modifications (e.g. changes from another process such as the SAE)
     // are "flushed" using touchDbAsync when the app re-enters the foreground.
 }
 
 - (void)uiDatabaseWillUpdate:(NSNotification *)notification
 {
+    if (!self.delegate.isObservingVMUpdates) {
+        return;
+    }
     [self.delegate conversationViewModelWillUpdate];
 }
 
@@ -698,6 +705,13 @@ static const int kYapDatabaseRangeMaxLength = 25000;
     OWSAssertDebug(oldItemIdList);
     OWSAssertDebug(updatedItemSetParam);
 
+    if (!self.delegate.isObservingVMUpdates) {
+        OWSLogVerbose(@"Skipping VM update.");
+        // We fire this event, but it will be ignored.
+        [self.delegate conversationViewModelDidUpdate:ConversationUpdate.minorUpdate];
+        return;
+    }
+
     if (oldItemIdList.count != [NSSet setWithArray:oldItemIdList].count) {
         OWSFailDebug(@"Old view item list has duplicates.");
         [self.delegate conversationViewModelDidUpdate:ConversationUpdate.reloadUpdate];