Cull cached cell media outside a load window.

// FREEBIE

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -165,7 +165,8 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
 @property (nonatomic, nullable) OWSAudioAttachmentPlayer *audioAttachmentPlayer;
 @property (nonatomic, nullable) NSUUID *voiceMessageUUID;
 
-@property (nonatomic) NSTimer *readTimer;
+@property (nonatomic, nullable) NSTimer *readTimer;
+@property (nonatomic, nullable) NSTimer *cellMediaCachesTimer;
 @property (nonatomic) ConversationHeaderView *navigationBarTitleView;
 @property (nonatomic) UILabel *navigationBarTitleLabel;
 @property (nonatomic) UILabel *navigationBarSubtitleLabel;
@@ -526,6 +527,7 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
         self.hasClearedUnreadMessagesIndicator = NO;
         [self.dynamicInteractions clearUnreadIndicatorState];
     }
+    [self clearCellMediaCaches];
 }
 
 - (void)applicationWillResignActive:(NSNotification *)notification
@@ -943,6 +945,69 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
 - (void)cancelReadTimer
 {
     [self.readTimer invalidate];
+    self.readTimer = nil;
+}
+
+- (void)startCellMediaCachesTimerIfNecessary
+{
+    if (self.cellMediaCachesTimer) {
+        return;
+    }
+    self.cellMediaCachesTimer = [NSTimer weakScheduledTimerWithTimeInterval:3.f
+                                                                     target:self
+                                                                   selector:@selector(auditCellMediaCaches)
+                                                                   userInfo:nil
+                                                                    repeats:NO];
+}
+
+- (void)auditCellMediaCaches
+{
+    [self.cellMediaCachesTimer invalidate];
+    self.cellMediaCachesTimer = nil;
+
+    NSIndexPath *_Nullable firstVisibleIndexPath = nil;
+    NSIndexPath *_Nullable lastVisibleIndexPath = nil;
+
+    for (NSIndexPath *indexPath in self.collectionView.indexPathsForVisibleItems) {
+        if (!firstVisibleIndexPath || firstVisibleIndexPath.row > indexPath.row) {
+            firstVisibleIndexPath = indexPath;
+        }
+        if (!lastVisibleIndexPath || lastVisibleIndexPath.row < indexPath.row) {
+            lastVisibleIndexPath = indexPath;
+        }
+    }
+
+    if (!firstVisibleIndexPath || !lastVisibleIndexPath) {
+        [self clearCellMediaCaches];
+        return;
+    }
+
+    // Only retain the cached cell media for N cells at a time.
+    const NSInteger kCellMediaCacheWindowSize = 24;
+    NSInteger centerRow = (firstVisibleIndexPath.row + lastVisibleIndexPath.row) / 2;
+    // Determine the first and law rows (inclusive) of the cached cell media load window.
+    // Note these row values may not correspond to actual rows in the collection view.
+    //
+    // Always retain the cached cell media for any visible cells.
+    NSInteger firstLoadWindowRow = MIN(firstVisibleIndexPath.row, centerRow - kCellMediaCacheWindowSize / 2);
+    NSInteger lastLoadWindowRow = MAX(lastVisibleIndexPath.row, centerRow + kCellMediaCacheWindowSize / 2);
+
+    // Cull cached cell media outside the load window.
+    for (ConversationViewItem *viewItem in self.viewItems) {
+        if (viewItem.row < firstLoadWindowRow || viewItem.row > lastLoadWindowRow) {
+            viewItem.cachedCellMedia = nil;
+        }
+    }
+}
+
+- (void)clearCellMediaCaches
+{
+    [self.cellMediaCachesTimer invalidate];
+    self.cellMediaCachesTimer = nil;
+
+    for (ConversationViewItem *viewItem in self.viewItems) {
+        viewItem.cachedCellMedia = nil;
+    }
 }
 
 - (void)viewDidAppear:(BOOL)animated
@@ -986,6 +1051,7 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
     [self saveDraft];
     [self markVisibleMessagesAsRead];
     [self cancelVoiceMemo];
+    [self clearCellMediaCaches];
 
     self.isUserScrolling = NO;
 }
@@ -3531,6 +3597,8 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView
 {
     [self updateLastVisibleTimestamp];
+
+    [self startCellMediaCachesTimerIfNecessary];
 }
 
 - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView

Signal/src/ViewControllers/ConversationView/ConversationViewItem.h

@@ -90,7 +90,7 @@ NSString *NSStringForOWSMessageCellType(OWSMessageCellType cellType);
 // media.  This cache is volatile and will get evacuated based
 // on scroll state, so that we only retain state for a sliding
 // window of cells that are almost on-screen.
-@property (nonatomic) id cachedCellMedia;
+@property (nonatomic, nullable) id cachedCellMedia;
 @property (nonatomic) BOOL didCellMediaFailToLoad;
 
 // TODO: