Add contentWidth property to ConversationViewCell.

// FREEBIE

Signal/src/ViewControllers/ConversationView/Cells/ConversationViewCell.h

@@ -54,9 +54,15 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property (nonatomic, nullable) ConversationViewItem *viewItem;
 
+// Cells are prefetched but expensive cells (e.g. media) should only load
+// when visible and unload when no longer visible.  Non-visible cells can
+// cache their contents on their ConversationViewItem, but that cache may
+// be evacuated before the cell becomes visible again.
 @property (nonatomic) BOOL isCellVisible;
 
-- (void)loadForDisplay:(int)contentWidth;
+@property (nonatomic) int contentWidth;
+
+- (void)loadForDisplay;
 
 - (CGSize)cellSizeForViewWidth:(int)viewWidth contentWidth:(int)contentWidth;
 

Signal/src/ViewControllers/ConversationView/Cells/ConversationViewCell.m

@@ -16,9 +16,10 @@ NS_ASSUME_NONNULL_BEGIN
     self.viewItem = nil;
     self.delegate = nil;
     self.isCellVisible = NO;
+    self.contentWidth = 0;
 }
 
-- (void)loadForDisplay:(int)contentWidth
+- (void)loadForDisplay
 {
     OWSFail(@"%@ This method should be overridden.", self.logTag);
 }

Signal/src/ViewControllers/ConversationView/Cells/OWSContactOffersCell.m

@@ -87,7 +87,7 @@ NS_ASSUME_NONNULL_BEGIN
     return NSStringFromClass([self class]);
 }
 
-- (void)loadForDisplay:(int)contentWidth
+- (void)loadForDisplay
 {
     OWSAssert(self.viewItem);
     OWSAssert([self.viewItem.interaction isKindOfClass:[OWSContactOffersInteraction class]]);

Signal/src/ViewControllers/ConversationView/Cells/OWSMessageCell.m

@@ -207,7 +207,7 @@ NS_ASSUME_NONNULL_BEGIN
     return (TSMessage *)self.viewItem.interaction;
 }
 
-- (void)loadForDisplay:(int)contentWidth
+- (void)loadForDisplay
 {
     OWSAssert(self.viewItem);
     OWSAssert(self.viewItem.interaction);
@@ -220,7 +220,7 @@ NS_ASSUME_NONNULL_BEGIN
         = isIncoming ? [self.bubbleFactory incoming] : [self.bubbleFactory outgoing];
     self.bubbleImageView.image = bubbleImageData.messageBubbleImage;
 
-    [self updateDateHeader:contentWidth];
+    [self updateDateHeader];
     [self updateFooter];
 
     switch (self.cellType) {
@@ -263,8 +263,10 @@ NS_ASSUME_NONNULL_BEGIN
     //    });
 }
 
-- (void)updateDateHeader:(int)contentWidth
+- (void)updateDateHeader
 {
+    OWSAssert(self.contentWidth > 0);
+
     static NSDateFormatter *dateHeaderDateFormatter = nil;
     static NSDateFormatter *dateHeaderTimeFormatter = nil;
     static dispatch_once_t onceToken;
@@ -312,7 +314,7 @@ NS_ASSUME_NONNULL_BEGIN
         self.dateHeaderConstraints = @[
             // Date headers should be visually centered within the conversation view,
             // so they need to extend outside the cell's boundaries.
-            [self.dateHeaderLabel autoSetDimension:ALDimensionWidth toSize:contentWidth],
+            [self.dateHeaderLabel autoSetDimension:ALDimensionWidth toSize:self.contentWidth],
             (self.isIncoming ? [self.dateHeaderLabel autoPinEdgeToSuperviewEdge:ALEdgeLeading]
                              : [self.dateHeaderLabel autoPinEdgeToSuperviewEdge:ALEdgeTrailing]),
             [self.dateHeaderLabel autoPinEdgeToSuperviewEdge:ALEdgeTop],

Signal/src/ViewControllers/ConversationView/Cells/OWSSystemMessageCell.m

@@ -85,7 +85,7 @@ NS_ASSUME_NONNULL_BEGIN
     return NSStringFromClass([self class]);
 }
 
-- (void)loadForDisplay:(int)contentWidth
+- (void)loadForDisplay
 {
     OWSAssert(self.viewItem);
 

Signal/src/ViewControllers/ConversationView/Cells/OWSUnreadIndicatorCell.m

@@ -84,7 +84,7 @@ NS_ASSUME_NONNULL_BEGIN
     return NSStringFromClass([self class]);
 }
 
-- (void)loadForDisplay:(int)contentWidth
+- (void)loadForDisplay
 {
     OWSAssert(self.viewItem);
     OWSAssert([self.viewItem.interaction isKindOfClass:[TSUnreadIndicatorInteraction class]]);

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -4025,8 +4025,9 @@ typedef NS_ENUM(NSInteger, MessagesRangeSizeMode) {
     }
     cell.viewItem = viewItem;
     cell.delegate = self;
+    cell.contentWidth = self.layout.contentWidth;
 
-    [cell loadForDisplay:self.layout.contentWidth];
+    [cell loadForDisplay];
 
     return cell;
 }