[ASRangeController] Fix stability of "minimum" rangeMode if the app h…

…as more than one layout before scrolling. (#790)

This should result in memory savings in many apps, since errant relayouts are pretty common.

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## master
 * Add your own contributions to the next release on the line below this with your name.
+- [ASRangeController] Fix stability of "minimum" rangeMode if the app has more than one layout before scrolling.
 - **Important** ASDisplayNode's cornerRadius is a new thread-safe bridged property that should be preferred over CALayer's. Use the latter at your own risk! [Huy Nguyen](https://github.com/nguyenhuy) [#749](https://github.com/TextureGroup/Texture/pull/749).
 - [ASCellNode] Adds mapping for UITableViewCell focusStyle [Alex Hill](https://github.com/alexhillc) [#727](https://github.com/TextureGroup/Texture/pull/727)
 - [ASNetworkImageNode] Fix capturing self in the block while loading image in ASNetworkImageNode. [Denis Mororozov](https://github.com/morozkin) [#777](https://github.com/TextureGroup/Texture/pull/777)

Source/ASCollectionView.mm

@@ -1549,13 +1549,10 @@ - (void)collectionView:(UICollectionView *)collectionView moveItemAtIndexPath:(N
 
 - (void)scrollViewDidScroll:(UIScrollView *)scrollView
 {
-  // If a scroll happenes the current range mode needs to go to full
   ASInterfaceState interfaceState = [self interfaceStateForRangeController:_rangeController];
   if (ASInterfaceStateIncludesVisible(interfaceState)) {
-    [_rangeController updateCurrentRangeWithMode:ASLayoutRangeModeFull];
     [self _checkForBatchFetching];
   }
-
   for (_ASCollectionViewCell *cell in _cellsForVisibilityUpdates) {
     // _cellsForVisibilityUpdates only includes cells for ASCellNode subclasses with overrides of the visibility method.
     [cell cellNodeVisibilityEvent:ASCellNodeVisibilityEventVisibleRectChanged inScrollView:scrollView];
@@ -1594,6 +1591,10 @@ - (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
 
 - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
 {
+  // If a scroll happens the current range mode needs to go to full
+  _rangeController.contentHasBeenScrolled = YES;
+  [_rangeController updateCurrentRangeWithMode:ASLayoutRangeModeFull];
+
   for (_ASCollectionViewCell *cell in _cellsForVisibilityUpdates) {
     [cell cellNodeVisibilityEvent:ASCellNodeVisibilityEventWillBeginDragging inScrollView:scrollView];
   }

Source/ASTableView.mm

@@ -1214,13 +1214,10 @@ - (void)scrollViewDidScroll:(UIScrollView *)scrollView
     [super scrollViewDidScroll:scrollView];
     return;
   }
-  // If a scroll happenes the current range mode needs to go to full
   ASInterfaceState interfaceState = [self interfaceStateForRangeController:_rangeController];
   if (ASInterfaceStateIncludesVisible(interfaceState)) {
-    [_rangeController updateCurrentRangeWithMode:ASLayoutRangeModeFull];
     [self _checkForBatchFetching];
-  }
-
+  }  
   for (_ASTableViewCell *tableCell in _cellsForVisibilityUpdates) {
     [[tableCell node] cellNodeVisibilityEvent:ASCellNodeVisibilityEventVisibleRectChanged
                                  inScrollView:scrollView
@@ -1272,6 +1269,10 @@ - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
     [super scrollViewWillBeginDragging:scrollView];
     return;
   }
+  // If a scroll happens the current range mode needs to go to full
+  _rangeController.contentHasBeenScrolled = YES;
+  [_rangeController updateCurrentRangeWithMode:ASLayoutRangeModeFull];
+
   for (_ASTableViewCell *tableViewCell in _cellsForVisibilityUpdates) {
     [[tableViewCell node] cellNodeVisibilityEvent:ASCellNodeVisibilityEventWillBeginDragging
                                           inScrollView:scrollView

Source/Details/ASRangeController.h

@@ -100,6 +100,11 @@ AS_SUBCLASSING_RESTRICTED
  */
 @property (nonatomic, weak) id<ASRangeControllerDelegate> delegate;
 
+/**
+ * Property that indicates whether the scroll view for this range controller has ever changed its contentOffset.
+ */
+@property (nonatomic, assign) BOOL contentHasBeenScrolled;
+
 @end
 
 

Source/Details/ASRangeController.mm

@@ -45,6 +45,7 @@ @interface ASRangeController ()
   NSSet<NSIndexPath *> *_allPreviousIndexPaths;
   NSHashTable<ASCellNode *> *_visibleNodes;
   ASLayoutRangeMode _currentRangeMode;
+  BOOL _contentHasBeenScrolled;
   BOOL _preserveCurrentRangeMode;
   BOOL _didRegisterForNodeDisplayNotifications;
   CFTimeInterval _pendingDisplayNodesTimestamp;
@@ -77,6 +78,7 @@ - (instancetype)init
 
   _rangeIsValid = YES;
   _currentRangeMode = ASLayoutRangeModeUnspecified;
+  _contentHasBeenScrolled = NO;
   _preserveCurrentRangeMode = NO;
   _previousScrollDirection = ASScrollDirectionDown | ASScrollDirectionRight;
 
@@ -222,10 +224,6 @@ - (void)_updateVisibleNodeIndexPaths
   auto visibleElements = [_dataSource visibleElementsForRangeController:self];
   NSHashTable *newVisibleNodes = [NSHashTable hashTableWithOptions:NSHashTableObjectPointerPersonality];
 
-  if (visibleElements.count == 0) { // if we don't have any visibleNodes currently (scrolled before or after content)...
-    [self _setVisibleNodes:newVisibleNodes];
-    return; // don't do anything for this update, but leave _rangeIsValid == NO to make sure we update it later
-  }
   ASSignpostStart(ASSignpostRangeControllerUpdate);
 
   // Get the scroll direction. Default to using the previous one, if they're not scrolling.
@@ -234,12 +232,28 @@ - (void)_updateVisibleNodeIndexPaths
     scrollDirection = _previousScrollDirection;
   }
   _previousScrollDirection = scrollDirection;
-
+
+  if (visibleElements.count == 0) { // if we don't have any visibleNodes currently (scrolled before or after content)...
+    // Verify the actual state by checking the layout with a "VisibleOnly" range.
+    // This allows us to avoid thrashing through -didExitVisibleState in the case of -reloadData, since that generates didEndDisplayingCell calls.
+    // Those didEndDisplayingCell calls result in items being removed from the visibleElements returned by the _dataSource, even though the layout remains correct.
+    visibleElements = [_layoutController elementsForScrolling:scrollDirection rangeMode:ASLayoutRangeModeVisibleOnly rangeType:ASLayoutRangeTypeDisplay map:map];
+    for (ASCollectionElement *element in visibleElements) {
+      [newVisibleNodes addObject:element.node];
+    }
+    [self _setVisibleNodes:newVisibleNodes];
+    return; // don't do anything for this update, but leave _rangeIsValid == NO to make sure we update it later
+  }
+
   ASInterfaceState selfInterfaceState = [self interfaceState];
   ASLayoutRangeMode rangeMode = _currentRangeMode;
-  // If the range mode is explicitly set via updateCurrentRangeWithMode: it will last in that mode until the
-  // range controller becomes visible again or explicitly changes the range mode again
-  if ((!_preserveCurrentRangeMode && ASInterfaceStateIncludesVisible(selfInterfaceState)) || [[self class] isFirstRangeUpdateForRangeMode:rangeMode]) {
+  BOOL updateRangeMode = (!_preserveCurrentRangeMode && _contentHasBeenScrolled);
+
+  // If we've never scrolled before, we never update the range mode, so it doesn't jump into Full too early.
+  // This can happen if we have multiple, noisy updates occurring from application code before the user has engaged.
+  // If the range mode is explicitly set via updateCurrentRangeWithMode:, we'll preserve that for at least one update cycle.
+  // Once the user has scrolled and the range is visible, we'll always resume managing the range mode automatically.
+  if ((updateRangeMode && ASInterfaceStateIncludesVisible(selfInterfaceState)) || [[self class] isFirstRangeUpdateForRangeMode:rangeMode]) {
     rangeMode = [ASRangeController rangeModeForInterfaceState:selfInterfaceState currentRangeMode:_currentRangeMode];
   }
 
@@ -412,7 +426,7 @@ - (void)_updateVisibleNodeIndexPaths
 //    NSLog(@"custom: %@", visibleNodePathsSet);
 //  }
   [modifiedIndexPaths sortUsingSelector:@selector(compare:)];
-  NSLog(@"Range update complete; modifiedIndexPaths: %@", [self descriptionWithIndexPaths:modifiedIndexPaths]);
+  NSLog(@"Range update complete; modifiedIndexPaths: %@, rangeMode: %d", [self descriptionWithIndexPaths:modifiedIndexPaths], rangeMode);
 #endif
 
   ASSignpostEnd(ASSignpostRangeControllerUpdate);