Make sure range controller listens to node display notifications if absolutely needed

AsyncDisplayKit/ASDisplayNode.mm

@@ -245,9 +245,9 @@ static ASDisplayNodeMethodOverrides GetASDisplayNodeMethodOverrides(Class c)
   dispatch_once(&onceToken, ^{
     renderQueue = [[ASRunLoopQueue<ASDisplayNode *> alloc] initWithRunLoop:CFRunLoopGetMain()
                                                                 andHandler:^(ASDisplayNode * _Nonnull dequeuedItem, BOOL isQueueDrained) {
-      CFAbsoluteTime timestamp = isQueueDrained ?  CFAbsoluteTimeGetCurrent() : 0;
       [dequeuedItem _recursivelyTriggerDisplayAndBlock:NO];
       if (isQueueDrained) {
+        CFAbsoluteTime timestamp = CFAbsoluteTimeGetCurrent();
         [[NSNotificationCenter defaultCenter] postNotificationName:ASRenderingEngineDidDisplayScheduledNodesNotification
                                                             object:nil
                                                           userInfo:@{ASRenderingEngineDidDisplayNodesScheduledBeforeTimestamp: @(timestamp)}];
@@ -2317,25 +2317,25 @@ void recursivelyTriggerDisplayForLayer(CALayer *layer, BOOL shouldBlock)
   });
 }
 
-- (void)recursivelySetInterfaceState:(ASInterfaceState)interfaceState
+- (void)recursivelySetInterfaceState:(ASInterfaceState)newInterfaceState
 {
-  ASInterfaceState oldState = self.interfaceState;
-  ASInterfaceState newState = interfaceState;
-  ASDisplayNodePerformBlockOnEveryNode(nil, self, ^(ASDisplayNode *node) {
-    node.interfaceState = interfaceState;
-  });
-  
-  if ([self supportsRangeManagedInterfaceState]) {
   // Instead of each node in the recursion assuming it needs to schedule itself for display,
   // setInterfaceState: skips this when handling range-managed nodes (our whole subtree has this set).
   // If our range manager intends for us to be displayed right now, and didn't before, get started!
-    
-    BOOL nowDisplay = ASInterfaceStateIncludesDisplay(newState);
-    BOOL wasDisplay = ASInterfaceStateIncludesDisplay(oldState);
-    if (nowDisplay && (nowDisplay != wasDisplay)) {
+  BOOL shouldScheduleDisplay = [self supportsRangeManagedInterfaceState] && [self shouldScheduleDisplayWithNewInterfaceState:newInterfaceState];
+  ASDisplayNodePerformBlockOnEveryNode(nil, self, ^(ASDisplayNode *node) {
+    node.interfaceState = newInterfaceState;
+  });
+  if (shouldScheduleDisplay) {
     [ASDisplayNode scheduleNodeForRecursiveDisplay:self];
   }
-  }
+}
+
+- (BOOL)shouldScheduleDisplayWithNewInterfaceState:(ASInterfaceState)newInterfaceState
+{
+  BOOL willDisplay = ASInterfaceStateIncludesDisplay(newInterfaceState);
+  BOOL nowDisplay = ASInterfaceStateIncludesDisplay(self.interfaceState);
+  return willDisplay && (willDisplay != nowDisplay);
 }
 
 - (ASHierarchyState)hierarchyState

AsyncDisplayKit/Details/ASRangeController.mm

@@ -27,7 +27,7 @@
   NSSet<NSIndexPath *> *_allPreviousIndexPaths;
   ASLayoutRangeMode _currentRangeMode;
   BOOL _didUpdateCurrentRange;
-  BOOL _didRegisterForNotifications;
+  BOOL _didRegisterForNodeDisplayNotifications;
   CFAbsoluteTime _pendingDisplayNodesTimestamp;
 }
 
@@ -56,7 +56,7 @@ static UIApplicationState __ApplicationState = UIApplicationStateActive;
 
 - (void)dealloc
 {
-  if (_didRegisterForNotifications) {
+  if (_didRegisterForNodeDisplayNotifications) {
     [[NSNotificationCenter defaultCenter] removeObserver:self name:ASRenderingEngineDidDisplayScheduledNodesNotification object:nil];
   }
 }
@@ -242,10 +242,6 @@ static UIApplicationState __ApplicationState = UIApplicationStateActive;
     [allIndexPaths addObjectsFromArray:ASIndexPathsForTwoDimensionalArray(allNodes)];
   }
   
-  // TODO Don't register for notifications if this range update doesn't cause any node to enter rendering pipeline.
-  // This can be done once there is an API to observe to (or be notified upon) interface state changes or pipeline enterings
-  [self registerForNotificationsForInterfaceStateIfNeeded:selfInterfaceState];
-  
 #if ASRangeControllerLoggingEnabled
   ASDisplayNodeAssertTrue([visibleIndexPaths isSubsetOfSet:displayIndexPaths]);
   NSMutableArray<NSIndexPath *> *modifiedIndexPaths = (ASRangeControllerLoggingEnabled ? [NSMutableArray array] : nil);
@@ -309,15 +305,20 @@ static UIApplicationState __ApplicationState = UIApplicationStateActive;
 #if ASRangeControllerLoggingEnabled
           [modifiedIndexPaths addObject:indexPath];
 #endif
-          [node recursivelySetInterfaceState:interfaceState];
-        }
-      }
-    }
-  }
           
-  if (_didRegisterForNotifications) {
+          BOOL nodeShouldScheduleDisplay = [node shouldScheduleDisplayWithNewInterfaceState:interfaceState];
+          [node recursivelySetInterfaceState:interfaceState];
+          
+          if (nodeShouldScheduleDisplay) {
+            [self registerForNodeDisplayNotificationsForInterfaceStateIfNeeded:selfInterfaceState];
+            if (_didRegisterForNodeDisplayNotifications) {
               _pendingDisplayNodesTimestamp = CFAbsoluteTimeGetCurrent();
             }
+          }
+        }
+      }
+    }
+  }
   
   _rangeIsValid = YES;
   _queuedRangeUpdate = NO;
@@ -338,9 +339,9 @@ static UIApplicationState __ApplicationState = UIApplicationStateActive;
 
 #pragma mark - Notification observers
 
-- (void)registerForNotificationsForInterfaceStateIfNeeded:(ASInterfaceState)interfaceState
+- (void)registerForNodeDisplayNotificationsForInterfaceStateIfNeeded:(ASInterfaceState)interfaceState
 {
-  if (!_didRegisterForNotifications) {
+  if (!_didRegisterForNodeDisplayNotifications) {
     ASLayoutRangeMode nextRangeMode = [ASRangeController rangeModeForInterfaceState:interfaceState
                                                                    currentRangeMode:_currentRangeMode];
     if (_currentRangeMode != nextRangeMode) {
@@ -348,7 +349,7 @@ static UIApplicationState __ApplicationState = UIApplicationStateActive;
                                                selector:@selector(scheduledNodesDidDisplay:)
                                                    name:ASRenderingEngineDidDisplayScheduledNodesNotification
                                                  object:nil];
-      _didRegisterForNotifications = YES;
+      _didRegisterForNodeDisplayNotifications = YES;
     }
   }
 }
@@ -359,7 +360,7 @@ static UIApplicationState __ApplicationState = UIApplicationStateActive;
   if (_pendingDisplayNodesTimestamp < notificationTimestamp) {
     // The rendering engine has processed all the nodes this range controller scheduled. Let's schedule a range update
     [[NSNotificationCenter defaultCenter] removeObserver:self name:ASRenderingEngineDidDisplayScheduledNodesNotification object:nil];
-    _didRegisterForNotifications = NO;
+    _didRegisterForNodeDisplayNotifications = NO;
     
     [self scheduleRangeUpdate];
   }

AsyncDisplayKit/Private/ASDisplayNode+FrameworkPrivate.h

@@ -135,6 +135,11 @@ inline BOOL ASHierarchyStateIncludesRangeManaged(ASHierarchyState hierarchyState
  */
 @property (nonatomic, assign) BOOL shouldBypassEnsureDisplay;
 
+/**
+ * @abstract Checks whether a node should be scheduled for display, considering its current and new interface states.
+ */
+- (BOOL)shouldScheduleDisplayWithNewInterfaceState:(ASInterfaceState)newInterfaceState;
+
 @end
 
 @interface UIView (ASDisplayNodeInternal)