Add experiment to defer requesting objects from data source until jus…

…t before diffing Summary: Inspired by recent performance investigations in Instagram, adding an experimental feature to defer requesting objects from the `IGListAdapterDataSource` until //just before// diffing is executed. If //n// updates are coalesced into one, this results in just a single request for objects from the data source. This is a **breaking change** to the public API, but since writing custom `IGListUpdatingDelegate`s is discouraged, I'm less worried about saving this for a major version launch. Reviewed By: manicakes Differential Revision: D7744518 fbshipit-source-id: f0001263cddda383e3dedd1d350a83d2cfb8362a
Instagram · Apr 26, 2018 · 3059c5e · 3059c5e
1 parent c189462
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Showing 9 changed files with 142 additions and 89 deletions.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -5,6 +5,10 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 4.0.0 (upcoming release)
 -----
 
+### Enhancements
+
+- Experimental performance improvement from deferring `-[IGListAdapterDataSource objectsForListAdapter:]` calls until just before diffing. [Ryan Nystrom](https://github.com/rnystrom) (tbd)
+
 3.3.0
 -----
 

diff --git a/Source/Common/IGListExperiments.h b/Source/Common/IGListExperiments.h
@@ -26,6 +26,8 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     IGListExperimentFasterVisibleSectionController = 1 << 4,
     /// Test deduping item-level updates.
     IGListExperimentDedupeItemUpdates = 1 << 5,
+    /// Test deferring object creation until just before diffing.
+    IGListExperimentDeferredToObjectCreation = 1 << 6,
 };
 
 /**

diff --git a/Source/IGListAdapter.m b/Source/IGListAdapter.m
@@ -324,14 +324,28 @@ - (void)performUpdatesAnimated:(BOOL)animated completion:(IGListUpdaterCompletio
     }
 
     NSArray *fromObjects = self.sectionMap.objects;
-    NSArray *newObjects = [dataSource objectsForListAdapter:self];
-
-    [self _enterBatchUpdates];
 
+    IGListToObjectBlock toObjectsBlock;
     __weak __typeof__(self) weakSelf = self;
+    if (IGListExperimentEnabled(self.experiments, IGListExperimentDeferredToObjectCreation)) {
+        toObjectsBlock = ^NSArray *{
+            __typeof__(self) strongSelf = weakSelf;
+            if (strongSelf == nil) {
+                return nil;
+            }
+            return [dataSource objectsForListAdapter:strongSelf];
+        };
+    } else {
+        NSArray *newObjects = [dataSource objectsForListAdapter:self];
+        toObjectsBlock = ^NSArray *{
+            return newObjects;
+        };
+    }
+
+    [self _enterBatchUpdates];
     [self.updater performUpdateWithCollectionView:collectionView
                                       fromObjects:fromObjects
-                                        toObjects:newObjects
+                                   toObjectsBlock:toObjectsBlock
                                          animated:animated
                             objectTransitionBlock:^(NSArray *toObjects) {
                                 // temporarily capture the item map that we are transitioning from in case

diff --git a/Source/IGListAdapterUpdater.m b/Source/IGListAdapterUpdater.m
@@ -41,7 +41,7 @@ - (BOOL)hasChanges {
     return self.hasQueuedReloadData
     || [self.batchUpdates hasChanges]
     || self.fromObjects != nil
-    || self.toObjects != nil;
+    || self.toObjectsBlock != nil;
 }
 
 - (void)performReloadDataWithCollectionView:(UICollectionView *)collectionView {
@@ -106,12 +106,25 @@ - (void)performBatchUpdatesWithCollectionView:(UICollectionView *)collectionView
     // create local variables so we can immediately clean our state but pass these items into the batch update block
     id<IGListAdapterUpdaterDelegate> delegate = self.delegate;
     NSArray *fromObjects = [self.fromObjects copy];
-    NSArray *toObjects = objectsWithDuplicateIdentifiersRemoved(self.toObjects);
+    IGListToObjectBlock toObjectsBlock = [self.toObjectsBlock copy];
     NSMutableArray *completionBlocks = [self.completionBlocks mutableCopy];
     void (^objectTransitionBlock)(NSArray *) = [self.objectTransitionBlock copy];
     const BOOL animated = self.queuedUpdateIsAnimated;
     IGListBatchUpdates *batchUpdates = self.batchUpdates;
 
+    NSArray *toObjects = nil;
+    if (toObjectsBlock != nil) {
+        toObjects = objectsWithDuplicateIdentifiersRemoved(toObjectsBlock());
+    }
+#ifdef DEBUG
+    for (id obj in toObjects) {
+        IGAssert([obj conformsToProtocol:@protocol(IGListDiffable)],
+                 @"In order to use IGListAdapterUpdater, object %@ must conform to IGListDiffable", obj);
+        IGAssert([obj diffIdentifier] != nil,
+                 @"Cannot have a nil diffIdentifier for object %@", obj);
+    }
+#endif
+
     // clean up all state so that new updates can be coalesced while the current update is in flight
     [self cleanStateBeforeUpdates];
 
@@ -340,7 +353,7 @@ - (void)cleanStateBeforeUpdates {
 
     // destroy to/from transition items
     self.fromObjects = nil;
-    self.toObjects = nil;
+    self.toObjectsBlock = nil;
 
     // destroy reloadData state
     self.reloadUpdates = nil;
@@ -408,11 +421,11 @@ - (NSPointerFunctions *)objectLookupPointerFunctions {
 }
 
 - (void)performUpdateWithCollectionView:(UICollectionView *)collectionView
-                            fromObjects:(nullable NSArray *)fromObjects
-                              toObjects:(nullable NSArray *)toObjects
+                            fromObjects:(NSArray *)fromObjects
+                         toObjectsBlock:(IGListToObjectBlock)toObjectsBlock
                                animated:(BOOL)animated
-                  objectTransitionBlock:(void (^)(NSArray *))objectTransitionBlock
-                             completion:(nullable void (^)(BOOL))completion {
+                  objectTransitionBlock:(IGListObjectTransitionBlock)objectTransitionBlock
+                             completion:(IGListUpdatingCompletion)completion {
     IGAssertMainThread();
     IGParameterAssert(collectionView != nil);
     IGParameterAssert(objectTransitionBlock != nil);
@@ -423,21 +436,12 @@ - (void)performUpdateWithCollectionView:(UICollectionView *)collectionView
     // if performBatchUpdates: hasn't applied the update block, then data source hasn't transitioned its state. if an
     // update is queued in between then we must use the pending toObjects
     self.fromObjects = self.fromObjects ?: self.pendingTransitionToObjects ?: fromObjects;
-    self.toObjects = toObjects;
+    self.toObjectsBlock = toObjectsBlock;
 
     // disabled animations will always take priority
     // reset to YES in -cleanupState
     self.queuedUpdateIsAnimated = self.queuedUpdateIsAnimated && animated;
 
-#ifdef DEBUG
-    for (id obj in toObjects) {
-        IGAssert([obj conformsToProtocol:@protocol(IGListDiffable)],
-                 @"In order to use IGListAdapterUpdater, object %@ must conform to IGListDiffable", obj);
-        IGAssert([obj diffIdentifier] != nil,
-                 @"Cannot have a nil diffIdentifier for object %@", obj);
-    }
-#endif
-
     // always use the last update block, even though this should always do the exact same thing
     self.objectTransitionBlock = objectTransitionBlock;
 

diff --git a/Source/IGListReloadDataUpdater.m b/Source/IGListReloadDataUpdater.m
@@ -19,11 +19,14 @@ - (NSPointerFunctions *)objectLookupPointerFunctions {
 
 - (void)performUpdateWithCollectionView:(UICollectionView *)collectionView
                             fromObjects:(NSArray *)fromObjects
-                              toObjects:(NSArray *)toObjects
+                         toObjectsBlock:(IGListToObjectBlock)toObjectsBlock
                                animated:(BOOL)animated
                   objectTransitionBlock:(IGListObjectTransitionBlock)objectTransitionBlock
                              completion:(IGListUpdatingCompletion)completion {
-    objectTransitionBlock(toObjects);
+    if (toObjectsBlock != nil) {
+        NSArray *toObjects = toObjectsBlock() ?: @[];
+        objectTransitionBlock(toObjects);
+    }
     [self _synchronousReloadDataWithCollectionView:collectionView];
     if (completion) {
         completion(YES);

diff --git a/Source/IGListUpdatingDelegate.h b/Source/IGListUpdatingDelegate.h
@@ -37,6 +37,10 @@ typedef void (^IGListItemUpdateBlock)(void);
 NS_SWIFT_NAME(ListReloadUpdateBlock)
 typedef void (^IGListReloadUpdateBlock)(void);
 
+/// A block that returns an array of objects to transition to.
+NS_SWIFT_NAME(ListToObjectBlock)
+typedef NSArray * _Nullable (^IGListToObjectBlock)(void);
+
 /**
  Implement this protocol in order to handle both section and row based update events. Implementation should forward or
  coalesce these events to a backing store or collection.
@@ -63,7 +67,7 @@ NS_SWIFT_NAME(ListUpdatingDelegate)
 
  @param collectionView The collection view to perform the transition on.
  @param fromObjects The previous objects in the collection view. Objects must conform to `IGListDiffable`.
- @param toObjects The new objects in collection view. Objects must conform to `IGListDiffable`.
+ @param toObjectsBlock A block returning the new objects in the collection view. Objects must conform to `IGListDiffable`.
  @param animated A flag indicating if the transition should be animated.
  @param objectTransitionBlock A block that must be called when the adapter applies changes to the collection view.
  @param completion A completion block to execute when the update is finished.
@@ -77,7 +81,7 @@ NS_SWIFT_NAME(ListUpdatingDelegate)
  */
 - (void)performUpdateWithCollectionView:(UICollectionView *)collectionView
                             fromObjects:(nullable NSArray<id <IGListDiffable>> *)fromObjects
-                              toObjects:(nullable NSArray<id <IGListDiffable>> *)toObjects
+                         toObjectsBlock:(nullable IGListToObjectBlock)toObjectsBlock
                                animated:(BOOL)animated
                   objectTransitionBlock:(IGListObjectTransitionBlock)objectTransitionBlock
                              completion:(nullable IGListUpdatingCompletion)completion;

diff --git a/Source/Internal/IGListAdapterUpdater+DebugDescription.m b/Source/Internal/IGListAdapterUpdater+DebugDescription.m
@@ -61,9 +61,9 @@ @implementation IGListAdapterUpdater (DebugDescription)
         [debug addObjectsFromArray:IGListDebugIndentedLines(linesFromObjects(self.fromObjects))];
     }
 
-    if (self.toObjects != nil) {
+    if (self.toObjectsBlock != nil) {
         [debug addObject:@"To objects:"];
-        [debug addObjectsFromArray:IGListDebugIndentedLines(linesFromObjects(self.toObjects))];
+        [debug addObjectsFromArray:IGListDebugIndentedLines(linesFromObjects(self.toObjectsBlock()))];
     }
 
     if (self.pendingTransitionToObjects != nil) {

diff --git a/Source/Internal/IGListAdapterUpdaterInternal.h b/Source/Internal/IGListAdapterUpdaterInternal.h
@@ -27,7 +27,7 @@ FOUNDATION_EXTERN void convertReloadToDeleteInsert(NSMutableIndexSet *reloads,
 @interface IGListAdapterUpdater ()
 
 @property (nonatomic, copy, nullable) NSArray *fromObjects;
-@property (nonatomic, copy, nullable) NSArray *toObjects;
+@property (nonatomic, copy, nullable) IGListToObjectBlock toObjectsBlock;
 @property (nonatomic, copy, nullable) NSArray *pendingTransitionToObjects;
 @property (nonatomic, strong) NSMutableArray<IGListUpdatingCompletion> *completionBlocks;