added experiment to make [IGListAdapter visibleSectionControllers] a bit faster

Summary:
* Issue: [IGListAdapter visibleSectionControllers] calls [UICollectionViewLayout layoutAttributesForElementsInRect:] which can be expensive. And since visibleSectionControllers is called in scrollViewDidScroll, it can add up and take a toll on scroll performance. On an iPhone 7, it's in 2-10% of the main-thread samples (using Time Profiler).
* Fix: IGListDisplayHandler conveniently keeps track of visible IGListSectionControllers, so let's just use that. Now, [IGListAdapter visibleSectionControllers] drops to ~0.3% of samples.

Reviewed By: rnystrom

Differential Revision: D6351474

fbshipit-source-id: 66914db95e08498927bfbceda4d2e9d58cbc7530

CHANGELOG.md

@@ -9,11 +9,12 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 
 - Added `-[IGListSectionController didHighlightItemAtIndex:]` and `-[IGListSectionController didUnhighlightItemAtIndex:]` APIs to support `UICollectionView` cell highlighting. [Kevin Delannoy](https://github.com/delannoyk) [(#933)](https://github.com/Instagram/IGListKit/pull/933)
 
-
 - Added a new listener API to be notified when `IGListAdapter` finishes updating. Add listeners via `-[IGListAdapter addUpdateListener:]` with objects conforming to the new `IGListAdapterUpdateListener` protocol. [Ryan Nystrom](https://github.com/rnystrom) [(tbd)](https://github.com/Instagram/IGListKit/pull/tbd)
 
 - Updated project settings for iOS 11. [Ryan Nystrom](https://github.com/rnystrom) [(#942)](https://github.com/Instagram/IGListKit/pull/942)
 
+- Added experiment to make  `-[IGListAdapter visibleSectionControllers:]` a bit faster. [Maxime Ollivier](https://github.com/maxoll) (tbd)
+
 ### Fixes
 
 - Duplicate objects for initial data source setup filtered out. [Mikhail Vashlyaev](https://github.com/yemodin) [(#993](https://github.com/Instagram/IGListKit/pull/993)

Source/Common/IGListExperiments.h

@@ -22,6 +22,8 @@ typedef NS_OPTIONS (NSInteger, IGListExperiment) {
     IGListExperimentBackgroundDiffing = 1 << 2,
     /// Test fallback to reloadData when "too many" update operations.
     IGListExperimentReloadDataFallback = 1 << 3,
+    /// Test a faster way to return visible section controllers.
+    IGListExperimentFasterVisibleSectionController = 1 << 4,
 };
 
 /**

Source/IGListAdapter.m

@@ -484,6 +484,14 @@
 
 - (NSArray<IGListSectionController *> *)visibleSectionControllers {
     IGAssertMainThread();
+    if (IGListExperimentEnabled(self.experiments, IGListExperimentFasterVisibleSectionController)) {
+        return [self visibleSectionControllersFromDisplayHandler];
+    } else {
+        return [self visibleSectionControllersFromLayoutAttributes];
+    }
+}
+
+- (NSArray<IGListSectionController *> *)visibleSectionControllersFromLayoutAttributes {
     NSMutableSet *visibleSectionControllers = [NSMutableSet new];
     NSArray<UICollectionViewLayoutAttributes *> *attributes =
     [self.collectionView.collectionViewLayout layoutAttributesForElementsInRect:self.collectionView.bounds];
@@ -497,6 +505,10 @@
     return [visibleSectionControllers allObjects];
 }
 
+- (NSArray<IGListSectionController *> *)visibleSectionControllersFromDisplayHandler {
+    return [[self.displayHandler visibleListSections] allObjects];
+}
+
 - (NSArray *)visibleObjects {
     IGAssertMainThread();
     NSArray<UICollectionViewCell *> *visibleCells = [self.collectionView visibleCells];

Source/Internal/IGListDisplayHandler.h

@@ -21,6 +21,11 @@ NS_ASSUME_NONNULL_BEGIN
 IGLK_SUBCLASSING_RESTRICTED
 @interface IGListDisplayHandler : NSObject
 
+/**
+ Counted set of the currently visible section controllers.
+ */
+@property (nonatomic, strong, readonly) NSCountedSet<IGListSectionController *> *visibleListSections;
+
 /**
  Tells the handler that a cell will be displayed in the IGListAdapter.
 

Source/Internal/IGListDisplayHandler.m

@@ -16,7 +16,6 @@
 
 @interface IGListDisplayHandler ()
 
-@property (nonatomic, strong) NSCountedSet *visibleListSections;
 @property (nonatomic, strong) NSMapTable *visibleViewObjectMap;
 
 @end

Tests/IGListAdapterTests.m

@@ -12,6 +12,7 @@
 #import <XCTest/XCTest.h>
 #import <OCMock/OCMock.h>
 
+#import <IGListKit/IGListExperiments.h>
 #import <IGListKit/IGListKit.h>
 
 #import "IGListAdapterInternal.h"
@@ -262,6 +263,22 @@
     XCTAssertTrue([visibleSectionControllers containsObject:[self.adapter sectionControllerForObject:@4]]);
 }
 
+- (void)test_whenCellsExtendBeyondBounds_withFasterExperiment_thatVisibleSectionControllersAreLimited {
+    // add experiment
+    self.adapter.experiments |= IGListExperimentFasterVisibleSectionController;
+    // # of items for each object == [item integerValue], so @2 has 2 items (cells)
+    self.dataSource.objects = @[@1, @2, @3, @4, @5, @6, @7, @8, @9, @10, @11, @12];
+    [self.adapter reloadDataWithCompletion:nil];
+    XCTAssertEqual([self.collectionView numberOfSections], 12);
+    NSArray *visibleSectionControllers = [self.adapter visibleSectionControllers];
+    // UIWindow is 100x100, each cell is 100x10 so should have the following section/cell count: 1 + 2 + 3 + 4 = 10 (100 tall)
+    XCTAssertEqual(visibleSectionControllers.count, 4);
+    XCTAssertTrue([visibleSectionControllers containsObject:[self.adapter sectionControllerForObject:@1]]);
+    XCTAssertTrue([visibleSectionControllers containsObject:[self.adapter sectionControllerForObject:@2]]);
+    XCTAssertTrue([visibleSectionControllers containsObject:[self.adapter sectionControllerForObject:@3]]);
+    XCTAssertTrue([visibleSectionControllers containsObject:[self.adapter sectionControllerForObject:@4]]);
+}
+
 - (void) test_withEmptySectionPlusFooter_thatVisibleSectionControllersAreCorrect {
     self.dataSource.objects = @[@0];
     [self.adapter reloadDataWithCompletion:nil];
@@ -279,6 +296,25 @@
     XCTAssertTrue(visibleSectionControllers.firstObject.supplementaryViewSource == supplementarySource);
 }
 
+- (void) test_withEmptySectionPlusFooter_withFasterExperiment_thatVisibleSectionControllersAreCorrect {
+    // add experiment
+    self.adapter.experiments |= IGListExperimentFasterVisibleSectionController;
+    self.dataSource.objects = @[@0];
+    [self.adapter reloadDataWithCompletion:nil];
+    IGTestSupplementarySource *supplementarySource = [IGTestSupplementarySource new];
+    supplementarySource.dequeueFromNib = YES;
+    supplementarySource.collectionContext = self.adapter;
+    supplementarySource.supportedElementKinds = @[UICollectionElementKindSectionFooter];
+    IGListSectionController *controller = [self.adapter sectionControllerForObject:@0];
+    controller.supplementaryViewSource = supplementarySource;
+    supplementarySource.sectionController = controller;
+    [self.adapter performUpdatesAnimated:NO completion:nil];
+    NSArray<IGListSectionController *> *visibleSectionControllers = [self.adapter visibleSectionControllers];
+
+    XCTAssertTrue([visibleSectionControllers count] == 1);
+    XCTAssertTrue(visibleSectionControllers.firstObject.supplementaryViewSource == supplementarySource);
+}
+
 - (void)test_whenCellsExtendBeyondBounds_thatVisibleCellsExistForSectionControllers {
     self.dataSource.objects = @[@2, @3, @4, @5, @6];
     [self.adapter reloadDataWithCompletion:nil];