goodbye allowsBackgroundReloading

Summary:
Originally, `allowsBackgroundReloading` was added to improve performance, but ironically, it's causing lots of performance issues among other issues.
* Performance: Looking back, it's not too surprising that it causes perf issues. We're falling back to a full `-reloadData` if the view is not in the window, which can happen pretty often. For example, if a view-controller is within a `UINavigationController` stack but not on top, or within a `UITabBarController`. Because a full `-reloadData` will re-query the cells and re-create the entire layout, it's going to be more expensive than an incremental update via `-performUpdatesAnimated`. The proof is in the data and we have a few examples where this flag was the cause of significant UI stalls.
* Bugs: Because we might reload cells often, it can create strange animation artifacts. Specifically, it was breaking the `UIView` snapshots just before a transition, like the new zoom animator.

Overall, we ended disabling this feature and I think most apps will be in the same boat.

But what if this flag does improve my app's performance?
* File an issue and lets chat! I'd be curious to understand why that's the case. If a full `-reloadData` is more performant than an incremental `-performUpdatesAnimated`, than something odd is happening and I don't think this flag is the right solution.

Reviewed By: joetam

Differential Revision: D25884777

fbshipit-source-id: c4626a52082ef4c7b7300b21077529f26c551e70

CHANGELOG.md

@@ -36,6 +36,8 @@ The changelog for `IGListKit`. Also see the [releases](https://github.com/instag
 - (void)performDataSourceChange:(IGListDataSourceChangeBlock)block;
 ```
 
+- Removed `allowsBackgroundReloading` from `IGListAdapterUpdater` because it's causing performance issues and other bugs. [Maxime Ollivier](https://github.com/maxolls) (tbd)
+
 ### Enhancements
 
 - Added `shouldSelectItemAtIndex:` to `IGListSectionController` . [dirtmelon](https://github.com/dirtmelon)

Source/IGListKit/IGListAdapterUpdater.h

@@ -62,19 +62,6 @@ NS_SWIFT_NAME(ListAdapterUpdater)
  */
 @property (nonatomic, assign) BOOL preferItemReloadsForSectionReloads;
 
-/**
- A flag indicating whether this updater should skip diffing and simply call
- `reloadData` for updates when the collection view is not in a window. The default value is `YES`.
-
- Default is YES.
-
- @note This will result in better performance, but will not generate the same delegate
- callbacks. If using a custom layout, it will not receive `prepareForCollectionViewUpdates:`.
-
- @warning On iOS < 8.3, this behavior is unsupported and will always be treated as `NO`.
- */
-@property (nonatomic, assign) BOOL allowsBackgroundReloading;
-
 /**
  If there's more than 100 diff updates, fallback to using `reloadData` to avoid stalling the main thread.
 

Source/IGListKit/IGListAdapterUpdater.m

@@ -31,7 +31,6 @@
 @synthesize sectionMovesAsDeletesInserts = _sectionMovesAsDeletesInserts;
 @synthesize singleItemSectionUpdates = _singleItemSectionUpdates;
 @synthesize preferItemReloadsForSectionReloads = _preferItemReloadsForSectionReloads;
-@synthesize allowsBackgroundReloading = _allowsBackgroundReloading;
 @synthesize allowsReloadingOnTooManyUpdates = _allowsReloadingOnTooManyUpdates;
 @synthesize experiments = _experiments;
 
@@ -40,7 +39,6 @@
 
     if (self = [super init]) {
         _transactionBuilder = [IGListUpdateTransactionBuilder new];
-        _allowsBackgroundReloading = YES;
         _allowsReloadingOnTooManyUpdates = YES;
     }
     return self;
@@ -91,7 +89,6 @@
         .sectionMovesAsDeletesInserts = _sectionMovesAsDeletesInserts,
         .singleItemSectionUpdates = _singleItemSectionUpdates,
         .preferItemReloadsForSectionReloads = _preferItemReloadsForSectionReloads,
-        .allowsBackgroundReloading = _allowsBackgroundReloading,
         .allowsReloadingOnTooManyUpdates = _allowsReloadingOnTooManyUpdates,
         .experiments = _experiments,
     };

Source/IGListKit/Internal/IGListAdapterUpdater+DebugDescription.m

@@ -22,7 +22,6 @@
         [NSString stringWithFormat:@"sectionMovesAsDeletesInserts: %@", IGListDebugBOOL(self.sectionMovesAsDeletesInserts)],
         [NSString stringWithFormat:@"singleItemSectionUpdates: %@", IGListDebugBOOL(self.singleItemSectionUpdates)],
         [NSString stringWithFormat:@"preferItemReloadsForSectionReloads: %@", IGListDebugBOOL(self.preferItemReloadsForSectionReloads)],
-        [NSString stringWithFormat:@"allowsBackgroundReloading: %@", IGListDebugBOOL(self.allowsBackgroundReloading)],
         [NSString stringWithFormat:@"allowsReloadingOnTooManyUpdates: %@", IGListDebugBOOL(self.allowsReloadingOnTooManyUpdates)]
     ];
     [debug addObjectsFromArray:IGListDebugIndentedLines(options)];

Source/IGListKit/Internal/IGListBatchUpdateTransaction.m

@@ -95,13 +95,6 @@ typedef NS_ENUM (NSInteger, IGListBatchUpdateTransactionMode) {
     // disables multiple performBatchUpdates: from happening at the same time
     self.state = IGListBatchUpdateStateQueuedBatchUpdate;
 
-    // if the collection view isn't in a visible window, skip diffing and batch updating. execute all transition blocks,
-    // reload data, execute completion blocks, and get outta here
-    if (self.config.allowsBackgroundReloading && self.collectionView.window == nil) {
-        [self _reload];
-        return;
-    }
-
     [self _diff];
 }
 

Source/IGListKit/Internal/IGListUpdateTransactable.h

@@ -19,7 +19,6 @@ typedef struct {
     BOOL sectionMovesAsDeletesInserts;
     BOOL singleItemSectionUpdates;
     BOOL preferItemReloadsForSectionReloads;
-    BOOL allowsBackgroundReloading;
     BOOL allowsReloadingOnTooManyUpdates;
     IGListExperiment experiments;
 } IGListUpdateTransactationConfig;

Tests/IGListAdapterUpdaterTests.m

@@ -533,7 +533,6 @@
 }
 
 - (void)test_whenCollectionViewNotInWindow_andBackgroundReloadFlag_isSetNO_diffHappens {
-    self.updater.allowsBackgroundReloading = NO;
     [self.collectionView removeFromSuperview];
 
     id mockDelegate = [OCMockObject niceMockForProtocol:@protocol(IGListAdapterUpdaterDelegate)];
@@ -557,57 +556,6 @@
     [mockDelegate verify];
 }
 
-- (void)test_whenCollectionViewNotInWindow_andBackgroundReloadFlag_isDefaultYES_fallbackToReload {
-    [self.collectionView removeFromSuperview];
-
-    id mockDelegate = [OCMockObject niceMockForProtocol:@protocol(IGListAdapterUpdaterDelegate)];
-    self.updater.delegate = mockDelegate;
-
-    [mockDelegate setExpectationOrderMatters:YES];
-    [[mockDelegate expect] listAdapterUpdater:self.updater willReloadDataWithCollectionView:self.collectionView isFallbackReload:YES];
-    [[mockDelegate expect] listAdapterUpdater:self.updater didReloadDataWithCollectionView:self.collectionView isFallbackReload:YES];
-
-    XCTestExpectation *expectation = genExpectation;
-    NSArray *to = @[
-        [IGSectionObject sectionWithObjects:@[]]
-    ];
-    [self.updater performUpdateWithCollectionViewBlock:[self collectionViewBlock]
-                                              animated:NO
-                                      sectionDataBlock:[self dataBlockFromObjects:self.dataSource.sections toObjects:to]
-                                 applySectionDataBlock:self.applySectionDataBlock
-                                            completion:^(BOOL finished) {
-        [expectation fulfill];
-    }];
-    waitExpectation;
-    [mockDelegate verify];
-}
-
-- (void)test_whenCollectionViewNotInWindow_andBackgroundReloadFlag_isDefaultYES_andDataSourceWasSetToNilBefore_fallbackToReload {
-    [self.collectionView removeFromSuperview];
-
-    id mockDelegate = [OCMockObject niceMockForProtocol:@protocol(IGListAdapterUpdaterDelegate)];
-    self.updater.delegate = mockDelegate;
-
-    [mockDelegate setExpectationOrderMatters:YES];
-    [[mockDelegate expect] listAdapterUpdater:self.updater willReloadDataWithCollectionView:self.collectionView isFallbackReload:YES];
-    [[mockDelegate expect] listAdapterUpdater:self.updater didReloadDataWithCollectionView:self.collectionView isFallbackReload:YES];
-
-    XCTestExpectation *expectation = genExpectation;
-    NSArray *to = @[
-        [IGSectionObject sectionWithObjects:@[]]
-    ];
-    self.collectionView.dataSource = nil;
-    [self.updater performUpdateWithCollectionViewBlock:[self collectionViewBlock]
-                                              animated:NO
-                                      sectionDataBlock:[self dataBlockFromObjects:self.dataSource.sections toObjects:to]
-                                 applySectionDataBlock:self.applySectionDataBlock
-                                            completion:^(BOOL finished) {
-        [expectation fulfill];
-    }];
-    waitExpectation;
-    [mockDelegate verify];
-}
-
 - (void)test_whenReloadBatchedWithUpdate_thatCompletionBlockStillExecuted {
     IGSectionObject *object = [IGSectionObject sectionWithObjects:@[@0, @1, @2]];
     self.dataSource.sections = @[object];

Tests/IGListBindingSectionControllerTests.m

@@ -413,7 +413,6 @@
 
     IGListCollectionViewLayout *layout = [[IGListCollectionViewLayout alloc] initWithStickyHeaders:NO topContentInset:0 stretchToEdge:NO];
     self.collectionView = [[UICollectionView alloc] initWithFrame:self.frame collectionViewLayout:layout];
-    [(IGListAdapterUpdater *)self.adapter.updater setAllowsBackgroundReloading:NO];
     self.adapter.experiments |= IGListExperimentInvalidateLayoutForUpdates;
 
     [self setupWithObjects:@[