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More on ASDataController's main-thread-only mode #1915

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Sep 18, 2020
Merged

More on ASDataController's main-thread-only mode #1915

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47 changes: 35 additions & 12 deletions Source/Details/ASDataController.mm
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Original file line number Diff line number Diff line change
Expand Up @@ -129,7 +129,16 @@ - (void)setLayoutDelegate:(id<ASDataControllerLayoutDelegate>)layoutDelegate

#pragma mark - Cell Layout

/**
* Allocates and layouts nodes from the given collection elements, and blocks the current thread while doing so.
*
* @param elements The elements from which nodes can be allocated and laid out.
* @param strictlyOnCurrentThread Whether or not all the work must be done strictly on the current thread.
* YES means all nodes will be allocated and laid out serially on the current thread.
* NO means the work can be offloaded to other thread(s), potentially reduce the blocking time on the calling thread.
*/
- (void)_allocateNodesFromElements:(NSArray<ASCollectionElement *> *)elements
strictlyOnCurrentThread:(BOOL)strictlyOnCurrentThread
{
NSUInteger nodeCount = elements.count;
__weak id<ASDataControllerSource> weakDataSource = _dataSource;
Expand All @@ -142,12 +151,7 @@ - (void)_allocateNodesFromElements:(NSArray<ASCollectionElement *> *)elements
{
as_activity_create_for_scope("Data controller batch");

dispatch_queue_t queue = dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0);
NSUInteger threadCount = 0;
if ([_dataSource dataControllerShouldSerializeNodeCreation:self]) {
threadCount = 1;
}
ASDispatchApply(nodeCount, queue, threadCount, ^(size_t i) {
void(^work)(size_t) = ^(size_t i) {
__strong id<ASDataControllerSource> strongDataSource = weakDataSource;
if (strongDataSource == nil) {
return;
Expand All @@ -166,7 +170,20 @@ - (void)_allocateNodesFromElements:(NSArray<ASCollectionElement *> *)elements
if (ASSizeRangeHasSignificantArea(sizeRange)) {
[self _layoutNode:node withConstrainedSize:sizeRange];
}
});
};

if (strictlyOnCurrentThread) {
for (NSUInteger i = 0; i < nodeCount; i++) {
work(i);
}
} else {
dispatch_queue_t queue = dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0);
NSUInteger threadCount = 0;
if ([_dataSource dataControllerShouldSerializeNodeCreation:self]) {
threadCount = 1;
}
ASDispatchApply(nodeCount, queue, threadCount, work);
}
}

ASSignpostEnd(DataControllerBatch, self, "count: %lu", (unsigned long)nodeCount);
Expand Down Expand Up @@ -620,8 +637,11 @@ - (void)updateWithChangeSet:(_ASHierarchyChangeSet *)changeSet
Class<ASDataControllerLayoutDelegate> layoutDelegateClass = [self.layoutDelegate class];

// Step 3: Call the layout delegate if possible. Otherwise, allocate and layout all elements
dispatch_block_t step3 = ^{
void (^step3)(BOOL) = ^(BOOL strictlyOnCurrentThread){
if (canDelegate) {
// Don't pass strictlyOnCurrentThread to the layout delegate. Instead give it
// total control over its threading behavior, as long as it blocks the
// calling thread while preparing the layout (which is part of the API contract).
[layoutDelegateClass calculateLayoutWithContext:layoutContext];
} else {
const auto elementsToProcess = [[NSMutableArray<ASCollectionElement *> alloc] init];
Expand All @@ -635,21 +655,24 @@ - (void)updateWithChangeSet:(_ASHierarchyChangeSet *)changeSet
[elementsToProcess addObject:element];
}
}
[self _allocateNodesFromElements:elementsToProcess];
[self _allocateNodesFromElements:elementsToProcess
strictlyOnCurrentThread:strictlyOnCurrentThread];
}
};

// Step 3 can be done on the main thread or on _editingTransactionQueue
// depending on an experiment.
BOOL mainThreadOnly = ASActivateExperimentalFeature(ASExperimentalMainThreadOnlyDataController);
if (mainThreadOnly) {
// We'll still dispatch to _editingTransactionQueue only to schedule a block
// In main-thread-only mode allocate and layout all nodes serially on the main thread.
//
// After this step, we'll still dispatch to _editingTransactionQueue only to schedule a block
// to _mainSerialQueue to execute next steps. This is not optimized because
// in theory we can skip _editingTransactionQueue entirely, but it's much safer
// because change sets will still flow through the pipeline in pretty the same way
// (main thread -> _editingTransactionQueue -> _mainSerialQueue) and so
// any methods that block on _editingTransactionQueue will still work.
step3();
step3(YES);
}

++_editingTransactionGroupCount;
Expand All @@ -658,7 +681,7 @@ - (void)updateWithChangeSet:(_ASHierarchyChangeSet *)changeSet
as_activity_scope_enter(as_activity_create("Prepare nodes for collection update", AS_ACTIVITY_CURRENT, OS_ACTIVITY_FLAG_DEFAULT), &preparationScope);

if (!mainThreadOnly) {
step3();
step3(NO);
}

// Step 4: Inform the delegate on main thread
Expand Down