Improvements to thread safety during closely spaced edit events.

This applies primarily when beginUpdates / endUpdates is not used.
Due to user interaction-driven edits, like reloads or adding content
at the bottom, sometimes this is unavoidable in app design and thus
critical.

I have a diff in flight to make ASDataController / ASRangeController
robust against very aggressive thrash testing, which will be added
both to the unit test suite and this new example project.

examples/ASTableViewStressTest/Sample/ViewController.m

@@ -0,0 +1,112 @@
+/* This file provided by Facebook is for non-commercial testing and evaluation
+ * purposes only.  Facebook reserves all rights not expressly granted.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * FACEBOOK BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#import "ViewController.h"
+
+#import <AsyncDisplayKit/AsyncDisplayKit.h>
+#import <AsyncDisplayKit/ASAssert.h>
+
+#define NumberOfSections 10
+#define NumberOfRowsPerSection 20
+#define NumberOfReloadIterations 50
+
+@interface ViewController () <ASTableViewDataSource, ASTableViewDelegate>
+{
+  ASTableView *_tableView;
+}
+
+@end
+
+
+@implementation ViewController
+
+#pragma mark -
+#pragma mark UIViewController.
+
+- (instancetype)init
+{
+  if (!(self = [super init]))
+    return nil;
+
+  _tableView = [[ASTableView alloc] initWithFrame:CGRectZero style:UITableViewStylePlain asyncDataFetching:YES];
+  _tableView.separatorStyle = UITableViewCellSeparatorStyleNone; // KittenNode has its own separator
+  _tableView.asyncDataSource = self;
+  _tableView.asyncDelegate = self;
+
+  return self;
+}
+
+- (void)viewDidLoad
+{
+  [super viewDidLoad];
+
+  [self.view addSubview:_tableView];
+}
+
+- (void)viewWillLayoutSubviews
+{
+  _tableView.frame = self.view.bounds;
+}
+
+- (void)viewDidAppear:(BOOL)animated
+{
+  [super viewDidAppear:animated];
+  
+  [self thrashTableView];
+}
+
+- (void)thrashTableView
+{
+  // Keep the viewport moderately sized so that new cells are loaded on scrolling
+  ASTableView *tableView = [[ASTableView alloc] initWithFrame:CGRectMake(0, 0, 100, 500)
+                                                        style:UITableViewStylePlain
+                                            asyncDataFetching:NO];
+  
+  tableView.asyncDelegate = self;
+  tableView.asyncDataSource = self;
+  
+  [tableView reloadData];
+  
+  [tableView reloadSections:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(1,2)] withRowAnimation:UITableViewRowAnimationNone];
+  
+  for (int i = 0; i < NumberOfReloadIterations; ++i) {
+    NSInteger randA = arc4random_uniform(NumberOfSections - 1);
+    NSInteger randB = arc4random_uniform(NumberOfSections - 1);
+    
+    [tableView reloadSections:[NSIndexSet indexSetWithIndexesInRange:NSMakeRange(MIN(randA, randB), MAX(randA, randB) - MIN(randA, randB))] withRowAnimation:UITableViewRowAnimationNone];
+    
+    BOOL animated = (arc4random_uniform(1) == 0 ? YES : NO);
+    
+    [tableView setContentOffset:CGPointMake(0, arc4random_uniform(tableView.contentSize.height - tableView.bounds.size.height)) animated:animated];
+    
+    [[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.01]];
+  }
+}
+
+- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
+{
+  return NumberOfSections;
+}
+
+- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
+{
+  return NumberOfRowsPerSection;
+}
+
+- (ASCellNode *)tableView:(ASTableView *)tableView nodeForRowAtIndexPath:(NSIndexPath *)indexPath
+{
+  ASTextCellNode *textCellNode = [ASTextCellNode new];
+  textCellNode.text = indexPath.description;
+  
+  return textCellNode;
+}
+
+@end