[ASDataController] Use 2 Threads Per CPU When Measuring Nodes (#2145)

* [ASDataController] Use custom apply function to control thread count

* Relax the test for stupid Travis CI

* Remove unneeded import

AsyncDisplayKitTests/ASDispatchTests.m

@@ -0,0 +1,38 @@
+//
+//  ASDispatchTests.m
+//  AsyncDisplayKit
+//
+//  Created by Adlai Holler on 8/25/16.
+//  Copyright © 2016 Facebook. All rights reserved.
+//
+
+#import <XCTest/XCTest.h>
+#import "ASDispatch.h"
+
+@interface ASDispatchTests : XCTestCase
+
+@end
+
+@implementation ASDispatchTests
+
+- (void)testDispatchApply
+{
+  dispatch_queue_t q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
+  NSInteger expectedThreadCount = [NSProcessInfo processInfo].activeProcessorCount * 2;
+  NSLock *lock = [NSLock new];
+  NSMutableSet *threads = [NSMutableSet set];
+  NSMutableIndexSet *indices = [NSMutableIndexSet indexSet];
+  
+  size_t const iterations = 1E5;
+  ASDispatchApply(iterations, q, 0, ^(size_t i) {
+    [lock lock];
+    [threads addObject:[NSThread currentThread]];
+    XCTAssertFalse([indices containsIndex:i]);
+    [indices addIndex:i];
+    [lock unlock];
+  });
+  XCTAssertLessThanOrEqual(threads.count, expectedThreadCount);
+  XCTAssertEqualObjects(indices, [NSIndexSet indexSetWithIndexesInRange:NSMakeRange(0, iterations)]);
+}
+
+@end