Add snapshot test for ASBackgroundLayoutSpec (#2131)

AsyncDisplayKitTests/ASBackgroundLayoutSpecSnapshotTests.mm

@@ -0,0 +1,42 @@
+//
+//  ASOverlayLayoutSpecSnapshotTests.mm
+//  AsyncDisplayKit
+//
+//  Copyright (c) 2014-present, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+
+#import "ASLayoutSpecSnapshotTestsHelper.h"
+
+#import "ASBackgroundLayoutSpec.h"
+#import "ASCenterLayoutSpec.h"
+
+static const ASSizeRange kSize = {{320, 320}, {320, 320}};
+
+@interface ASBackgroundLayoutSpecSnapshotTests : ASLayoutSpecSnapshotTestCase
+
+@end
+
+@implementation ASBackgroundLayoutSpecSnapshotTests
+
+- (void)testBackground
+{
+  ASDisplayNode *backgroundNode = ASDisplayNodeWithBackgroundColor([UIColor blueColor]);
+  ASStaticSizeDisplayNode *foregroundNode = ASDisplayNodeWithBackgroundColor([UIColor blackColor]);
+  foregroundNode.staticSize = {20, 20};
+  
+  ASLayoutSpec *layoutSpec =
+  [ASBackgroundLayoutSpec
+   backgroundLayoutSpecWithChild:
+   [ASCenterLayoutSpec
+    centerLayoutSpecWithCenteringOptions:ASCenterLayoutSpecCenteringXY
+    sizingOptions:{}
+    child:foregroundNode]
+   background:backgroundNode];
+  
+  [self testLayoutSpec:layoutSpec sizeRange:kSize subnodes:@[backgroundNode, foregroundNode] identifier: nil];
+}
+
+@end