Fixes #606 (#610)

Summary:
Fixes #606.

If there are no subviews in `UIView`, yoga assumes that `sizeThatFits:` returns `CGSizeZero`. However, according to [the documentation](https://developer.apple.com/documentation/uikit/uiview/1622625-sizethatfits), `UIView` returns current size if there are no subviews.

This diff adds a check - if there are no subviews, `sizeThatFits:` doesn't get called, and CGSizeZero is returned.
Pull Request resolved: https://github.com/facebook/yoga/pull/610

Reviewed By: davidaurelio

Differential Revision: D6807406

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 9189cf14c393f840122bc365d3827881bf03548c

YogaKit/Source/YGLayout.m

@@ -129,6 +129,7 @@ static YGConfigRef globalConfig;
 @interface YGLayout ()
 
 @property (nonatomic, weak, readonly) UIView *view;
+@property(nonatomic, assign, readonly) BOOL isUIView;
 
 @end
 
@@ -153,6 +154,7 @@ static YGConfigRef globalConfig;
     YGNodeSetContext(_node, (__bridge void *) view);
     _isEnabled = NO;
     _isIncludedInLayout = YES;
+    _isUIView = [view isMemberOfClass:[UIView class]];
   }
 
   return self;
@@ -331,10 +333,20 @@ static YGSize YGMeasureView(
   const CGFloat constrainedHeight = (heightMode == YGMeasureModeUndefined) ? CGFLOAT_MAX: height;
 
   UIView *view = (__bridge UIView*) YGNodeGetContext(node);
-  const CGSize sizeThatFits = [view sizeThatFits:(CGSize) {
+  CGSize sizeThatFits = CGSizeZero;
+
+  // The default implementation of sizeThatFits: returns the existing size of
+  // the view. That means that if we want to layout an empty UIView, which
+  // already has got a frame set, its measured size should be CGSizeZero, but
+  // UIKit returns the existing size.
+  //
+  // See https://github.com/facebook/yoga/issues/606 for more information.
+  if (!view.yoga.isUIView || [view.subviews count] > 0) {
+    sizeThatFits = [view sizeThatFits:(CGSize){
                                           .width = constrainedWidth,
                                           .height = constrainedHeight,
                                       }];
+  }
 
   return (YGSize) {
     .width = YGSanitizeMeasurement(constrainedWidth, sizeThatFits.width, widthMode),

YogaKit/Tests/YogaKitTests.m

@@ -127,6 +127,15 @@
   XCTAssertEqual(longTextLabelSize.width + textBadgeView.yoga.intrinsicSize.width, containerSize.width);
 }
 
+- (void)testSizeThatFitsEmptyView {
+  UIView* view = [[UIView alloc] initWithFrame:CGRectMake(10, 10, 200, 200)];
+  view.yoga.isEnabled = YES;
+
+  const CGSize viewSize = view.yoga.intrinsicSize;
+  XCTAssertEqual(viewSize.height, 0);
+  XCTAssertEqual(viewSize.width, 0);
+}
+
 - (void)testPreservingOrigin
 {
   UIView *container = [[UIView alloc] initWithFrame:CGRectMake(0,0,50,75)];