Add Sizing API that doesn't change view's frame.

Summary: This exposes an API so people can find out the resulting size of a layout calculation without changing the frame of the view.

Reviewed By: emilsjolander

Differential Revision: D4124630

fbshipit-source-id: f2b28d8a5474857cb1c92e021a1f161806826cda

uikit/CSSLayout/Tests/CSSLayoutTests.m

@@ -58,4 +58,24 @@
   XCTAssertFalse([view css_usesFlexbox]);
 }
 
+- (void)testSizeThatFitsAsserts
+{
+  UIView *view = [[UIView alloc] initWithFrame:CGRectZero];
+  XCTAssertThrows([view css_sizeThatFits:CGSizeZero]);
+
+  dispatch_sync(dispatch_queue_create("com.facebook.CSSLayout.testing", DISPATCH_QUEUE_SERIAL), ^(void){
+    XCTAssertThrows([view css_sizeThatFits:CGSizeZero]);
+  });
+}
+
+- (void)testSizeThatFitsSmoke
+{
+  UIView *view = [[UIView alloc] initWithFrame:CGRectZero];
+  [view css_setUsesFlexbox:YES];
+
+  const CGSize constrainedSize = CGSizeMake(50, 50);
+  const CGSize actualSize = [view css_sizeThatFits:constrainedSize];
+  XCTAssertTrue(CGSizeEqualToSize(constrainedSize, actualSize), @"Actual Size: %@", NSStringFromCGSize(actualSize));
+}
+
 @end

uikit/CSSLayout/UIView+CSSLayout.h

@@ -45,7 +45,10 @@
 // Get the resolved direction of this node. This won't be CSSDirectionInherit
 - (CSSDirection)css_resolvedDirection;
 
-// Perform a layout calculation and update the frames of the views in the hierarchy with th results
+//! @abstract Perform a layout calculation and update the frames of the views in the hierarchy with th results
 - (void)css_applyLayout;
 
+//! @abstract Compute the size of a layout with a constrained size.
+- (CGSize)css_sizeThatFits:(CGSize)constrainedSize;
+
 @end

uikit/CSSLayout/UIView+CSSLayout.m

@@ -180,20 +180,29 @@
   return CSSNodeLayoutGetDirection([self cssNode]);
 }
 
-- (void)css_applyLayout
+- (CGSize)css_sizeThatFits:(CGSize)constrainedSize
 {
-  NSAssert([NSThread isMainThread], @"Method called using a thread other than main!");
-  NSAssert([self css_usesFlexbox], @"css_applyLayout must be called on a node using css styling!");
+  NSAssert([NSThread isMainThread], @"CSS Layout calculation must be done on main.");
+  NSAssert([self css_usesFlexbox], @"CSS Layout is not enabled for this view.");
 
   _attachNodesRecursive(self);
 
-  CSSNodeRef node = [self cssNode];
+  const CSSNodeRef node = [self cssNode];
   CSSNodeCalculateLayout(
-    [self cssNode],
-    CSSNodeStyleGetWidth(node),
-    CSSNodeStyleGetHeight(node),
+    node,
+    constrainedSize.width,
+    constrainedSize.height,
     CSSNodeStyleGetDirection(node));
 
+  return (CGSize) {
+    .width = CSSNodeLayoutGetWidth(node),
+    .height = CSSNodeLayoutGetHeight(node),
+  };
+}
+
+- (void)css_applyLayout
+{
+  [self css_sizeThatFits:self.bounds.size];
   _updateFrameRecursive(self);
 }