Take care of pixel rounding for UIView.

Summary: You can lose a lot of performance on UIView's if they are not pixel-aligned. This protects it from happening in views which use css-layout.

Reviewed By: emilsjolander

Differential Revision: D4140306

fbshipit-source-id: 53493c08c084a7e3dcd4e05f6a665a99340ea5a6

uikit/CSSLayout/Tests/CSSLayoutTests.m

@@ -78,4 +78,33 @@
   XCTAssertTrue(CGSizeEqualToSize(constrainedSize, actualSize), @"Actual Size: %@", NSStringFromCGSize(actualSize));
 }
 
+- (void)testFrameAndOriginPlacement
+{
+  const CGSize containerSize = CGSizeMake(320, 50);
+
+  UIView *container = [[UIView alloc] initWithFrame:CGRectMake(0, 0, containerSize.width, containerSize.height)];
+  [container css_setUsesFlexbox:YES];
+  [container css_setFlexDirection:CSSFlexDirectionRow];
+
+  for (int i = 0; i < 3; i++) {
+    UIView *subview = [[UIView alloc] initWithFrame:CGRectZero];
+    [subview css_setUsesFlexbox:YES];
+    [subview css_setFlexGrow:1];
+
+    [container addSubview:subview];
+  }
+  [container css_applyLayout];
+
+  XCTAssertFalse(CGRectIntersectsRect([container.subviews objectAtIndex:0].frame, [container.subviews objectAtIndex:1].frame));
+  XCTAssertFalse(CGRectIntersectsRect([container.subviews objectAtIndex:1].frame, [container.subviews objectAtIndex:2].frame));
+  XCTAssertFalse(CGRectIntersectsRect([container.subviews objectAtIndex:0].frame, [container.subviews objectAtIndex:2].frame));
+
+  CGFloat totalWidth = 0;
+  for (UIView *view in container.subviews) {
+    totalWidth += view.bounds.size.width;
+  }
+
+  XCTAssertEqual(containerSize.width, totalWidth, @"The container's width is %.6f, the subviews take up %.6f", containerSize.width, totalWidth);
+}
+
 @end

uikit/CSSLayout/UIView+CSSLayout.m

@@ -277,22 +277,43 @@ static void _attachNodesRecursive(UIView *view) {
   }
 }
 
+static CGFloat _roundPixelValue(CGFloat value)
+{
+  static CGFloat scale;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^(){
+    scale = [UIScreen mainScreen].scale;
+  });
+
+  return round(value * scale) / scale;
+}
+
 static void _updateFrameRecursive(UIView *view) {
+  NSAssert([NSThread isMainThread], @"Framesetting should only be done on the main thread.");
   CSSNodeRef node = [view cssNode];
-  const BOOL usesFlexbox = [view css_usesFlexbox];
+
-  const BOOL isLeaf = !usesFlexbox || view.subviews.count == 0;
+  const CGPoint topLeft = {
+    CSSNodeLayoutGetLeft(node),
+    CSSNodeLayoutGetTop(node),
+  };
+
+  const CGPoint bottomRight = {
+    topLeft.x + CSSNodeLayoutGetWidth(node),
+    topLeft.y + CSSNodeLayoutGetHeight(node),
+  };
 
   view.frame = (CGRect) {
     .origin = {
-      .x = CSSNodeLayoutGetLeft(node),
+      .x = _roundPixelValue(topLeft.x),
-      .y = CSSNodeLayoutGetTop(node),
+      .y = _roundPixelValue(topLeft.y),
     },
     .size = {
-      .width = CSSNodeLayoutGetWidth(node),
+      .width = _roundPixelValue(bottomRight.x) - _roundPixelValue(topLeft.x),
-      .height = CSSNodeLayoutGetHeight(node),
+      .height = _roundPixelValue(bottomRight.y) - _roundPixelValue(topLeft.y),
     },
   };
 
+  const BOOL isLeaf = ![view css_usesFlexbox] || view.subviews.count == 0;
   if (!isLeaf) {
     for (NSUInteger i = 0; i < view.subviews.count; i++) {
       _updateFrameRecursive(view.subviews[i]);