Set nodes dirty when sizeThatFits changes

Summary:
In YogaKit, nodes are not dirtied when `sizeThatFits` of leaf nodes change. This makes it difficult to reuse views like `UILabel` when their content (text) changes. I wrote a failing test to prove it and wrote a fix by caching the latest value of `sizeThatFits` in `YGNodeBridge` and checking for changes in `YGAttachNodesFromViewHierachy` (renamed to `YGUpdateNodesFromViewHierachy` to make it clear it's not simply reattaching nodes anymore). When a new size is detected, I call `YGNodeMarkDirty` on the leaf node.

**Unfortunately**, this makes another test fail for no logical reason (`testThatViewNotIncludedInFirstLayoutPassAreIncludedInSecond`). Either there's something I don't understand or I've un-advertedly unearthed a bug in the C implementation, but in what case would setting a node dirty make the layout return a different size?
Closes https://github.com/facebook/yoga/pull/308

Reviewed By: emilsjolander

Differential Revision: D4378826

Pulled By: dshahidehpour

fbshipit-source-id: cd0640bc863debb9e434370739416e3e33e57a08

YogaKit/Tests/YogaKitTests.m

@@ -96,6 +96,29 @@
   XCTAssertTrue(CGSizeEqualToSize(CGSizeMake(514,21), containerSize), @"Size is actually %@", NSStringFromCGSize(containerSize));
 }
 
+- (void)testThatMarkingLeafsAsDirtyWillTriggerASizeRecalculation
+{
+  UIView *container = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 500, 50)];
+  [container yg_setUsesYoga:YES];
+  [container yg_setFlexDirection:YGFlexDirectionRow];
+  [container yg_setAlignItems:YGAlignFlexStart];
+
+  UILabel *label = [[UILabel alloc] initWithFrame:CGRectZero];
+  label.text = @"This is a short text.";
+  label.numberOfLines = 1;
+  [label yg_setUsesYoga:YES];
+  [container addSubview:label];
+
+  [container yg_applyLayout];
+  XCTAssertTrue(CGSizeEqualToSize(CGSizeMake(146,21), label.bounds.size), @"Size is actually %@", NSStringFromCGSize(label.bounds.size));
+
+  label.text = @"This is a slightly longer text.";
+  [label yg_markDirty];
+
+  [container yg_applyLayout];
+  XCTAssertTrue(CGSizeEqualToSize(CGSizeMake(213,21), label.bounds.size), @"Size is actually %@", NSStringFromCGSize(label.bounds.size));
+}
+
 - (void)testFrameAndOriginPlacement
 {
   const CGSize containerSize = CGSizeMake(320, 50);

YogaKit/UIView+Yoga.h

@@ -75,4 +75,9 @@
  */
 - (BOOL)yg_isLeaf;
 
+/**
+ Mark that a view's layout needs to be recalculated. Only works for leaf views.
+ */
+- (void)yg_markDirty;
+
 @end

YogaKit/UIView+Yoga.m

@@ -11,6 +11,8 @@
 
 #import <objc/runtime.h>
 
+static const void *kYGNodeBridgeAssociatedKey = &kYGNodeBridgeAssociatedKey;
+
 @interface YGNodeBridge : NSObject
 @property (nonatomic, assign, readonly) YGNodeRef cnode;
 @end
@@ -39,6 +41,14 @@
 
 @implementation UIView (Yoga)
 
+- (void)yg_markDirty
+{
+  YGNodeBridge *const bridge = objc_getAssociatedObject(self, kYGNodeBridgeAssociatedKey);
+  if (bridge != nil && [self yg_isLeaf]) {
+    YGNodeMarkDirty(bridge.cnode);
+  }
+}
+
 - (BOOL)yg_usesYoga
 {
   NSNumber *usesYoga = objc_getAssociatedObject(self, @selector(yg_usesYoga));
@@ -226,13 +236,12 @@
 
 - (YGNodeRef)ygNode
 {
-  YGNodeBridge *node = objc_getAssociatedObject(self, @selector(ygNode));
+  YGNodeBridge *node = objc_getAssociatedObject(self, kYGNodeBridgeAssociatedKey);
   if (!node) {
     node = [YGNodeBridge new];
     YGNodeSetContext(node.cnode, (__bridge void *) self);
-    objc_setAssociatedObject(self, @selector(ygNode), node, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
+    objc_setAssociatedObject(self, kYGNodeBridgeAssociatedKey, node, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
   }
-
   return node.cnode;
 }