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/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;
 }