Add support for native animations on iOS

Summary:
Currently on iOS animations are being performed on the JS thread. This ports animations over to the native thread and performs them natively. A lot of this work has already been done on Android, but this PR enables a few animation nodes that Android doesn't yet support such as Transform, Multiplication, and Addition nodes.
Also there is a demo of the native animations added to the UIExplorer app.
Closes https://github.com/facebook/react-native/pull/7884

Differential Revision: D3401811

Pulled By: nicklockwood

fbshipit-source-id: c8d750b75e4410923e17eaeb6dcaf079a09942e2

Libraries/NativeAnimation/Nodes/RCTAnimatedNode.m

@@ -0,0 +1,135 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ */
+
+#import "RCTAnimatedNode.h"
+
+#import "RCTDefines.h"
+
+@implementation RCTAnimatedNode
+{
+  NSMutableDictionary<NSNumber *, RCTAnimatedNode *> *_childNodes;
+  NSMutableDictionary<NSNumber *, RCTAnimatedNode *> *_parentNodes;
+}
+
+- (instancetype)initWithTag:(NSNumber *)tag
+                     config:(NSDictionary<NSString *, id> *)config
+{
+  if ((self = [super init])) {
+    _nodeTag = tag;
+    _config = [config copy];
+  }
+  return self;
+}
+
+RCT_NOT_IMPLEMENTED(- (instancetype)init)
+
+- (NSDictionary<NSNumber *, RCTAnimatedNode *> *)childNodes
+{
+  return _childNodes;
+}
+
+- (NSDictionary<NSNumber *, RCTAnimatedNode *> *)parentNodes
+{
+  return _parentNodes;
+}
+
+- (void)addChild:(RCTAnimatedNode *)child
+{
+  if (!_childNodes) {
+    _childNodes = [NSMutableDictionary new];
+  }
+  if (child) {
+    _childNodes[child.nodeTag] = child;
+    [child onAttachedToNode:self];
+  }
+}
+
+- (void)removeChild:(RCTAnimatedNode *)child
+{
+  if (!_childNodes) {
+    return;
+  }
+  if (child) {
+    [_childNodes removeObjectForKey:child.nodeTag];
+    [child onDetachedFromNode:self];
+  }
+}
+
+- (void)onAttachedToNode:(RCTAnimatedNode *)parent
+{
+  if (!_parentNodes) {
+    _parentNodes = [NSMutableDictionary new];
+  }
+  if (parent) {
+    _parentNodes[parent.nodeTag] = parent;
+  }
+}
+
+- (void)onDetachedFromNode:(RCTAnimatedNode *)parent
+{
+  if (!_parentNodes) {
+    return;
+  }
+  if (parent) {
+    [_parentNodes removeObjectForKey:parent.nodeTag];
+  }
+}
+
+- (void)detachNode
+{
+  for (RCTAnimatedNode *parent in _parentNodes.allValues) {
+    [parent removeChild:self];
+  }
+  for (RCTAnimatedNode *child in _childNodes.allValues) {
+    [self removeChild:child];
+  }
+}
+
+- (void)setNeedsUpdate
+{
+  if (_needsUpdate) {
+    // Has already been marked. Stop branch.
+    return;
+  }
+  _needsUpdate = YES;
+  for (RCTAnimatedNode *child in _childNodes.allValues) {
+    [child setNeedsUpdate];
+  }
+}
+
+- (void)cleanupAnimationUpdate
+{
+  if (_hasUpdated) {
+    _needsUpdate = NO;
+    _hasUpdated = NO;
+    for (RCTAnimatedNode *child in _childNodes.allValues) {
+      [child cleanupAnimationUpdate];
+    }
+  }
+}
+
+- (void)updateNodeIfNecessary
+{
+  if (_needsUpdate && !_hasUpdated) {
+    for (RCTAnimatedNode *parent in _parentNodes.allValues) {
+      [parent updateNodeIfNecessary];
+    }
+    [self performUpdate];
+  }
+}
+
+- (void)performUpdate
+{
+  _hasUpdated = YES;
+  // To be overidden by subclasses
+  // This method is called on a node only if it has been marked for update
+  // during the current update loop
+}
+
+@end