[ASDisplayNode] Provide safeAreaInsets and layoutMargins bridge (#685)

* [ASDisplayNode] Add safeAreaInsets, layoutMargins and related properties to ASDisplayNode

* Add layoutMargins bridged to the underlying view

* Add safeAreaInsets bridged to the underlying view

* Add fallback calculation of safeAreaInsets for old iOS versions

* Add automaticallyRelayoutOnSafeAreaChanges and automaticallyRelayoutOnLayoutMarginsChanges properties

* Add additionalSafeAreaInsets property to ASViewController for compatibility with old iOS versions

* Provide safeAreaInsets for layer-backed nodes.

This also fixes tests.

* Fix crash when insetsLayoutMarginsFromSafeArea is set from a background thread

* Changes requested at code review:
* Update documentation for layoutMargins and safeAreaInsets properties. Suggest that users set the automaticallyRelayout* flags to ensure that their layout is synchronized to the margin's values.
* Fix accessing ASDisplayNode internal structures without a lock.
* Add shortcut in -[ASDisplayNode _fallbackUpdateSafeAreaOnChildren] to skip a child when possible.
* Add shortcut in ASViewController to avoid fallback safe area insets recalculation in iOS 11. Fix fallback safe area insets recalculation when the additionalSafeAreaInsets are set.
* Add debug check that a view controller's node is never reused without its view controller, so the viewControllerRoot flag value is always consistent.
* Use getters instead of reading ivars directly in -layoutMarginsDidChange and -safeAreaInsetsDidChange.

* Minor change in CHANGELOG

* Minor change in ASDisplayNodeTests.mm

Source/ASViewController.mm

@@ -32,6 +32,7 @@
   NSInteger _visibilityDepth;
   BOOL _selfConformsToRangeModeProtocol;
   BOOL _nodeConformsToRangeModeProtocol;
+  UIEdgeInsets _fallbackAdditionalSafeAreaInsets;
 }
 
 - (instancetype)initWithNibName:(NSString *)nibNameOrNil bundle:(NSBundle *)nibBundleOrNil
@@ -73,10 +74,14 @@
   if (_node == nil) {
     return;
   }
+
+  _node.viewControllerRoot = YES;
   
   _selfConformsToRangeModeProtocol = [self conformsToProtocol:@protocol(ASRangeControllerUpdateRangeProtocol)];
   _nodeConformsToRangeModeProtocol = [_node conformsToProtocol:@protocol(ASRangeControllerUpdateRangeProtocol)];
   _automaticallyAdjustRangeModeBasedOnViewEvents = _selfConformsToRangeModeProtocol || _nodeConformsToRangeModeProtocol;
+
+  _fallbackAdditionalSafeAreaInsets = UIEdgeInsetsZero;
   
   // In case the node will get loaded
   if (_node.nodeLoaded) {
@@ -159,6 +164,20 @@
     [_node recursivelyEnsureDisplaySynchronously:YES];
   }
   [super viewDidLayoutSubviews];
+
+  if (!AS_AT_LEAST_IOS11) {
+    [self _updateNodeFallbackSafeArea];
+  }
+}
+
+- (void)_updateNodeFallbackSafeArea
+{
+  UIEdgeInsets safeArea = UIEdgeInsetsMake(self.topLayoutGuide.length, 0, self.bottomLayoutGuide.length, 0);
+  UIEdgeInsets additionalInsets = self.additionalSafeAreaInsets;
+
+  safeArea = ASConcatInsets(safeArea, additionalInsets);
+
+  _node.fallbackSafeAreaInsets = safeArea;
 }
 
 ASVisibilityDidMoveToParentViewController;
@@ -264,6 +283,25 @@ ASVisibilityDepthImplementation;
   return _node.interfaceState;
 }
 
+- (UIEdgeInsets)additionalSafeAreaInsets
+{
+  if (AS_AVAILABLE_IOS(11.0)) {
+    return super.additionalSafeAreaInsets;
+  }
+
+  return _fallbackAdditionalSafeAreaInsets;
+}
+
+- (void)setAdditionalSafeAreaInsets:(UIEdgeInsets)additionalSafeAreaInsets
+{
+  if (AS_AVAILABLE_IOS(11.0)) {
+    [super setAdditionalSafeAreaInsets:additionalSafeAreaInsets];
+  } else {
+    _fallbackAdditionalSafeAreaInsets = additionalSafeAreaInsets;
+    [self _updateNodeFallbackSafeArea];
+  }
+}
+
 #pragma mark - ASTraitEnvironment
 
 - (ASPrimitiveTraitCollection)primitiveTraitCollectionForUITraitCollection:(UITraitCollection *)traitCollection