exposed a way to trigger layout on the uimanager

Summary:
A component can be backed by native "node" that can change its internal state, which would result in a new UI after the next layout. Since js has no way of knowing that this has happened it wouldn't trigger a layout if nothing in js world has changed. Therefore we need a way how to trigger layout from native code.

This diff does it by adding methods `layoutIfNeeded` on the uimanager and `isBatchActive` on the bridge.
When `layoutIfNeeded` is called it checks whether a batch is in progress. If it is we do nothing, since at it's end layout happens. If a batch is not in progress we immidiately do layout.

I went with the easiest way how to implement this - `isBatchActive` is a public method on the bridge. It's not ideal, but consistent with other methods for modules.
public

Reviewed By: jspahrsummers, nicklockwood

Differential Revision: D2748896

fb-gh-sync-id: f3664c4af980d40a463b538e069b26c9ebad6300

React/Base/RCTBatchedBridge.m

@@ -982,6 +982,11 @@ RCT_NOT_IMPLEMENTED(- (instancetype)initWithBundleURL:(__unused NSURL *)bundleUR
   }];
 }
 
+- (BOOL)isBatchActive
+{
+  return _wasBatchActive;
+}
+
 @end
 
 @implementation RCTBatchedBridge(Deprecated)

React/Base/RCTBridge.h

@@ -163,6 +163,11 @@ RCT_EXTERN BOOL RCTBridgeModuleClassIsRegistered(Class);
  */
 - (void)reload;
 
+/**
+ * Says whether bridge has started recieving calls from javascript.
+ */
+- (BOOL)isBatchActive;
+
 @end
 
 /**

React/Base/RCTBridge.m

@@ -287,6 +287,11 @@ RCT_NOT_IMPLEMENTED(- (instancetype)init)
   return self.batchedBridge.valid;
 }
 
+- (BOOL)isBatchActive
+{
+  return [_batchedBridge isBatchActive];
+}
+
 - (void)invalidate
 {
   RCTBatchedBridge *batchedBridge = self.batchedBridge;

React/Modules/RCTUIManager.h

@@ -88,6 +88,12 @@ RCT_EXTERN NSString *const RCTUIManagerRootViewKey;
  */
 @property (atomic, assign) BOOL unsafeFlushUIChangesBeforeBatchEnds;
 
+/**
+ * In some cases we might want to trigger layout from native side.
+ * React won't be aware of this, so we need to make sure it happens.
+ */
+- (void)setNeedsLayout;
+
 @end
 
 /**

React/Modules/RCTUIManager.m

@@ -888,6 +888,15 @@ RCT_EXPORT_METHOD(findSubviewIn:(nonnull NSNumber *)reactTag atPoint:(CGPoint)po
 }
 
 - (void)batchDidComplete
+{
+  [self _layoutAndMount];
+}
+
+/**
+ * Sets up animations, computes layout, creates UI mounting blocks for computed layout,
+ * runs these blocks and all other already existing blocks.
+ */
+- (void)_layoutAndMount
 {
   // Gather blocks to be executed now that all view hierarchy manipulations have
   // been completed (note that these may still take place before layout has finished)
@@ -962,6 +971,15 @@ RCT_EXPORT_METHOD(findSubviewIn:(nonnull NSNumber *)reactTag atPoint:(CGPoint)po
   }
 }
 
+- (void)setNeedsLayout
+{
+  // If there is an active batch layout will happen when batch finished, so we will wait for that.
+  // Otherwise we immidiately trigger layout.
+  if (![_bridge isBatchActive]) {
+    [self _layoutAndMount];
+  }
+}
+
 RCT_EXPORT_METHOD(measure:(nonnull NSNumber *)reactTag
                   callback:(RCTResponseSenderBlock)callback)
 {