Fabric: Trivial implementation of prelumiary view allocation on iOS

Summary:
@public
We have this feature in the current version of RN, so it would be nice to support that in Fabric as well. This should save us tens of ms of views creation during mounting.
And that's quite easy to do!

Reviewed By: fkgozali

Differential Revision: D8701992

fbshipit-source-id: 4e3049df009ffd65bb43043de388e81795e5e559

Author: shergin

React/Fabric/Mounting/RCTComponentViewRegistry.h

@@ -43,6 +43,11 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (ReactTag)tagByComponentView:(UIView<RCTComponentViewProtocol> *)componentView;
 
+/**
+ * Creates a component view with a given type and puts it to the recycle pool.
+ */
+- (void)preliminaryCreateComponentViewWithName:(NSString *)componentName;
+
 @end
 
 NS_ASSUME_NONNULL_END

React/Fabric/Mounting/RCTComponentViewRegistry.mm

@@ -64,7 +64,7 @@ + (void)unregisterView:(UIView *)view
 
 #endif
 
-const NSInteger RCTComponentViewRegistryRecyclePoolMaxSize = 256;
+const NSInteger RCTComponentViewRegistryRecyclePoolMaxSize = 1024;
 
 @implementation RCTComponentViewRegistry {
   NSMapTable<id, UIView<RCTComponentViewProtocol> *> *_registry;
@@ -121,6 +121,13 @@ - (void)enqueueComponentViewWithName:(NSString *)componentName
   [self _enqueueComponentViewWithName:componentName componentView:componentView];
 }
 
+- (void)preliminaryCreateComponentViewWithName:(NSString *)componentName
+{
+  RCTAssertMainQueue();
+  [self _enqueueComponentViewWithName:componentName
+                        componentView:[self _createComponentViewWithName:componentName]];
+}
+
 - (UIView<RCTComponentViewProtocol> *)componentViewByTag:(ReactTag)tag
 {
   RCTAssertMainQueue();

React/Fabric/Mounting/RCTMountingManager.h

@@ -26,10 +26,18 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * Transfroms mutation insturctions to mount items and execute them.
  * The order of mutation tnstructions matters.
+ * Can be called from any thread.
  */
 - (void)mutateComponentViewTreeWithMutationInstructions:(facebook::react::TreeMutationInstructionList)instructions
                                                 rootTag:(ReactTag)rootTag;
 
+/**
+ * Suggests preliminary creation of a component view of given type.
+ * The receiver is free to ignore the request.
+ * Can be called from any thread.
+ */
+- (void)preliminaryCreateComponentViewWithName:(NSString *)componentName;
+
 @end
 
 NS_ASSUME_NONNULL_END

React/Fabric/Mounting/RCTMountingManager.mm

@@ -181,4 +181,11 @@ - (void)_performMountItems:(NSArray<RCTMountItemProtocol> *)mountItems
   [self.delegate mountingManager:self didMountComponentsWithRootTag:rootTag];
 }
 
+- (void)preliminaryCreateComponentViewWithName:(NSString *)componentName
+{
+  RCTExecuteOnMainQueue(^{
+    [self->_componentViewRegistry preliminaryCreateComponentViewWithName:componentName];
+  });
+}
+
 @end

React/Fabric/RCTScheduler.h

@@ -19,7 +19,7 @@ NS_ASSUME_NONNULL_BEGIN
 @class RCTMountingManager;
 
 /**
- * Exacly same semantic as `facebook::react::SchedulerDelegate`.
+ * Exactly same semantic as `facebook::react::SchedulerDelegate`.
  */
 @protocol RCTSchedulerDelegate
 

React/Fabric/RCTSurfacePresenter.mm

@@ -67,15 +67,18 @@ - (void)dealloc
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 }
 
-- (void)schedulerDidComputeMutationInstructions:(facebook::react::TreeMutationInstructionList)instructions rootTag:(ReactTag)rootTag
+#pragma mark - RCTSchedulerDelegate
+
+- (void)schedulerDidComputeMutationInstructions:(facebook::react::TreeMutationInstructionList)instructions
+                                        rootTag:(ReactTag)rootTag
 {
   [_mountingManager mutateComponentViewTreeWithMutationInstructions:instructions
                                                             rootTag:rootTag];
 }
 
 - (void)schedulerDidRequestPreliminaryViewAllocationWithComponentName:(NSString *)componentName
 {
-  // TODO: To be implemeted.
+  [_mountingManager preliminaryCreateComponentViewWithName:componentName];
 }
 
 #pragma mark - Internal Surface-dedicated Interface