Speed up Flipper by using background queue for serialization

Reviewed By: jknoxville

Differential Revision: D21290732

fbshipit-source-id: 21fcb793900a6517d764fa3a581255fd75a39801

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutPlugin/FlipperKitLayoutPlugin.h

@@ -10,6 +10,7 @@
 #import <Foundation/Foundation.h>
 
 #import <FlipperKit/FlipperPlugin.h>
+#import <FlipperKit/SKMacros.h>
 
 #import "SKDescriptorMapper.h"
 #import "SKInvalidation.h"
@@ -29,4 +30,7 @@
 
 @end
 
+/** Exposed for tests only. */
+SK_EXTERN_C dispatch_queue_t SKLayoutPluginSerialBackgroundQueue(void);
+
 #endif

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutPlugin/FlipperKitLayoutPlugin.mm

@@ -258,7 +258,10 @@
     [elements addObject:node];
   }
 
+  // Converting to folly::dynamic is expensive, do it on a bg queue:
+  dispatch_async(SKLayoutPluginSerialBackgroundQueue(), ^{
     [responder success:@{@"elements" : elements}];
+  });
 }
 
 - (void)onCallSetData:(NSString*)objectId
@@ -560,4 +563,22 @@
 
 @end
 
+/**
+ Operations like converting NSDictionary to folly::dynamic can be expensive.
+ Do them on this serial background queue to avoid blocking the main thread.
+ (Of course, ideally we wouldn't bother with building NSDictionary objects
+ in the first place, in favor of just using folly::dynamic directly...)
+ */
+dispatch_queue_t SKLayoutPluginSerialBackgroundQueue(void) {
+  static dispatch_queue_t queue;
+  static dispatch_once_t onceToken;
+  dispatch_once(&onceToken, ^{
+    queue = dispatch_queue_create("flipper.layout.bg", DISPATCH_QUEUE_SERIAL);
+    // This should be relatively high priority, to prevent Flipper lag.
+    dispatch_set_target_queue(
+        queue, dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0));
+  });
+  return queue;
+}
+
 #endif

iOS/Plugins/FlipperKitLayoutPlugin/SonarKitLayoutPluginTests/SonarKitLayoutPluginTests.m

@@ -67,6 +67,11 @@
   SonarReceiver receiver = connection.receivers[@"getNodes"];
   receiver(@{@"ids" : @[]}, responder);
 
+  dispatch_barrier_sync(
+      SKLayoutPluginSerialBackgroundQueue(),
+      ^{
+      });
+
   XCTAssertTrue(([responder.successes containsObject:@{@"elements" : @[]}]));
 }
 
@@ -96,6 +101,11 @@
   receiver(
       @{@"ids" : @[ @"testNode1", @"testNode2", @"testNode3" ]}, responder);
 
+  dispatch_barrier_sync(
+      SKLayoutPluginSerialBackgroundQueue(),
+      ^{
+      });
+
   XCTAssertTrue(([responder.successes containsObject:@{
     @"elements" : @[
       @{
@@ -149,6 +159,11 @@
   SonarReceiver getNodesCall = connection.receivers[@"getNodes"];
   getNodesCall(@{@"ids" : @[ @"testNode1", @"testNode2" ]}, responder);
 
+  dispatch_barrier_sync(
+      SKLayoutPluginSerialBackgroundQueue(),
+      ^{
+      });
+
   SonarReceiver setHighlighted = connection.receivers[@"setHighlighted"];
   setHighlighted(@{@"id" : @"testNode2"}, responder);
 
@@ -249,6 +264,11 @@
   connection.receivers[@"getRoot"](@{}, responder);
   connection.receivers[@"getNodes"](@{@"ids" : @[ @"testNode2" ]}, responder);
 
+  dispatch_barrier_sync(
+      SKLayoutPluginSerialBackgroundQueue(),
+      ^{
+      });
+
   // Modify the name of testNode3
   connection.receivers[@"setData"](
       @{