From 68461492bc8b885d96c22ae0e4847dd32add6f51 Mon Sep 17 00:00:00 2001
From: Adam Comella <adcom@microsoft.com>
Date: Mon, 28 Aug 2017 22:37:51 -0700
Subject: [PATCH] iOS: Warn about slow main thread React methods

Summary:
When code blocks the UI thread for too long, it's a bad sign because this can prevent the app from remaining responsive. This change helps detect such responsiveness issues by warning when a React method executes on the UI thread longer than some threshold.

**Test Plan**

Changed AppState's getCurrentAppState method to sleep for 500 ms and verified that a warning was emitted:

```
2017-08-17 19:45:29.479 [warn][tid:main][RCTModuleMethod.mm:527] mainThreadWatchdog: invocation of [RCTAppState getCurrentAppState:error:] took 501ms
```

Adam Comella
Microsoft Corp.
Closes https://github.com/facebook/react-native/pull/15542

Differential Revision: D5724764

Pulled By: shergin

fbshipit-source-id: f1dc4bf17d3657c397720a47fabc7f32bf81b7ac
---
 React/Base/RCTModuleMethod.mm | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/React/Base/RCTModuleMethod.mm b/React/Base/RCTModuleMethod.mm
index 1309db17b..755b8d40e 100644
--- a/React/Base/RCTModuleMethod.mm
+++ b/React/Base/RCTModuleMethod.mm
@@ -22,6 +22,10 @@
 #import "RCTProfile.h"
 #import "RCTUtils.h"
 
+#if !defined(RCT_MAIN_THREAD_WATCH_DOG_THRESHOLD) && defined(RCT_DEBUG)
+#define RCT_MAIN_THREAD_WATCH_DOG_THRESHOLD 0.020 // seconds
+#endif
+
 typedef BOOL (^RCTArgumentBlock)(RCTBridge *, NSUInteger, id);
 
 @implementation RCTMethodArgument
@@ -516,7 +520,22 @@ RCT_EXTERN_C_END
   }
 
   // Invoke method
+#ifdef RCT_MAIN_THREAD_WATCH_DOG_THRESHOLD
+  if (RCTIsMainQueue()) {
+    CFTimeInterval start = CACurrentMediaTime();
+    [_invocation invokeWithTarget:module];
+    CFTimeInterval duration = CACurrentMediaTime() - start;
+    if (duration > RCT_MAIN_THREAD_WATCH_DOG_THRESHOLD) {
+      RCTLogWarn(@"mainThreadWatchdog: invocation of %@ blocked the main thread for %dms. Consider spending less time on the main thread to keep the app's UI responsive.",
+                 [self methodName],
+                 (int)(duration * 1000));
+    }
+  } else {
+    [_invocation invokeWithTarget:module];
+  }
+#else
   [_invocation invokeWithTarget:module];
+#endif
 
   index = 2;
   [_retainedObjects removeAllObjects];