Add _inBackground for app events to indicate whether the events were …

…logged when app in background mode

Summary: As title

Reviewed By: YOUDAN

Differential Revision: D14576211

fbshipit-source-id: 1217b0fbbe94d6d5f604bf8943e8d61b40a9f6c3

FBSDKCoreKit/FBSDKCoreKit/AppEvents/FBSDKAppEvents.m

@@ -235,6 +235,7 @@
 NSString *const FBSDKAppEventParameterLogTime = @"_logTime";
 NSString *const FBSDKAppEventParameterEventName = @"_eventName";
 NSString *const FBSDKAppEventParameterImplicitlyLogged = @"_implicitlyLogged";
+NSString *const FBSDKAppEventParameterInBackground = @"_inBackground";
 
 NSString *const FBSDKAppEventParameterLiveStreamingPrevStatus    = @"live_streaming_prev_status";
 NSString *const FBSDKAppEventParameterLiveStreamingStatus        = @"live_streaming_status";
@@ -348,6 +349,7 @@ @implementation FBSDKAppEvents
 #endif
   NSString *_userID;
   BOOL _isUnityInit;
+  UIApplicationState _applicationState;
 }
 
 #pragma mark - Object Lifecycle
@@ -397,6 +399,12 @@ - (void)registerNotifications {
    selector:@selector(applicationDidBecomeActive)
    name:UIApplicationDidBecomeActiveNotification
    object:NULL];
+
+  [[NSNotificationCenter defaultCenter]
+   addObserver:self
+   selector:@selector(applicationDidEnterBackground)
+   name:UIApplicationDidEnterBackgroundNotification
+   object:NULL];
 }
 
 - (void)dealloc
@@ -1047,6 +1055,10 @@ - (void)instanceLogEvent:(FBSDKAppEventName)eventName
     eventDictionary[FBSDKAppEventParameterImplicitlyLogged] = @"1";
   }
 
+  if (_applicationState == UIApplicationStateBackground) {
+    eventDictionary[FBSDKAppEventParameterInBackground] = @"1";
+  }
+
   NSString *currentViewControllerName;
   if ([NSThread isMainThread]) {
     // We only collect the view controller when on the main thread, as the behavior off
@@ -1277,6 +1289,8 @@ - (void)flushTimerFired:(id)arg
 
 - (void)applicationDidBecomeActive
 {
+  _applicationState = UIApplicationStateActive;
+
   [FBSDKAppEventsUtility ensureOnMainThread:NSStringFromSelector(_cmd) className:NSStringFromClass([self class])];
 
   [self checkPersistedEvents];
@@ -1285,6 +1299,11 @@ - (void)applicationDidBecomeActive
   [FBSDKTimeSpentData restore:NO];
 }
 
+- (void)applicationDidEnterBackground
+{
+  _applicationState = UIApplicationStateBackground;
+}
+
 - (void)applicationMovingFromActiveStateOrTerminating
 {
   // When moving from active state, we don't have time to wait for the result of a flush, so