Checks for connectivity when App is moving to foreground. (#4985)

* Checks for connectivity when App is moving to foreground.

* Split MaybeInvokeCallbacks and invoke them when foregrounding and network is available.

Author: wu-hui

Firestore/core/src/firebase/firestore/remote/connectivity_monitor.cc

@@ -33,8 +33,12 @@ void ConnectivityMonitor::MaybeInvokeCallbacks(NetworkStatus new_status) {
   if (new_status == status_) {
     return;
   }
-  status_ = new_status;
 
+  InvokeCallbacks(new_status);
+}
+
+void ConnectivityMonitor::InvokeCallbacks(NetworkStatus new_status) {
+  status_ = new_status;
   for (auto& callback : callbacks_) {
     callback(status_.value());
   }

Firestore/core/src/firebase/firestore/remote/connectivity_monitor.h

@@ -71,6 +71,9 @@ class ConnectivityMonitor {
   // Invokes callbacks only if the status changed.
   void MaybeInvokeCallbacks(NetworkStatus new_status);
 
+  // Invokes callbacks and sets net status to `new_status`.
+  void InvokeCallbacks(NetworkStatus new_status);
+
   const std::shared_ptr<util::AsyncQueue>& queue() {
     return worker_queue_;
   }

Firestore/core/src/firebase/firestore/remote/connectivity_monitor_apple.mm

@@ -18,6 +18,10 @@
 
 #if defined(__APPLE__)
 
+#if TARGET_OS_IOS || TARGET_OS_TV
+#import <UIKit/UIKit.h>
+#endif
+
 #include <SystemConfiguration/SystemConfiguration.h>
 #include <dispatch/dispatch.h>
 #include <netinet/in.h>
@@ -83,7 +87,7 @@ explicit ConnectivityMonitorApple(
       return;
     }
 
-    SCNetworkReachabilityFlags flags;
+    SCNetworkReachabilityFlags flags{};
     if (SCNetworkReachabilityGetFlags(reachability_, &flags)) {
       SetInitialStatus(ToNetworkStatus(flags));
     }
@@ -107,9 +111,23 @@ explicit ConnectivityMonitorApple(
       LOG_DEBUG("Couldn't set reachability queue");
       return;
     }
+
+#if TARGET_OS_IOS || TARGET_OS_TV
+    this->observer_ = [[NSNotificationCenter defaultCenter]
+        addObserverForName:UIApplicationWillEnterForegroundNotification
+                    object:nil
+                     queue:[NSOperationQueue mainQueue]
+                usingBlock:^(NSNotification* note) {
+                  this->OnEnteredForeground();
+                }];
+#endif
   }
 
   ~ConnectivityMonitorApple() {
+#if TARGET_OS_IOS || TARGET_OS_TV
+    [[NSNotificationCenter defaultCenter] removeObserver:this->observer_];
+#endif
+
     if (reachability_) {
       bool success =
           SCNetworkReachabilitySetDispatchQueue(reachability_, nullptr);
@@ -121,13 +139,37 @@ explicit ConnectivityMonitorApple(
     }
   }
 
+#if TARGET_OS_IOS || TARGET_OS_TV
+  void OnEnteredForeground() {
+    SCNetworkReachabilityFlags flags{};
+    if (!SCNetworkReachabilityGetFlags(reachability_, &flags)) return;
+
+    queue()->Enqueue([this, flags] {
+      auto status = ToNetworkStatus(flags);
+      if (status != NetworkStatus::Unavailable) {
+        // There may have been network changes while Firestore was in the
+        // background for which we did not get OnReachabilityChangedCallback
+        // notifications. If entering the foreground and we have a connection,
+        // reset the connection to ensure that RPCs don't have to wait for TCP
+        // timeouts.
+        this->InvokeCallbacks(status);
+      } else {
+        this->MaybeInvokeCallbacks(status);
+      }
+    });
+  }
+#endif
+
   void OnReachabilityChanged(SCNetworkReachabilityFlags flags) {
     queue()->Enqueue(
         [this, flags] { MaybeInvokeCallbacks(ToNetworkStatus(flags)); });
   }
 
  private:
   SCNetworkReachabilityRef reachability_ = nil;
+#if TARGET_OS_IOS || TARGET_OS_TV
+  id<NSObject> observer_ = nil;
+#endif
 };
 
 namespace {