[Core] Add async flush method

FirebaseCore/Extension/FIRHeartbeatLogger.h

@@ -68,13 +68,23 @@ NSString *_Nullable FIRHeaderValueFromHeartbeatsPayload(FIRHeartbeatsPayload *he
 - (void)log;
 
 #ifndef FIREBASE_BUILD_CMAKE
-/// Flushes heartbeats from storage into a structured payload of heartbeats.
+/// Synchronously flushes heartbeats from storage into a structured payload of heartbeats.
 ///
 /// This API is for clients using platform logging V2.
 ///
 /// @note This API is thread-safe.
 /// @return A payload of heartbeats.
 - (FIRHeartbeatsPayload *)flushHeartbeatsIntoPayload;
+
+/// Asynchronously flushes heartbeats from storage into a structured payload of heartbeats.
+///
+/// This API is for clients using platform logging V2.
+///
+/// @note This API is thread-safe.
+/// @param completionHandler A completion handler to process the flushed payload of heartbeats.
+- (void)flushHeartbeatsIntoPayloadWithCompletionHandler:
+    (void (^)(FIRHeartbeatsPayload *))completionHandler
+    API_AVAILABLE(ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0));
 #endif  // FIREBASE_BUILD_CMAKE
 
 /// Gets today's corresponding heartbeat code.

FirebaseCore/Sources/FIRHeartbeatLogger.m

@@ -78,6 +78,13 @@ - (FIRHeartbeatsPayload *)flushHeartbeatsIntoPayload {
   FIRHeartbeatsPayload *payload = [_heartbeatController flush];
   return payload;
 }
+
+- (void)flushHeartbeatsIntoPayloadWithCompletionHandler:
+    (void (^)(FIRHeartbeatsPayload *))completionHandler {
+  [_heartbeatController flushAsyncWithCompletionHandler:^(FIRHeartbeatsPayload *payload) {
+    completionHandler(payload);
+  }];
+}
 #endif  // FIREBASE_BUILD_CMAKE
 
 - (FIRDailyHeartbeatCode)heartbeatCodeForToday {

FirebaseCore/Tests/Unit/FIRHeartbeatLoggerTests.m

@@ -125,6 +125,30 @@ - (void)testFlushing_UsingV2API_WhenHeartbeatsAreStored_ReturnsNonEmptyPayload {
               }];
 }
 
+- (void)testFlushingAsync_UsingV2API_WhenHeartbeatsAreStored_ReturnsNonEmptyPayload API_AVAILABLE(
+    ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0)) {
+  // Given
+  FIRHeartbeatLogger *heartbeatLogger = self.heartbeatLogger;
+  NSString *expectedDate = [[self class] formattedStringForDate:[NSDate date]];
+  // When
+  [heartbeatLogger log];
+  XCTestExpectation *expectation = [self expectationWithDescription:@"async flush"];
+  [heartbeatLogger
+      flushHeartbeatsIntoPayloadWithCompletionHandler:^(FIRHeartbeatsPayload *heartbeatsPayload) {
+        // Then
+        [self assertEncodedPayloadHeader:FIRHeaderValueFromHeartbeatsPayload(heartbeatsPayload)
+                    isEqualToPayloadJSON:@{
+                      @"version" : @2,
+                      @"heartbeats" : @[
+                        @{@"agent" : @"dummy_agent",
+                          @"dates" : @[ expectedDate ]}
+                      ]
+                    }];
+        [expectation fulfill];
+      }];
+  [self waitForExpectations:@[ expectation ] timeout:1.0];
+}
+
 - (void)testFlushing_UsingV2API_WhenNoHeartbeatsAreStored_ReturnsEmptyPayload {
   // Given
   FIRHeartbeatLogger *heartbeatLogger = self.heartbeatLogger;
@@ -134,6 +158,21 @@ - (void)testFlushing_UsingV2API_WhenNoHeartbeatsAreStored_ReturnsEmptyPayload {
   [self assertHeartbeatsPayloadIsEmpty:heartbeatsPayload];
 }
 
+- (void)testFlushingAsync_UsingV2API_WhenNoHeartbeatsAreStored_ReturnsEmptyPayload API_AVAILABLE(
+    ios(13.0), macosx(10.15), macCatalyst(13.0), tvos(13.0), watchos(6.0)) {
+  // Given
+  FIRHeartbeatLogger *heartbeatLogger = self.heartbeatLogger;
+  // When
+  XCTestExpectation *expectation = [self expectationWithDescription:@"async flush"];
+  [heartbeatLogger
+      flushHeartbeatsIntoPayloadWithCompletionHandler:^(FIRHeartbeatsPayload *heartbeatsPayload) {
+        // Then
+        [self assertHeartbeatsPayloadIsEmpty:heartbeatsPayload];
+        [expectation fulfill];
+      }];
+  [self waitForExpectations:@[ expectation ] timeout:1.0];
+}
+
 - (void)testLogAndFlushUsingV1API_AndThenFlushAgainUsingV2API_FlushesHeartbeatInTheFirstFlush {
   // Given
   FIRHeartbeatLogger *heartbeatLogger = self.heartbeatLogger;