Added a DartExecutor API for querying # of pending channel callbacks (#…

…10021)

shell/platform/android/io/flutter/embedding/engine/dart/DartExecutor.java

@@ -196,6 +196,26 @@ public void send(@NonNull String channel, @Nullable ByteBuffer message, @Nullabl
   public void setMessageHandler(@NonNull String channel, @Nullable BinaryMessenger.BinaryMessageHandler handler) {
     messenger.setMessageHandler(channel, handler);
   }
+
+  /**
+   * Returns the number of pending channel callback replies.
+   *
+   * <p>When sending messages to the Flutter application using {@link BinaryMessenger#send(String,
+   * ByteBuffer, io.flutter.plugin.common.BinaryMessenger.BinaryReply)}, developers can optionally
+   * specify a reply callback if they expect a reply from the Flutter application.
+   *
+   * <p>This method tracks all the pending callbacks that are waiting for response, and is supposed
+   * to be called from the main thread (as other methods). Calling from a different thread could
+   * possibly capture an indeterministic internal state, so don't do it.
+   *
+   * <p>Currently, it's mainly useful for a testing framework like Espresso to determine whether all
+   * the async channel callbacks are handled and the app is idle.
+   */
+  @UiThread
+  public int getPendingChannelResponseCount() {
+    return messenger.getPendingChannelResponseCount();
+  }
+
   //------ END BinaryMessenger -----
 
   /**

shell/platform/android/io/flutter/embedding/engine/dart/DartMessenger.java

@@ -118,6 +118,22 @@ public void handlePlatformMessageResponse(int replyId, @Nullable byte[] reply) {
     }
   }
 
+  /**
+   * Returns the number of pending channel callback replies.
+   *
+   * <p>When sending messages to the Flutter application using {@link BinaryMessenger#send(String,
+   * ByteBuffer, io.flutter.plugin.common.BinaryMessenger.BinaryReply)}, developers can optionally
+   * specify a reply callback if they expect a reply from the Flutter application.
+   *
+   * <p>This method tracks all the pending callbacks that are waiting for response, and is supposed
+   * to be called from the main thread (as other methods). Calling from a different thread could
+   * possibly capture an indeterministic internal state, so don't do it.
+   */
+  @UiThread
+  public int getPendingChannelResponseCount() {
+    return pendingReplies.size();
+  }
+
   private static class Reply implements BinaryMessenger.BinaryReply {
     @NonNull
     private final FlutterJNI flutterJNI;
@@ -141,4 +157,4 @@ public void reply(@Nullable ByteBuffer reply) {
       }
     }
   }
-}
+}