1 files changed, 67 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/query2/engine/QueryExpressionEvalListener.java b/src/main/java/com/google/devtools/build/lib/query2/engine/QueryExpressionEvalListener.java
new file mode 100644
index 0000000000..e6bdaef7a9
--- /dev/null
+++ b/src/main/java/com/google/devtools/build/lib/query2/engine/QueryExpressionEvalListener.java
@@ -0,0 +1,67 @@
+// Copyright 2016 The Bazel Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package com.google.devtools.build.lib.query2.engine;
+
+import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
+import java.util.concurrent.ForkJoinPool;
+
+/** Listener for calls to the internal methods of {@link QueryExpression} used for evaluation. */
+@ThreadSafe
+public interface QueryExpressionEvalListener<T> {
+  /** Called right before {@link QueryExpression#evalImpl} is called. */
+  void onEval(
+      QueryExpression expr,
+      QueryEnvironment<T> env,
+      VariableContext<T> context,
+      Callback<T> callback);
+
+  /** Called right before {@link QueryExpression#parEvalImpl} is called. */
+  void onParEval(
+      QueryExpression expr,
+      QueryEnvironment<T> env,
+      VariableContext<T> context,
+      ThreadSafeCallback<T> callback,
+      ForkJoinPool forkJoinPool);
+
+  /** A {@link QueryExpressionEvalListener} that does nothing. */
+  class NullListener<T> implements QueryExpressionEvalListener<T> {
+    private static final NullListener<?> INSTANCE = new NullListener<>();
+
+    private NullListener() {
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <T> NullListener<T> instance() {
+      return (NullListener<T>) INSTANCE;
+    }
+
+    @Override
+    public void onEval(
+        QueryExpression expr,
+        QueryEnvironment<T> env,
+        VariableContext<T> context,
+        Callback<T> callback) {
+    }
+
+    @Override
+    public void onParEval(
+        QueryExpression expr,
+        QueryEnvironment<T> env,
+        VariableContext<T> context,
+        ThreadSafeCallback<T> callback,
+        ForkJoinPool forkJoinPool) {
+    }
+  }
+}
+