Introduce a variant of RepositoryWatcher which returns watched files …

client/java/src/main/java/com/linecorp/centraldogma/internal/client/AbstractWatcher.java

@@ -106,7 +106,7 @@ private enum State {
     }
 
     private final CentralDogma client;
-    private final ScheduledExecutorService watchScheduler;
+    protected final ScheduledExecutorService watchScheduler;
     private final String projectName;
     private final String repositoryName;
     private final String pathPattern;

client/java/src/main/java/com/linecorp/centraldogma/internal/client/FilesWatcher.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright 2021 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you 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:
+ *
+ *   https://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.linecorp.centraldogma.internal.client;
+
+import static java.util.Objects.requireNonNull;
+
+import java.util.AbstractMap.SimpleEntry;
+import java.util.Map;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.function.Function;
+
+import com.google.common.collect.ImmutableMap;
+
+import com.linecorp.centraldogma.client.CentralDogma;
+import com.linecorp.centraldogma.client.Latest;
+import com.linecorp.centraldogma.common.Entry;
+import com.linecorp.centraldogma.common.Revision;
+
+/**
+ * Similar to {@link RepositoryWatcher} but retrieves files after receive a change notification.
+ * @param <T>
+ */
+public final class FilesWatcher<T> extends AbstractWatcher<Map<String, Entry<T>>> {
+    private final String pathPattern;
+    private final Function<Entry<?>, ? extends Entry<T>> function;
+
+    /**
+     * Creates a new instance.
+     */
+    public FilesWatcher(CentralDogma client, ScheduledExecutorService watchScheduler,
+                        String projectName, String repositoryName,
+                        String pathPattern, Function<Entry<?>, ? extends Entry<T>> function) {
+        super(client, watchScheduler, projectName, repositoryName, pathPattern);
+        this.pathPattern = requireNonNull(pathPattern, "pathPattern");
+        this.function = requireNonNull(function, "function");
+    }
+
+    @Override
+    protected CompletableFuture<Latest<Map<String, Entry<T>>>> doWatch(CentralDogma client, String projectName,
+                                                                       String repositoryName,
+                                                                       Revision lastKnownRevision) {
+        return client.watchRepository(projectName, repositoryName, lastKnownRevision, pathPattern)
+                     .thenComposeAsync(revision -> {
+                         if (revision == null) {
+                             return CompletableFuture.completedFuture(null);
+                         }
+                         return client.getFiles(projectName, repositoryName, revision, pathPattern)
+                                      .thenApply(files -> {
+                                          if (files == null) {
+                                              return null;
+                                          }
+                                          return new Latest<>(revision, convert(files));
+                                      });
+                     }, watchScheduler);
+    }
+
+    private Map<String, Entry<T>> convert(Map<String, Entry<?>> files) {
+        return files.entrySet().stream()
+                    .map(e -> new SimpleEntry<>(e.getKey(), function.apply(e.getValue())))
+                    .collect(ImmutableMap.toImmutableMap(SimpleEntry::getKey,
+                                                         SimpleEntry::getValue));
+    }
+}