Add ServerOperation and queue.

pkg/analysis_server/lib/src/operation/operation_queue.dart

+// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library operation.queue;
+
+import 'dart:collection';
+
+import 'package:analysis_server/src/analysis_server.dart';
+import 'package:analysis_server/src/operation/operation.dart';
+
+
+/**
+ * A queue of operations in an [AnalysisServer].
+ */
+class ServerOperationQueue {

[Brian Wilkerson, 2014/06/02 14:32:18]

The implementation of this class makes the assumption that the priority of an
operation cannot change over time, but I believe that the ability to set
priority files makes that assumption wrong.

This might be an indication that an explicit queue is not the best approach.
(Something to think about.)

[scheglov, 2014/06/02 17:56:59]

Done.
Now we do two passes - during the first we look for a priority analysis
operations, and during the second we use operation priorities.

+  final List<Queue<ServerOperation>> queues = <Queue<ServerOperation>>[];
+
+  ServerOperationQueue() {
+    for (int i = 0; i < ServerOperationPriority.COUNT; i++) {
+      var queue = new DoubleLinkedQueue<ServerOperation>();
+      queues.add(queue);
+    }
+  }
+
+  /**
+   * Adds the given operation to this queue. The exact position in the queue
+   * depends on the priority of the given operation relative to the priorities
+   * of the other operations in the queue. If there is already an operation with
+   * which the given one can be merge, it will be merged.
+   */
+   void add(ServerOperation operation) {
+    int queueIndex = operation.priority.ordinal;
+    Queue<ServerOperation> queue = queues[queueIndex];
+    // check if can be merged
+    if (operation is MergeableOperation) {
+      for (ServerOperation existingOperation in queue) {
+        if (existingOperation is MergeableOperation) {
+          bool merged = existingOperation.mergeWith(operation);
+          if (merged) {
+            return;
+          }
+        }
+      }
+    }
+    // add to the end
+    queue.addLast(operation);
+  }
+
+  /**
+   * Returns `true` if there are no queued [ServerOperation]s.
+   */
+  bool get isEmpty {
+    return queues.every((queue) => queue.isEmpty);
+  }
+
+  /**
+   * Returns the next operation to perform or `null` if empty.
+   */
+  ServerOperation take() {
+    for (Queue<ServerOperation> queue in queues) {
+      if (!queue.isEmpty) {
+        return queue.removeFirst();
+      }
+    }
+    return null;
+  }
+}
+