Upstream Chrome on iOS source code [1/11].

ios/chrome/browser/callback_counter.mm

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ios/chrome/browser/callback_counter.h"
+
+CallbackCounter::CallbackCounter(const FinalCallback& final_callback)
+    : callback_count_(0U), final_callback_(final_callback) {
+  DCHECK(!final_callback.is_null());
+}
+
+CallbackCounter::~CallbackCounter() {
+  DCHECK_EQ(0U, callback_count_);
+}
+
+void CallbackCounter::IncrementCount() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(!final_callback_.is_null());
+  ++callback_count_;
+}
+
+void CallbackCounter::DecrementCount() {
+  DCHECK(thread_checker_.CalledOnValidThread());
+  DCHECK(callback_count_);
+
+  --callback_count_;
+  if (callback_count_ == 0) {
+    final_callback_.Run();
+    final_callback_.Reset();
+  }
+}