ozone: Initialize a subsystem only if necessary.

ui/events/ozone/device/udev/device_manager_udev.cc

 // Copyright 2014 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 "ui/events/ozone/device/udev/device_manager_udev.h"
 
 #include <libudev.h>
 
 #include "base/debug/trace_event.h"
 #include "base/strings/stringprintf.h"
@@ skipping 60 matching lines @@
       LOG(ERROR) << "Failed to start receiving events from udev";
   } else {
     LOG(ERROR) << "Failed to create udev monitor";
   }
 
   return scoped_udev_monitor(monitor);
 }
 
 }  // namespace
 
-DeviceManagerUdev::DeviceManagerUdev()
-  : udev_(UdevCreate()),
-    monitor_(UdevCreateMonitor(udev_.get())) {
+DeviceManagerUdev::DeviceManagerUdev() : udev_(UdevCreate()) {
+}
 
+DeviceManagerUdev::~DeviceManagerUdev() {
+}
+
+void DeviceManagerUdev::CreateMonitor() {
+  if (monitor_)
+    return;
+  monitor_ = UdevCreateMonitor(udev_.get());
   if (monitor_) {
     int fd = udev_monitor_get_fd(monitor_.get());
     CHECK_GT(fd, 0);
     base::MessageLoopForUI::current()->WatchFileDescriptor(
         fd, true, base::MessagePumpLibevent::WATCH_READ, &controller_, this);
   }
 }
 
-DeviceManagerUdev::~DeviceManagerUdev() {}
+void DeviceManagerUdev::ScanDevices(DeviceEventObserver* observer) {
+  CreateMonitor();
 
-void DeviceManagerUdev::ScanDevices(DeviceEventObserver* observer) {
   scoped_udev_enumerate enumerate(udev_enumerate_new(udev_.get()));
   if (!enumerate)
     return;
 
   for (size_t i = 0; i < arraysize(kSubsystems); ++i)
     udev_enumerate_add_match_subsystem(enumerate.get(), kSubsystems[i]);
   udev_enumerate_scan_devices(enumerate.get());
 
   struct udev_list_entry* devices =
       udev_enumerate_get_list_entry(enumerate.get());
@@ skipping 64 matching lines @@
   else if (!strcmp(action, "change"))
     action_type = DeviceEvent::CHANGE;
   else
     return scoped_ptr<DeviceEvent>();
 
   return scoped_ptr<DeviceEvent>(
       new DeviceEvent(device_type, action_type, base::FilePath(path)));
 }
 
 }  // namespace ui