Remove BrowserThread dependency from usb_service.

Instead of explicitly depending on specific browser threads the USB
service can assume that it is instantiated on BrowserThread::FILE (or
equivalent) and save a TaskRunner reference from this instantiation for
later use when called from other threads.

To reach BrowserThread::UI (required for DBus on Chrome OS) a reference
to the appropriate TaskRunner must be provided when calling
UsbService::GetInstance().

BUG=

Review URL: https://codereview.chromium.org/507503002

Cr-Commit-Position: refs/heads/master@{#292546}

4 files changed, 109 insertions, 0 deletions

diff --git a/device/core/BUILD.gn b/device/core/BUILD.gn
new file mode 100644
index 0000000..7d1671f
--- /dev/null
+++ b/device/core/BUILD.gn
@@ -0,0 +1,10 @@
+# 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.
+
+source_set("core") {
+  sources = [
+    "device_client.cc",
+    "device_client.h",
+  ]
+}
diff --git a/device/core/core.gyp b/device/core/core.gyp
new file mode 100644
index 0000000..09bd315
--- /dev/null
+++ b/device/core/core.gyp
@@ -0,0 +1,22 @@
+# 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.
+
+{
+  'variables': {
+    'chromium_code': 1,
+  },
+  'targets': [
+    {
+      'target_name': 'device_core',
+      'type': 'static_library',
+      'include_dirs': [
+        '../..',
+      ],
+      'sources': [
+        'device_client.cc',
+        'device_client.h',
+      ],
+    },
+  ],
+}
diff --git a/device/core/device_client.cc b/device/core/device_client.cc
new file mode 100644
index 0000000..ad96b1e
--- /dev/null
+++ b/device/core/device_client.cc
@@ -0,0 +1,38 @@
+// 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 "device/core/device_client.h"
+
+#include "base/logging.h"
+
+namespace device {
+
+namespace {
+
+DeviceClient* g_instance = NULL;
+
+}  // namespace
+
+DeviceClient::DeviceClient() {
+  DCHECK(!g_instance);
+  g_instance = this;
+}
+
+DeviceClient::~DeviceClient() {
+  g_instance = NULL;
+}
+
+/* static */
+DeviceClient* DeviceClient::Get() {
+  DCHECK(g_instance);
+  return g_instance;
+}
+
+usb_service::UsbService* DeviceClient::GetUsbService() {
+  // This should never be called by clients which do not support the USB API.
+  NOTREACHED();
+  return NULL;
+}
+
+}  // namespace device
diff --git a/device/core/device_client.h b/device/core/device_client.h
new file mode 100644
index 0000000..d8d938b
--- /dev/null
+++ b/device/core/device_client.h
@@ -0,0 +1,39 @@
+// 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.
+
+#ifndef DEVICE_CORE_DEVICE_CLIENT_H_
+#define DEVICE_CORE_DEVICE_CLIENT_H_
+
+#include "base/macros.h"
+
+namespace usb_service {
+class UsbService;
+}
+
+namespace device {
+
+// Interface used by consumers of //device APIs to get pointers to the service
+// singletons appropriate for a given embedding application. For an example see
+// //chrome/browser/chrome_device_client.h.
+class DeviceClient {
+ public:
+  // Construction sets the single instance.
+  DeviceClient();
+
+  // Destruction clears the single instance.
+  ~DeviceClient();
+
+  // Returns the single instance of |this|.
+  static DeviceClient* Get();
+
+  // Returns the UsbService instance for this embedder.
+  virtual usb_service::UsbService* GetUsbService();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DeviceClient);
+};
+
+}  // namespace device
+
+#endif  // DEVICE_CORE_DEVICE_CLIENT_H_