Client Hints

chrome/browser/net/client_hints.cc

+// Copyright 2013 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 "chrome/browser/net/client_hints.h"
+
+#include <locale>
+
+#include "base/bind.h"
+#include "base/strings/stringprintf.h"
+#include "content/public/browser/browser_thread.h"
+#include "ui/gfx/screen.h"
+
+namespace {
+
+void FetchScreenInfoOnUIThread(float* device_pixel_ratio) {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::UI));
+  // TODO(bengr): Consider multi-display scenarios.
+  gfx::Display display = gfx::Screen::GetNativeScreen()->GetPrimaryDisplay();
+  *device_pixel_ratio = display.device_scale_factor();
+}
+
+}  // namespace
+
+ClientHints::ClientHints()
+    : weak_ptr_factory_(this) {
+}
+
+ClientHints::~ClientHints() {
+}
+
+void ClientHints::Init() {
+  // TODO(bengr): Observe the device pixel ratio in case it changes during
+  // the Chrome session.
+  RetrieveScreenInfo();
+}
+
+bool ClientHints::RetrieveScreenInfo() {
+  DCHECK(content::BrowserThread::CurrentlyOn(content::BrowserThread::IO));
+  float* devicePixelRatio = new float;
+  return content::BrowserThread::PostTaskAndReply(
+      content::BrowserThread::UI,
+      FROM_HERE,
+      base::Bind(&FetchScreenInfoOnUIThread, devicePixelRatio),
+      base::Bind(&ClientHints::UpdateScreenInfo, weak_ptr_factory_.GetWeakPtr(),
+                 base::Owned(devicePixelRatio)));

[mmenke, 2013/09/25 16:26:19]

How about just using PostTaskAndReplyWithResult in src/base/task_runner_util.h
(It does the same thing more intuitively - though, admittedly, the adapter it
uses internally to convert a return value to an argument adds more overhead).

[bengr, 2013/09/25 17:25:52]

Done. I hadn't done this before b/c I was returning a bunch of values, but it
definitely makes sense now. (It probably made sense to do so even when I was
returning multiple values.)

+}
+
+const std::string& ClientHints::GetString() const {
+  return screen_hints_;
+}
+
+void ClientHints::UpdateScreenInfo(const float* device_pixel_ratio_value) {
+  if (device_pixel_ratio_value && *device_pixel_ratio_value > 0.0) {

[mmenke, 2013/09/25 16:26:19]

If something can't happen, you shouldn't be testing for it.  In this case, I
don't think we should have the "device_pixel_ratio_value" check.  Using
PostTaskAndReplyWithResult would make this a moot point, since you'd just pass
in a float, of course.

[bengr, 2013/09/25 17:25:52]

Done.

+    std::string device_pixel_ratio = base::StringPrintf("%.2f",
+        *device_pixel_ratio_value);
+    // Make sure the Client Hints value doesn't change
+    // according to the machine's locale
+    std::locale locale;
+    for (std::string::iterator it = device_pixel_ratio.begin();
+         it != device_pixel_ratio.end(); ++it)
+      if (!std::isdigit(*it, locale))
+        *it = '.';
+    screen_hints_ = device_pixel_ratio;
+  }
+}
+