[Mojo Video Capture] Introduce abstraction VideoCaptureSystem

media/capture/video/video_capture_system.cc

+// Copyright 2017 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 "media/capture/video/video_capture_system.h"
+
+#include "media/base/bind_to_current_loop.h"
+
+namespace {
+
+// Compares two VideoCaptureFormat by checking smallest frame_size area, then
+// by _largest_ frame_rate. Used to order a VideoCaptureFormats vector so that
+// the first entry for a given resolution has the largest frame rate, as needed
+// by the ConsolidateCaptureFormats() method.
+bool IsCaptureFormatSmaller(const media::VideoCaptureFormat& format1,
+                            const media::VideoCaptureFormat& format2) {
+  DCHECK(format1.frame_size.GetCheckedArea().IsValid());
+  DCHECK(format2.frame_size.GetCheckedArea().IsValid());
+  if (format1.frame_size.GetCheckedArea().ValueOrDefault(0) ==
+      format2.frame_size.GetCheckedArea().ValueOrDefault(0)) {
+    return format1.frame_rate > format2.frame_rate;
+  }
+  return format1.frame_size.GetCheckedArea().ValueOrDefault(0) <
+         format2.frame_size.GetCheckedArea().ValueOrDefault(0);
+}
+
+bool IsCaptureFormatSizeEqual(const media::VideoCaptureFormat& format1,
+                              const media::VideoCaptureFormat& format2) {
+  DCHECK(format1.frame_size.GetCheckedArea().IsValid());
+  DCHECK(format2.frame_size.GetCheckedArea().IsValid());
+  return format1.frame_size.GetCheckedArea().ValueOrDefault(0) ==
+         format2.frame_size.GetCheckedArea().ValueOrDefault(0);
+}
+
+// This function receives a list of capture formats, removes duplicated
+// resolutions while keeping the highest frame rate for each, and forcing I420
+// pixel format.
+void ConsolidateCaptureFormats(media::VideoCaptureFormats* formats) {
+  if (formats->empty())
+    return;
+  std::sort(formats->begin(), formats->end(), IsCaptureFormatSmaller);
+  // Due to the ordering imposed, the largest frame_rate is kept while removing
+  // duplicated resolutions.
+  media::VideoCaptureFormats::iterator last =
+      std::unique(formats->begin(), formats->end(), IsCaptureFormatSizeEqual);
+  formats->erase(last, formats->end());
+  // Mark all formats as I420, since this is what the renderer side will get
+  // anyhow: the actual pixel format is decided at the device level.
+  // Don't do this for Y16 format as it is handled separatelly.
+  for (auto& format : *formats) {
+    if (format.pixel_format != media::PIXEL_FORMAT_Y16)
+      format.pixel_format = media::PIXEL_FORMAT_I420;
+  }
+}
+
+}  // anonymous namespace
+
+namespace media {
+
+VideoCaptureSystem::VideoCaptureSystem(
+    std::unique_ptr<VideoCaptureDeviceFactory> factory)
+    : factory_(std::move(factory)) {}
+
+VideoCaptureSystem::~VideoCaptureSystem() = default;
+
+void VideoCaptureSystem::GetDeviceInfosAsync(
+    const DeviceInfoCallback& result_callback) {
+  factory_->EnumerateDeviceDescriptors(media::BindToCurrentLoop(

[emircan, 2017/04/03 19:04:28]

It seems like VideoCaptureDeviceFactory::EnumerateDeviceDescriptors() runs
synchronously and can return the result immediately. That would make these extra
posttasks unnecessary. Consider refactoring this method and the callers. 
https://cs.chromium.org/chromium/src/media/capture/video/video_capture_device...

[chfremer, 2017/04/04 19:05:33]

Agreed that this is the case. I'd like to defer this work. Will open a bug entry
for tracking this.

+      base::Bind(&VideoCaptureSystem::OnDescriptorsReceived,
+                 base::Unretained(this), result_callback)));

[emircan, 2017/04/03 19:04:28]

Can you add a comment explaining base::Unretained(this) is safe?

[chfremer, 2017/04/04 19:05:33]

Done.

+}
+
+// Creates a VideoCaptureDevice object. Returns NULL if something goes wrong.
+std::unique_ptr<VideoCaptureDevice> VideoCaptureSystem::CreateDevice(
+    const std::string& device_id) {
+  const VideoCaptureDeviceInfo* device_info = LookupDeviceInfoFromId(device_id);
+  if (!device_info)
+    return nullptr;
+  return factory_->CreateDevice(device_info->descriptor);
+}
+
+void VideoCaptureSystem::OnDescriptorsReceived(
+    const DeviceInfoCallback& result_callback,
+    std::unique_ptr<VideoCaptureDeviceDescriptors> descriptors) {

[emircan, 2017/04/03 19:04:28]

Can you add thread_checker to this class? It would make it clear that these
methods are not overlapping and |devices_info_cache_| access is ok.

[chfremer, 2017/04/04 19:05:33]

Done.

+  // For devices for which we already have an entry in |devices_info_cache_|,
+  // we do not want to query the |factory_| for supported formats again. We
+  // simply copy them from |devices_info_cache_|.
+  std::vector<VideoCaptureDeviceInfo> new_devices_info_cache;
+  new_devices_info_cache.reserve(descriptors->size());
+  for (const auto& descriptor : *descriptors) {
+    if (auto* cached_info = LookupDeviceInfoFromId(descriptor.device_id)) {
+      new_devices_info_cache.push_back(*cached_info);
+    } else {
+      // Query for supported formats in order to create the entry.
+      VideoCaptureDeviceInfo device_info(descriptor);
+      factory_->GetSupportedFormats(descriptor, &device_info.supported_formats);
+      ConsolidateCaptureFormats(&device_info.supported_formats);
+      new_devices_info_cache.push_back(device_info);
+    }
+  }
+
+  devices_info_cache_.swap(new_devices_info_cache);
+  result_callback.Run(devices_info_cache_);
+}
+
+const VideoCaptureDeviceInfo* VideoCaptureSystem::LookupDeviceInfoFromId(
+    const std::string& device_id) {
+  auto iter = std::find_if(
+      devices_info_cache_.begin(), devices_info_cache_.end(),
+      [&device_id](const media::VideoCaptureDeviceInfo& device_info) {
+        return device_info.descriptor.device_id == device_id;
+      });
+  if (iter == devices_info_cache_.end())
+    return nullptr;
+  return &(*iter);
+}
+
+}  // namespace media