Implement the content/renderer and content/browser part of the Device Motion API.

content/renderer/device_motion_shared_memory_reader.cc

+// Copyright (c) 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 "device_motion_shared_memory_reader.h"
+
+#include "base/debug/trace_event.h"
+#include "base/metrics/histogram.h"
+#include "content/common/device_motion_hardware_buffer.h"
+#include "content/common/device_motion_messages.h"
+#include "content/public/renderer/render_thread.h"
+#include "ipc/ipc_sync_message_filter.h"
+#include "third_party/WebKit/Source/Platform/chromium/public/WebDeviceMotionData.h"
+
+namespace content {
+
+bool DeviceMotionSharedMemoryReader::latestDeviceMotionData(
+    WebKit::WebDeviceMotionData& data) {
+
+  WebKit::WebDeviceMotionData read_into;
+  TRACE_EVENT0("DEVICEMOTION", "lastDeviceMotionData");
+
+  if (!base::SharedMemory::IsHandleValid(renderer_shared_memory_handle_))
+    return false;
+
+  // Only try to read this many times before failing to avoid waiting here
+  // very long in case of contention with the writer.
+  bool is_ready_for_read = false;
+  const int kMaximumContentionCount = 10;
+  int contention_count = -1;
+  base::subtle::Atomic32 version;
+  do {
+    version = device_motion_hardware_buffer_->sequence.ReadBegin();
+    is_ready_for_read = device_motion_hardware_buffer_->is_ready_for_read;
+    memcpy(&read_into, &device_motion_hardware_buffer_->buffer,
+        sizeof(read_into));
+    ++contention_count;
+    if (contention_count == kMaximumContentionCount)
+      break;
+  } while (device_motion_hardware_buffer_->sequence.ReadRetry(version));
+  UMA_HISTOGRAM_COUNTS("DeviceMotion.ReadContentionCount", contention_count);
+
+  if (contention_count >= kMaximumContentionCount) {
+    // We failed to successfully read, presumably because the hardware
+    // thread was taking unusually long. Don't copy the data to the output
+    // buffer, and simply leave what was there before.
+    return false;
+  }
+
+  // New data was read successfully, copy it into the output buffer.
+  memcpy(&data, &read_into, sizeof(data));
+
+  return is_ready_for_read;
+}
+
+DeviceMotionSharedMemoryReader::DeviceMotionSharedMemoryReader()
+    : device_motion_hardware_buffer_(NULL) {
+}
+
+void DeviceMotionSharedMemoryReader::startUpdating() {
+  CHECK(RenderThread::Get()->Send(
+      new DeviceMotionHostMsg_StartPolling(&renderer_shared_memory_handle_)));
+
+  bool valid_handle = base::SharedMemory::IsHandleValid(
+      renderer_shared_memory_handle_);
+  if (!valid_handle)
+    return;
+  renderer_shared_memory_.reset(new base::SharedMemory(
+      renderer_shared_memory_handle_, true));
+  CHECK(renderer_shared_memory_->Map(sizeof(DeviceMotionHardwareBuffer)));
+  void *memory = renderer_shared_memory_->memory();
+  CHECK(memory);
+  device_motion_hardware_buffer_ =
+      static_cast<DeviceMotionHardwareBuffer*>(memory);
+
+}
+
+void DeviceMotionSharedMemoryReader::stopUpdating() {
+  CHECK(RenderThread::Get()->Send(new DeviceMotionHostMsg_StopPolling()));
+}
+
+} // namespace content