cc: Rename LayerTreeHost to LayerTreeHostInProcess.

cc/trees/layer_tree_host.h

-// Copyright 2011 The Chromium Authors. All rights reserved.
+// Copyright 2016 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 CC_TREES_LAYER_TREE_HOST_H_
 #define CC_TREES_LAYER_TREE_HOST_H_
 
-#include <stddef.h>
-#include <stdint.h>
-
-#include <limits>
-#include <memory>
-#include <set>
-#include <string>
-#include <unordered_map>
-#include <vector>
-
-#include "base/cancelable_callback.h"
 #include "base/macros.h"
-#include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
-#include "base/time/time.h"
-#include "cc/animation/target_property.h"
 #include "cc/base/cc_export.h"
 #include "cc/debug/micro_benchmark.h"
-#include "cc/debug/micro_benchmark_controller.h"
-#include "cc/input/event_listener_properties.h"
-#include "cc/input/input_handler.h"
-#include "cc/input/layer_selection_bound.h"
-#include "cc/input/scrollbar.h"
 #include "cc/input/top_controls_state.h"
-#include "cc/layers/layer_collections.h"
-#include "cc/layers/layer_list_iterator.h"
-#include "cc/output/output_surface.h"
-#include "cc/output/swap_promise.h"
-#include "cc/resources/resource_format.h"
-#include "cc/trees/compositor_mode.h"
-#include "cc/trees/layer_tree.h"
-#include "cc/trees/layer_tree_host_client.h"
-#include "cc/trees/layer_tree_host_interface.h"
-#include "cc/trees/layer_tree_settings.h"
-#include "cc/trees/proxy.h"
-#include "cc/trees/surface_sequence_generator.h"
-#include "cc/trees/swap_promise_manager.h"
-#include "third_party/skia/include/core/SkColor.h"
-#include "ui/gfx/geometry/rect.h"
+
+namespace base {
+class TimeTicks;
+}  // namespace base
+
+namespace gfx {
+class Rect;
+}  // namespace gfx
 
 namespace gpu {
 class GpuMemoryBufferManager;
-}
+}  // namespace gpu
 
 namespace cc {
-
-class AnimationEvents;
-class AnimationHost;
-class BeginFrameSource;
-class ClientPictureCache;
-class EnginePictureCache;
-class HeadsUpDisplayLayer;
-class ImageSerializationProcessor;
-class Layer;
-class LayerTreeHostImpl;
-class LayerTreeHostImplClient;
-class LayerTreeHostSingleThreadClient;
+class InputHandler;
+class LayerTree;
+class LayerTreeDebugState;
 class LayerTreeMutator;
-class PropertyTrees;
-class Region;
-class RemoteProtoChannel;
-class RenderingStatsInstrumentation;
-class ResourceProvider;
-class ResourceUpdateQueue;
-class SharedBitmapManager;
-class TaskGraphRunner;
-class TopControlsManager;
-struct PendingPageScaleAnimation;
-struct RenderingStats;
-struct ScrollAndScaleSet;
-
-namespace proto {
-class LayerTreeHost;
-}
-
-// This class is being refactored to an interface, see LayerTreeHostInterface,
-// which is the API for the cc embedder. When adding new code to this class,
-// consider the following:
-// 1) If its state/data that gets pushed to the LayerTreeImpl during commit, add
-//    it to the LayerTree.
-// 2) If it's a call from the embedder, add it to the LayerTreeHostInterface.
-// 3) If it's a call from any of the internal cc classes, i.e., LayerTree or
-//    PropertyTreeBuilder, etc., add it to the LayerTreeHostInterface.
-// 4) If it's a notification from the impl thread or a call from Proxy, add it
-//    to this class.
-// This class will be renamed to LayerTreeHostInProcess and will be the
-// LayerTreeHost implementation for when the impl thread for the compositor runs
-// in the same process, and thus uses a Proxy.
-class CC_EXPORT LayerTreeHost : public LayerTreeHostInterface {
+class LayerTreeSettings;
+class OutputSurface;
+class SurfaceSequenceGenerator;
+class SwapPromise;
+class SwapPromiseManager;
+class SwapPromiseMonitor;
+class TaskRunnerProvider;
+class UIResourceManager;
+
+// This encapsulates the API for any embedder to use cc. The
+// LayerTreeHostInProcess provides the implementation where the compositor
+// thread components of this host run within the same process. Use
+// LayerTreeHostInProcess::CreateThreaded/CreateSingleThread to get either.
+class CC_EXPORT LayerTreeHost {
  public:
-  // TODO(sad): InitParams should be a movable type so that it can be
-  // std::move()d to the Create* functions.
-  struct CC_EXPORT InitParams {
-    LayerTreeHostClient* client = nullptr;
-    SharedBitmapManager* shared_bitmap_manager = nullptr;
-    gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager = nullptr;
-    TaskGraphRunner* task_graph_runner = nullptr;
-    LayerTreeSettings const* settings = nullptr;
-    scoped_refptr<base::SingleThreadTaskRunner> main_task_runner;
-    std::unique_ptr<BeginFrameSource> external_begin_frame_source;
-    ImageSerializationProcessor* image_serialization_processor = nullptr;
-    std::unique_ptr<AnimationHost> animation_host;
-
-    InitParams();
-    ~InitParams();
-  };
-
-  // The SharedBitmapManager will be used on the compositor thread.
-  static std::unique_ptr<LayerTreeHostInterface> CreateThreaded(
-      scoped_refptr<base::SingleThreadTaskRunner> impl_task_runner,
-      InitParams* params);
-
-  static std::unique_ptr<LayerTreeHost> CreateSingleThreaded(
-      LayerTreeHostSingleThreadClient* single_thread_client,
-      InitParams* params);
-
-  static std::unique_ptr<LayerTreeHostInterface> CreateRemoteServer(
-      RemoteProtoChannel* remote_proto_channel,
-      InitParams* params);
-
-  // The lifetime of this LayerTreeHost is tied to the lifetime of the remote
-  // server LayerTreeHost. It should be created on receiving
-  // CompositorMessageToImpl::InitializeImpl message and destroyed on receiving
-  // a CompositorMessageToImpl::CloseImpl message from the server. This ensures
-  // that the client will not send any compositor messages once the
-  // LayerTreeHost on the server is destroyed.
-  static std::unique_ptr<LayerTreeHostInterface> CreateRemoteClient(
-      RemoteProtoChannel* remote_proto_channel,
-      scoped_refptr<base::SingleThreadTaskRunner> impl_task_runner,
-      InitParams* params);
-
-  ~LayerTreeHost() override;
-
-  // LayerTreeHostInterface implementation.
-  int GetId() const override;
-  int SourceFrameNumber() const override;
-  LayerTree* GetLayerTree() override;
-  const LayerTree* GetLayerTree() const override;
-  UIResourceManager* GetUIResourceManager() const override;
-  TaskRunnerProvider* GetTaskRunnerProvider() const override;
-  const LayerTreeSettings& GetSettings() const override;
-  void SetSurfaceClientId(uint32_t client_id) override;
-  void SetLayerTreeMutator(std::unique_ptr<LayerTreeMutator> mutator) override;
-  void QueueSwapPromise(std::unique_ptr<SwapPromise> swap_promise) override;
-  SwapPromiseManager* GetSwapPromiseManager() override;
-  void SetHasGpuRasterizationTrigger(bool has_trigger) override;
-  void SetVisible(bool visible) override;
-  bool IsVisible() const override;
-  void SetOutputSurface(std::unique_ptr<OutputSurface> output_surface) override;
-  std::unique_ptr<OutputSurface> ReleaseOutputSurface() override;
-  void SetNeedsAnimate() override;
-  void SetNeedsUpdateLayers() override;
-  void SetNeedsCommit() override;
-  bool BeginMainFrameRequested() const override;
-  bool CommitRequested() const override;
-  void SetDeferCommits(bool defer_commits) override;
-  void LayoutAndUpdateLayers() override;
-  void Composite(base::TimeTicks frame_begin_time) override;
-  void SetNeedsRedraw() override;
-  void SetNeedsRedrawRect(const gfx::Rect& damage_rect) override;
-  void SetNextCommitForcesRedraw() override;
-  void NotifyInputThrottledUntilCommit() override;
-  void UpdateTopControlsState(TopControlsState constraints,
-                              TopControlsState current,
-                              bool animate) override;
-  const base::WeakPtr<InputHandler>& GetInputHandler() const override;
-  void DidStopFlinging() override;
-  void SetDebugState(const LayerTreeDebugState& debug_state) override;
-  const LayerTreeDebugState& GetDebugState() const override;
-  int ScheduleMicroBenchmark(
+  virtual ~LayerTreeHost() {}
+
+  // Returns the process global unique identifier for this LayerTreeHost.
+  virtual int GetId() const = 0;
+
+  // The current source frame number. This is incremented for each main frame
+  // update(commit) pushed to the compositor thread.
+  virtual int SourceFrameNumber() const = 0;
+
+  // Returns the LayerTree that holds the main frame state pushed to the
+  // LayerTreeImpl on commit.
+  virtual LayerTree* GetLayerTree() = 0;
+  virtual const LayerTree* GetLayerTree() const = 0;
+
+  // Returns the UIResourceManager used to create UIResources for
+  // UIResourceLayers pushed to the LayerTree.
+  virtual UIResourceManager* GetUIResourceManager() const = 0;
+
+  // Returns the TaskRunnerProvider used to access the main and compositor
+  // thread task runners.
+  virtual TaskRunnerProvider* GetTaskRunnerProvider() const = 0;
+
+  // Returns the settings used by this host.
+  virtual const LayerTreeSettings& GetSettings() const = 0;
+
+  // Sets the client id used to generate the SurfaceId that uniquely identifies
+  // the Surfaces produced by this compositor.
+  virtual void SetSurfaceClientId(uint32_t client_id) = 0;
+
+  // Sets the LayerTreeMutator interface used to directly mutate the compositor
+  // state on the compositor thread. (Compositor-Worker)
+  virtual void SetLayerTreeMutator(
+      std::unique_ptr<LayerTreeMutator> mutator) = 0;
+
+  // Call this function when you expect there to be a swap buffer.
+  // See swap_promise.h for how to use SwapPromise.
+  virtual void QueueSwapPromise(std::unique_ptr<SwapPromise> swap_promise) = 0;
+
+  // Returns the SwapPromiseManager used to create SwapPromiseMonitors for this
+  // host.
+  virtual SwapPromiseManager* GetSwapPromiseManager() = 0;
+
+  // Sets whether the content is suitable to use Gpu Rasterization.
+  virtual void SetHasGpuRasterizationTrigger(bool has_trigger) = 0;
+
+  // Visibility and OutputSurface -------------------------------
+
+  virtual void SetVisible(bool visible) = 0;
+  virtual bool IsVisible() const = 0;
+
+  // Called in response to an OutputSurface request made to the client using
+  // LayerTreeHostClient::RequestNewOutputSurface. The client will be informed
+  // of the OutputSurface initialization status using DidInitializaOutputSurface
+  // or DidFailToInitializeOutputSurface. The request is completed when the host
+  // successfully initializes an OutputSurface.
+  virtual void SetOutputSurface(
+      std::unique_ptr<OutputSurface> output_surface) = 0;
+
+  // Forces the host to immediately release all references to the OutputSurface,
+  // if any. Can be safely called any time.
+  virtual std::unique_ptr<OutputSurface> ReleaseOutputSurface() = 0;
+
+  // Frame Scheduling (main and compositor frames) requests -------
+
+  // Requests a main frame update even if no content has changed. This is used,
+  // for instance in the case of RequestAnimationFrame from blink to ensure the
+  // main frame update is run on the next tick without pre-emptively forcing a
+  // full commit synchronization or layer updates.
+  virtual void SetNeedsAnimate() = 0;
+
+  // Requests a main frame update and also ensure that the host pulls layer
+  // updates from the client, even if no content might have changed, without
+  // forcing a full commit synchronization.
+  virtual void SetNeedsUpdateLayers() = 0;
+
+  // Requests that the next main frame update performs a full commit
+  // synchronization.
+  virtual void SetNeedsCommit() = 0;
+
+  // Returns true if a main frame (for any pipeline stage above) has been
+  // requested.
+  virtual bool BeginMainFrameRequested() const = 0;
+
+  // Returns true if a main frame with commit synchronization has been
+  // requested.
+  virtual bool CommitRequested() const = 0;
+
+  // Enables/disables the compositor from requesting main frame updates from the
+  // client.
+  virtual void SetDeferCommits(bool defer_commits) = 0;
+
+  // Synchronously performs a main frame update and layer updates. Used only in
+  // single threaded mode when the compositor's internal scheduling is disabled.
+  virtual void LayoutAndUpdateLayers() = 0;
+
+  // Synchronously performs a complete main frame update, commit and compositor
+  // frame. Used only in single threaded mode when the compositor's internal
+  // scheduling is disabled.
+  virtual void Composite(base::TimeTicks frame_begin_time) = 0;
+
+  // Requests a redraw (compositor frame) for the complete viewport.
+  virtual void SetNeedsRedraw() = 0;
+
+  // Requests a redraw (compositor frame) for the given rect.
+  virtual void SetNeedsRedrawRect(const gfx::Rect& damage_rect) = 0;
+
+  // Requests a main frame (including layer updates) and ensures that this main
+  // frame results in a redraw for the complete viewport when producing the
+  // CompositorFrame.
+  virtual void SetNextCommitForcesRedraw() = 0;
+
+  // Input Handling ---------------------------------------------
+
+  // Notifies the compositor that input from the browser is being throttled till
+  // the next commit. The compositor will prioritize activation of the pending
+  // tree so a commit can be performed.
+  virtual void NotifyInputThrottledUntilCommit() = 0;
+
+  // Sets the state of the top controls. (Used for URL bar animations on
+  // android).
+  virtual void UpdateTopControlsState(TopControlsState constraints,
+                                      TopControlsState current,
+                                      bool animate) = 0;
+
+  // Returns a reference to the InputHandler used to respond to input events on
+  // the compositor thread.
+  virtual const base::WeakPtr<InputHandler>& GetInputHandler() const = 0;
+
+  // Informs the compositor that an active fling gesture being processed on the
+  // main thread has been finished.
+  virtual void DidStopFlinging() = 0;
+
+  // Debugging and benchmarks ---------------------------------
+  virtual void SetDebugState(const LayerTreeDebugState& debug_state) = 0;
+  virtual const LayerTreeDebugState& GetDebugState() const = 0;
+
+  // Returns the id of the benchmark on success, 0 otherwise.
+  virtual int ScheduleMicroBenchmark(
       const std::string& benchmark_name,
       std::unique_ptr<base::Value> value,
-      const MicroBenchmark::DoneCallback& callback) override;
-  bool SendMessageToMicroBenchmark(int id,
-                                   std::unique_ptr<base::Value> value) override;
-  SurfaceSequenceGenerator* GetSurfaceSequenceGenerator() override;
-
-  // LayerTreeHost interface to Proxy.
-  void WillBeginMainFrame();
-  void DidBeginMainFrame();
-  void BeginMainFrame(const BeginFrameArgs& args);
-  void BeginMainFrameNotExpectedSoon();
-  void AnimateLayers(base::TimeTicks monotonic_frame_begin_time);
-  void RequestMainFrameUpdate();
-  void FinishCommitOnImplThread(LayerTreeHostImpl* host_impl);
-  void WillCommit();
-  void CommitComplete();
-  void RequestNewOutputSurface();
-  void DidInitializeOutputSurface();
-  void DidFailToInitializeOutputSurface();
-  virtual std::unique_ptr<LayerTreeHostImpl> CreateLayerTreeHostImpl(
-      LayerTreeHostImplClient* client);
-  void DidLoseOutputSurface();
-  void DidCommitAndDrawFrame() { client_->DidCommitAndDrawFrame(); }
-  void DidCompleteSwapBuffers() { client_->DidCompleteSwapBuffers(); }
-  bool UpdateLayers();
-  // Called when the compositor completed page scale animation.
-  void DidCompletePageScaleAnimation();
-  void ApplyScrollAndScale(ScrollAndScaleSet* info);
-
-  LayerTreeHostClient* client() { return client_; }
-
-  bool gpu_rasterization_histogram_recorded() const {
-    return gpu_rasterization_histogram_recorded_;
-  }
-
-  void CollectRenderingStats(RenderingStats* stats) const;
-
-  RenderingStatsInstrumentation* rendering_stats_instrumentation() const {
-    return rendering_stats_instrumentation_.get();
-  }
-
-  void SetNextCommitWaitsForActivation();
-
-  void SetAnimationEvents(std::unique_ptr<AnimationEvents> events);
-
-  bool has_gpu_rasterization_trigger() const {
-    return has_gpu_rasterization_trigger_;
-  }
-
-  Proxy* proxy() const { return proxy_.get(); }
-
-  // Serializes the parts of this LayerTreeHost that is needed for a commit to a
-  // protobuf message. Not all members are serialized as they are not helpful
-  // for remote usage.
-  // The |swap_promise_list_| is transferred to the serializer in
-  // |swap_promises|.
-  void ToProtobufForCommit(
-      proto::LayerTreeHost* proto,
-      std::vector<std::unique_ptr<SwapPromise>>* swap_promises);
-
-  // Deserializes the protobuf into this LayerTreeHost before a commit. The
-  // expected input is a serialized remote LayerTreeHost. After deserializing
-  // the protobuf, the normal commit-flow should continue.
-  void FromProtobufForCommit(const proto::LayerTreeHost& proto);
-
-  bool IsSingleThreaded() const;
-  bool IsThreaded() const;
-  bool IsRemoteServer() const;
-  bool IsRemoteClient() const;
-
-  ImageSerializationProcessor* image_serialization_processor() const {
-    return image_serialization_processor_;
-  }
-
-  EnginePictureCache* engine_picture_cache() const {
-    return engine_picture_cache_ ? engine_picture_cache_.get() : nullptr;
-  }
-
-  ClientPictureCache* client_picture_cache() const {
-    return client_picture_cache_ ? client_picture_cache_.get() : nullptr;
-  }
-
-  void ResetGpuRasterizationTracking();
-
- protected:
-  // Allow tests to inject the LayerTree.
-  LayerTreeHost(InitParams* params,
-                CompositorMode mode,
-                std::unique_ptr<LayerTree> layer_tree);
-  LayerTreeHost(InitParams* params, CompositorMode mode);
-
-  void InitializeThreaded(
-      scoped_refptr<base::SingleThreadTaskRunner> main_task_runner,
-      scoped_refptr<base::SingleThreadTaskRunner> impl_task_runner,
-      std::unique_ptr<BeginFrameSource> external_begin_frame_source);
-  void InitializeSingleThreaded(
-      LayerTreeHostSingleThreadClient* single_thread_client,
-      scoped_refptr<base::SingleThreadTaskRunner> main_task_runner,
-      std::unique_ptr<BeginFrameSource> external_begin_frame_source);
-  void InitializeRemoteServer(
-      RemoteProtoChannel* remote_proto_channel,
-      scoped_refptr<base::SingleThreadTaskRunner> main_task_runner);
-  void InitializeRemoteClient(
-      RemoteProtoChannel* remote_proto_channel,
-      scoped_refptr<base::SingleThreadTaskRunner> main_task_runner,
-      scoped_refptr<base::SingleThreadTaskRunner> impl_task_runner);
-  void InitializeForTesting(
-      std::unique_ptr<TaskRunnerProvider> task_runner_provider,
-      std::unique_ptr<Proxy> proxy_for_testing,
-      std::unique_ptr<BeginFrameSource> external_begin_frame_source);
-  void InitializePictureCacheForTesting();
-  void SetTaskRunnerProviderForTesting(
-      std::unique_ptr<TaskRunnerProvider> task_runner_provider);
-  void SetUIResourceManagerForTesting(
-      std::unique_ptr<UIResourceManager> ui_resource_manager);
-
-  // shared_bitmap_manager(), gpu_memory_buffer_manager(), and
-  // task_graph_runner() return valid values only until the LayerTreeHostImpl is
-  // created in CreateLayerTreeHostImpl().
-  SharedBitmapManager* shared_bitmap_manager() const {
-    return shared_bitmap_manager_;
-  }
-  gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager() const {
-    return gpu_memory_buffer_manager_;
-  }
-  TaskGraphRunner* task_graph_runner() const { return task_graph_runner_; }
-
-  void OnCommitForSwapPromises();
-
-  void RecordGpuRasterizationHistogram();
-
-  MicroBenchmarkController micro_benchmark_controller_;
-
-  std::unique_ptr<LayerTree> layer_tree_;
-
-  base::WeakPtr<InputHandler> input_handler_weak_ptr_;
-
- private:
-  friend class LayerTreeHostSerializationTest;
-
-  // This is the number of consecutive frames in which we want the content to be
-  // suitable for GPU rasterization before re-enabling it.
-  enum { kNumFramesToConsiderBeforeGpuRasterization = 60 };
-
-  void ApplyViewportDeltas(ScrollAndScaleSet* info);
-  void ApplyPageScaleDeltaFromImplSide(float page_scale_delta);
-  void InitializeProxy(
-      std::unique_ptr<Proxy> proxy,
-      std::unique_ptr<BeginFrameSource> external_begin_frame_source);
-
-  bool DoUpdateLayers(Layer* root_layer);
-  void UpdateHudLayer();
-
-  bool AnimateLayersRecursive(Layer* current, base::TimeTicks time);
-
-  void CalculateLCDTextMetricsCallback(Layer* layer);
-
-  void SetPropertyTreesNeedRebuild();
-
-  const CompositorMode compositor_mode_;
-
-  std::unique_ptr<UIResourceManager> ui_resource_manager_;
-
-  LayerTreeHostClient* client_;
-  std::unique_ptr<Proxy> proxy_;
-  std::unique_ptr<TaskRunnerProvider> task_runner_provider_;
-
-  int source_frame_number_;
-  std::unique_ptr<RenderingStatsInstrumentation>
-      rendering_stats_instrumentation_;
-
-  SwapPromiseManager swap_promise_manager_;
-
-  // |current_output_surface_| can't be updated until we've successfully
-  // initialized a new output surface. |new_output_surface_| contains the
-  // new output surface that is currently being initialized. If initialization
-  // is successful then |new_output_surface_| replaces
-  // |current_output_surface_|.
-  std::unique_ptr<OutputSurface> new_output_surface_;
-  std::unique_ptr<OutputSurface> current_output_surface_;
-
-  const LayerTreeSettings settings_;
-  LayerTreeDebugState debug_state_;
-
-  bool visible_;
-
-  bool has_gpu_rasterization_trigger_;
-  bool content_is_suitable_for_gpu_rasterization_;
-  bool gpu_rasterization_histogram_recorded_;
-
-  // If set, then page scale animation has completed, but the client hasn't been
-  // notified about it yet.
-  bool did_complete_scale_animation_;
-
-  int id_;
-  bool next_commit_forces_redraw_;
-
-  SharedBitmapManager* shared_bitmap_manager_;
-  gpu::GpuMemoryBufferManager* gpu_memory_buffer_manager_;
-  TaskGraphRunner* task_graph_runner_;
-
-  ImageSerializationProcessor* image_serialization_processor_;
-  std::unique_ptr<EnginePictureCache> engine_picture_cache_;
-  std::unique_ptr<ClientPictureCache> client_picture_cache_;
-
-  SurfaceSequenceGenerator surface_sequence_generator_;
-  uint32_t num_consecutive_frames_suitable_for_gpu_ = 0;
-
-  DISALLOW_COPY_AND_ASSIGN(LayerTreeHost);
+      const MicroBenchmark::DoneCallback& callback) = 0;
+
+  // Returns true if the message was successfully delivered and handled.
+  virtual bool SendMessageToMicroBenchmark(
+      int id,
+      std::unique_ptr<base::Value> value) = 0;
+
+  // Methods used internally in cc. These are not intended to be a part of the
+  // public API for use by the embedder ----------------------
+  virtual SurfaceSequenceGenerator* GetSurfaceSequenceGenerator() = 0;
+
+  // When the main thread informs the impl thread that it is ready to commit,
+  // generally it would remain blocked till the main thread state is copied to
+  // the pending tree. Calling this would ensure that the main thread remains
+  // blocked till the pending tree is activated.
+  virtual void SetNextCommitWaitsForActivation() = 0;
+
+  // The LayerTreeHost tracks whether the content is suitable for Gpu raster.
+  // Calling this will reset it back to not suitable state.
+  virtual void ResetGpuRasterizationTracking() = 0;
 };
 
 }  // namespace cc
 
 #endif  // CC_TREES_LAYER_TREE_HOST_H_