[translate] Add translate ranker model loader.

components/translate/core/browser/ranker_model_loader.h

Index: components/translate/core/browser/ranker_model_loader.h
diff --git a/components/translate/core/browser/ranker_model_loader.h b/components/translate/core/browser/ranker_model_loader.h
new file mode 100644
index 0000000000000000000000000000000000000000..df43d34eb115fc4349d736c96fcae7c2d040b872
--- /dev/null
+++ b/components/translate/core/browser/ranker_model_loader.h
@@ -0,0 +1,139 @@
+// 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 COMPONENTS_TRANSLATE_CORE_BROWSER_RANKER_MODEL_LOADER_H_
+#define COMPONENTS_TRANSLATE_CORE_BROWSER_RANKER_MODEL_LOADER_H_
+
+#include <memory>
+#include <string>
+
+#include "base/callback.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/weak_ptr.h"
+#include "base/sequence_checker.h"
+
+class GURL;
+
+namespace base {
+class FilePath;
+class SequencedTaskRunner;
+}  // namespace base
+
+namespace chrome_intelligence {
+class RankerModel;
+}  // namespace chrome_intelligence
+
+namespace translate {
+
+// Enumeration denoting the outcome of an attempt to download the model. This
+// must be kept in sync with the RankerModelStatus enum in histograms.xml
+enum class RankerModelStatus {
+  OK = 0,
+  DOWNLOAD_THROTTLED = 1,
+  DOWNLOAD_FAILED = 2,
+  PARSE_FAILED = 3,
+  VALIDATION_FAILED = 4,
+  INCOMPATIBLE = 5,
+
+  // Insert new values above this line.
+  MAX
+};
+
+// If enabled, downloads a translate ranker model and uses it to determine
+// whether the user should be given a translation prompt or not.
+class RankerModelLoader {
+ public:
+  // Callback to validate a ranker model on behalf of the model loader client.
+  // For example, the callback might validate that the model is compatible with
+  // the features generated when ranking translation offerings. This callback
+  // may be called on any sequence and must, therefore, be thread-safe. This
+  // callback may be invoked after the RankerModelLoader has been destroyed
+  // and must, therefore, only access memory that it can guarantee to be
+  // valid.
+  using ValidateModelCallback = base::Callback<RankerModelStatus(
+      const chrome_intelligence::RankerModel& model)>;
+
+  // Called to transfer ownership of a loaded model back to the model loader
+  // client. This will be called on the sequence on which the model loader was
+  // constructed.  This callback may be invoked after the RankerModelLoader has
+  // been destroyed and must, therefore, only access memory that it can
+  // guarantee to be valid.
+  using OnModelAvailableCallback = base::Callback<void(
+      std::unique_ptr<chrome_intelligence::RankerModel> model)>;
+
+  // |validate_model_callback| may be called on any sequence; it must be thread
+  // safe.
+  //
+  // |on_model_available_callback| will be called on the sequence on which the
+  // ranker model loader is constructed.
+  //
+  // |model_path| denotes the file path at which the model is cached. The loader
+  // will attempt to load the model from this path first, falling back to the
+  // |model_url| if the model cannot be loaded or has expired. Upon downloading
+  // a fresh model from |model_url| the model will be persisted to |model_path|
+  // for subsequent caching.
+  //
+  // |model_url| denotes the URL from which the model should be loaded, if it
+  // has not already been cached at |model_path|.
+  //
+  // |uma_prefix| will be used as a prefix for the names of all UMA metrics
+  // generated by this loader.
+  RankerModelLoader(const ValidateModelCallback& validate_model_callback,
+                    const OnModelAvailableCallback& on_model_available_callback,
+                    const base::FilePath& model_path,
+                    const GURL& model_url,
+                    const std::string& uma_prefix);
+
+  ~RankerModelLoader();
+
+  // Asynchronously initiates loading the model from the cache file path and URL
+  // previously configured.
+  void Start();
+
+  // Call this method periodically to notify the model loader the ranker is
+  // actively in use. The user's engagement with the feature is used as a proxy
+  // for network activity. If a model download is pending, this will trigger
+  // (subject to retry and frequency limits) a model download attempt.
+  void NotifyOfRankerActivity();
+
+ private:
+  // The model loader backend to which the actual loading functionality is
+  // delegated.
+  class Backend;
+  friend class Backend;
+
+  // A enum to track the current loader state.
+  enum class LoaderState { NOT_STARTED, RUNNING, FINISHED };
+
+  // Helper method to forward OnModelAvailable callbacks while noting whether
+  // or not the loader has finished its work.
+  void InternalOnModelAvailable(
+      const OnModelAvailableCallback& callback,
+      std::unique_ptr<chrome_intelligence::RankerModel> model,
+      bool finished);
+
+  // Validates that ranker model loader tasks are all performed on the same
+  // sequence.
+  base::SequenceChecker sequence_checker_;
+
+  // The task runner on which backend tasks are performed.
+  const scoped_refptr<base::SequencedTaskRunner> backend_task_runner_;
+
+  // The model loader backend to which the actual loading functionality is
+  // delegated. |backend_| may outlive its RankerModelLoader. When the
+  // RankerModelLoader is destroyed, ownership of |backend_| is transferred
+  // to a delete task posted |backend_task_runner_|.
+  std::unique_ptr<Backend> backend_;
+
+  // The current state of the loader.
+  LoaderState state_ = LoaderState::NOT_STARTED;
+
+  base::WeakPtrFactory<RankerModelLoader> weak_ptr_factory_;
+
+  DISALLOW_COPY_AND_ASSIGN(RankerModelLoader);
+};
+
+}  // namespace translate
+
+#endif  // COMPONENTS_TRANSLATE_CORE_BROWSER_RANKER_MODEL_LOADER_H_