Add a Model to the download component.

components/download/internal/model.h

Index: components/download/internal/model.h
diff --git a/components/download/internal/model.h b/components/download/internal/model.h
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+++ b/components/download/internal/model.h
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+// 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.
+
+#ifndef COMPONENTS_DOWNLOAD_INTERNAL_MODEL_H_
+#define COMPONENTS_DOWNLOAD_INTERNAL_MODEL_H_
+
+#include <memory>
+#include <string>
+#include <vector>
+
+#include "components/download/public/clients.h"
+
+namespace download {
+
+struct Entry;
+class Store;
+
+// The model that contains a runtime representation of Entry entries. Any
+// updates to the model will be persisted to a backing |Store| as necessary.
+class Model {
+ public:
+  // The Client which is responsible for handling all relevant messages from the
+  // model.
+  class Client {
+   public:
+    virtual ~Client() = default;
+
+    // Called asynchronously in response to a Model::Initialize call.  If
+    // |success| is |false|, initialization of the Model and/or the underlying
+    // Store failed.  Initialization of the Model is complete after this
+    // callback.  If |success| is true it can be accessed now.
+    virtual void OnInitialized(bool success) = 0;
+
+    // Called asynchronously in response to a Model::Destroy call.  If |success|
+    // is |false|, destruction of the Model and/or the underlying Store failed.
+    // Destruction of the Model is effectively complete after this callback.
+    virtual void OnDestroyed(bool success) = 0;
+
+    // Called when an Entry addition is complete.  |success| determines whether
+    // or not the entry has been successfully persisted to the Store.
+    virtual void OnItemAdded(bool success,
+                             DownloadClient client,
+                             const std::string& guid) = 0;
+
+    // Called when an Entry update is complete.  |success| determines whether or
+    // not the update has been successfully persisted to the Store.
+    virtual void OnItemUpdated(bool success,
+                               DownloadClient client,
+                               const std::string& guid) = 0;
+
+    // Called when an Entry removal is complete.  |success| determines whether
+    // or not the entry has been successfully removed from the Store.
+    virtual void OnItemRemoved(bool success,
+                               DownloadClient client,
+                               const std::string& guid) = 0;
+  };
+
+  using EntryList = std::vector<Entry*>;
+
+  virtual ~Model() = default;
+
+  // Initializes the Model.  Client::OnInitialized() will be called in response.
+  // The Model can be used after that call.
+  virtual void Initialize() = 0;
+
+  // Destroys the Model.  Client::OnDestroyed() will be called in response.
+  virtual void Destroy() = 0;
+
+  // Adds |entry| to this Model and attempts to write |entry| to the Store.
+  // Client::OnItemAdded() will be called in response asynchronously.
+  virtual void Add(const Entry& entry) = 0;
+
+  // Updates |entry| in this Model and attempts to write |entry| to the Store.
+  // Client::OnItemUpdated() will be called in response asynchronously.
+  virtual void Update(const Entry& entry) = 0;
+
+  // Removes the Entry specified by |guid| from this Model and attempts to
+  // remove that entry from the Store.  Client::OnItemRemoved() will be called
+  // in response asynchronously.
+  virtual void Remove(const std::string& guid) = 0;
+
+  // Retrieves an Entry identified by |guid| or |nullptr| if no entry exists.
+  // IMPORTANT NOTE: The result of this method should be used immediately and
+  // NOT stored.  The underlying data may get updated or removed by any other
+  // modifications to this model.
+  virtual Entry* Get(const std::string& guid) = 0;
+
+  // Returns a temporary list of Entry objects that this Model stores.
+  // IMPORTANT NOTE: Like Get(), the result of this method should be used
+  // immediately and NOT stored.  The underlying data may get updated or removed
+  // by any other modifications to this model.
+  virtual EntryList PeekEntries() = 0;
+};
+
+}  // namespace download
+
+#endif  // COMPONENTS_DOWNLOAD_INTERNAL_MODEL_H_