text: // Copyright 2018 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 CONTENT_BROWSER_FRAME_HOST_BACK_FORWARD_CACHE_H_
  #define CONTENT_BROWSER_FRAME_HOST_BACK_FORWARD_CACHE_H_
  
  #include <list>
  #include <memory>
  
  #include "base/macros.h"
  #include "content/common/content_export.h"
  
  namespace content {
  
  class RenderFrameHostImpl;
  
  // BackForwardCache:
  //
  // After the user navigates away from a document, the old one goes into the
  // frozen state and is kept in this object. They can potentially be reused
  // after an history navigation. Reusing a document means swapping it back with
  // the current_frame_host.
  class CONTENT_EXPORT BackForwardCache {
   public:
    BackForwardCache();
    ~BackForwardCache();
  
    // Returns true when a RenderFrameHost can be stored into the
    // BackForwardCache. Depends on the |render_frame_host| and its children's
    // state.
    bool CanStoreDocument(RenderFrameHostImpl* render_frame_host);
  
    // Moves |render_frame_host| into the BackForwardCache. It can be reused in
    // a future history navigation by using RestoreDocument(). When the
    // BackForwardCache is full, the least recently used document is evicted.
    // Precondition: CanStoreDocument(render_frame_host).
    void StoreDocument(std::unique_ptr<RenderFrameHostImpl>);
  
    // During a history navigation, move a document out of the BackForwardCache
    // knowing its navigation entry ID. Returns nullptr when none is found.
    std::unique_ptr<RenderFrameHostImpl> RestoreDocument(int navigation_entry_id);
  
    // Remove all entries from the BackForwardCache.
    void Flush();
  
   private:
    // Contains the set of stored RenderFrameHost.
    // Invariant:
    // - Ordered from the most recently used to the last recently used.
    // - Once the list is full, the least recently used document is evicted.
    std::list<std::unique_ptr<RenderFrameHostImpl>> render_frame_hosts_;
  
    DISALLOW_COPY_AND_ASSIGN(BackForwardCache);
  };
  }  // namespace content
  
  #endif  // CONTENT_BROWSER_FRAME_HOST_BACK_FORWARD_CACHE_H_
  