Allows the PDF engine return the page index it is scrolling to

pdf/pdfium/pdfium_engine.h

 // Copyright (c) 2012 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 PDF_PDFIUM_PDFIUM_ENGINE_H_
 #define PDF_PDFIUM_PDFIUM_ENGINE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <map>
 #include <memory>
 #include <string>
 #include <utility>
 #include <vector>
 
 #include "base/macros.h"
+#include "base/optional.h"
 #include "base/time/time.h"
 #include "pdf/document_loader.h"
 #include "pdf/pdf_engine.h"
 #include "pdf/pdfium/pdfium_page.h"
 #include "pdf/pdfium/pdfium_range.h"
 #include "ppapi/cpp/completion_callback.h"
 #include "ppapi/cpp/dev/buffer_dev.h"
 #include "ppapi/cpp/image_data.h"
 #include "ppapi/cpp/point.h"
 #include "ppapi/cpp/var_array.h"
@@ skipping 214 matching lines @@
   // document size.
   void LoadPageInfo(bool reload);
 
   // Calculates which pages should be displayed right now.
   void CalculateVisiblePages();
 
   // Returns true iff the given page index is visible.  CalculateVisiblePages
   // must have been called first.
   bool IsPageVisible(int index) const;
 
+  // Internal interface that caches the page index requested by the PDFium
+  // engine
+  // to be used during the interval the client has not finished handling it.

[Lei Zhang, 2016/08/27 01:20:54]

nit: funny line break.

+  void ScrollToPage(int page);
+
   // Checks if a page is now available, and if so marks it as such and returns
   // true.  Otherwise, it will return false and will add the index to the given
   // array if it's not already there.
   bool CheckPageAvailable(int index, std::vector<int>* pending);
 
   // Helper function to get a given page's size in pixels.  This is not part of
   // PDFiumPage because we might not have that structure when we need this.
   pp::Size GetPageSize(int index);
 
   void GetAllScreenRectsUnion(std::vector<PDFiumRange>* rect_range,
@@ skipping 409 matching lines @@
   std::map<int, std::pair<int, TimerCallback> > timers_;
   int next_timer_id_;
 
   // Holds the zero-based page index of the last page that the mouse clicked on.
   int last_page_mouse_down_;
 
   // Holds the zero-based page index of the most visible page; refreshed by
   // calling CalculateVisiblePages()
   int most_visible_page_;
 
+  // Holds the page index requested by PDFium while there are in-flight
+  // messages being sent by the Browser <-> PDFium engine.

[Lei Zhang, 2016/08/27 01:20:54]

The other side is not the Browser. It's Chromium's "pdf.js" which is still part
of the PDF Plugin. We wouldn't run JS in the browser process. The JS and C++
portions of the PDF plugin indeed sends each other messages asynchronously.

+  base::Optional<int> in_flight_visible_page_;
+
   // Set to true after FORM_DoDocumentJSAction/FORM_DoDocumentOpenAction have
   // been called. Only after that can we call FORM_DoPageAAction.
   bool called_do_document_action_;
 
   // Records parts of form fields that need to be highlighted at next paint, in
   // screen coordinates.
   std::vector<pp::Rect> form_highlights_;
 
   // Whether to render in grayscale or in color.
   bool render_grayscale_;
@@ skipping 68 matching lines @@
   bool GetPDFPageSizeByIndex(const void* pdf_buffer,
                              int pdf_buffer_size,
                              int page_number,
                              double* width,
                              double* height) override;
 };
 
 }  // namespace chrome_pdf
 
 #endif  // PDF_PDFIUM_PDFIUM_ENGINE_H_