Offline Page Cache uses user interaction triggers for saving pages.

components/offline_pages/core/snapshot_controller.h

 // 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_OFFLINE_PAGES_CORE_SNAPSHOT_CONTROLLER_H_
 #define COMPONENTS_OFFLINE_PAGES_CORE_SNAPSHOT_CONTROLLER_H_
 
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "base/single_thread_task_runner.h"
@@ skipping 12 matching lines @@
 //   OnLoad is currently such signal.
 // - Once Reset() is called on the SnapshotController, the delayed tasks are
 //   reset so no StartSnapshot calls is made 'cross-session'.
 class SnapshotController {
  public:
   enum class State {
     READY,             // Listening to input, will start snapshot when needed.
     SNAPSHOT_PENDING,  // Snapshot is in progress, don't start another.
     STOPPED,           // Terminal state, no snapshots until reset.
   };
+  // The expected quality of a page based on its current loading progress.
+  enum class PageQuality {
+    // Not loaded enough to reach a minimum level of quality. Snapshots taken at
+    // this point are expected to be useless.
+    POOR = 0,
+    // A minimal level of quality has been attained but the page is still
+    // loading so its quality is continuously increasing. One or more snapshots
+    // could be taken at this stage as later ones are expected to have higher
+    // quality.
+    FAIR_AND_IMPROVING,
+    // The page is loaded enough and has attained its peak expected quality.
+    // Snapshots taken at this point are not expected to increase in quality
+    // after the first one.
+    HIGH,
+  };
 
   // Client of the SnapshotController.
   class Client {
    public:
     // Invoked at a good moment to start a snapshot. May be invoked multiple
     // times, but not in overlapping manner - waits until
     // PreviousSnapshotCompleted() before the next StatrSnapshot().
     // Client should overwrite the result of previous snapshot with the new one,
     // it is assumed that later snapshots are better then previous.
     virtual void StartSnapshot() = 0;
@@ skipping 26 matching lines @@
 
   // Invoked from WebContentObserver::DocumentAvailableInMainFrame
   void DocumentAvailableInMainFrame();
 
   // Invoked from WebContentObserver::DocumentOnLoadCompletedInMainFrame
   void DocumentOnLoadCompletedInMainFrame();
 
   size_t GetDelayAfterDocumentAvailableForTest();
   size_t GetDelayAfterDocumentOnLoadCompletedForTest();
 
+  PageQuality current_page_quality() const { return current_page_quality_; }
+
  private:
-  void MaybeStartSnapshot();
+  void MaybeStartSnapshot(PageQuality updated_page_quality);
   void MaybeStartSnapshotThenStop();
 
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
   // Client owns this class.
   SnapshotController::Client* client_;
   SnapshotController::State state_;
   size_t delay_after_document_available_ms_;
   size_t delay_after_document_on_load_completed_ms_;
 
+  // The expected quality of a snapshot taken at the moment this value is
+  // queried.
+  PageQuality current_page_quality_ = PageQuality::POOR;
+
   base::WeakPtrFactory<SnapshotController> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SnapshotController);
 };
 
 }  // namespace offline_pages
 
 #endif  // COMPONENTS_OFFLINE_PAGES_CORE_SNAPSHOT_CONTROLLER_H_