navigation: Retain a screenshot of the page before it unloads

content/browser/web_contents/navigation_controller_impl.cc

Index: content/browser/web_contents/navigation_controller_impl.cc
diff --git a/content/browser/web_contents/navigation_controller_impl.cc b/content/browser/web_contents/navigation_controller_impl.cc
index 9481ed69c3db1a195ad77c892530b855e293adf8..2e83475be34cb8e4345929747d52b3e5dc053d44 100644
--- a/content/browser/web_contents/navigation_controller_impl.cc
+++ b/content/browser/web_contents/navigation_controller_impl.cc
@@ -5,6 +5,7 @@
 #include "content/browser/web_contents/navigation_controller_impl.h"
 
 #include "base/bind.h"
+#include "base/command_line.h"
 #include "base/file_util.h"
 #include "base/logging.h"
 #include "base/string_number_conversions.h"  // Temporary
@@ -28,15 +29,20 @@
 #include "content/public/browser/navigation_details.h"
 #include "content/public/browser/notification_service.h"
 #include "content/public/browser/notification_types.h"
+#include "content/public/browser/render_widget_host.h"
+#include "content/public/browser/render_widget_host_view.h"
 #include "content/public/browser/storage_partition.h"
 #include "content/public/browser/user_metrics.h"
 #include "content/public/browser/web_contents_delegate.h"
 #include "content/public/common/content_client.h"
 #include "content/public/common/content_constants.h"
+#include "content/public/common/content_switches.h"
 #include "content/public/common/url_constants.h"
 #include "net/base/escape.h"
 #include "net/base/mime_util.h"
 #include "net/base/net_util.h"
+#include "skia/ext/platform_canvas.h"
+#include "ui/gfx/codec/png_codec.h"
 #include "webkit/glue/glue_serialize.h"
 
 namespace content {
@@ -470,6 +476,64 @@ int NavigationControllerImpl::GetIndexForOffset(int offset) const {
   return GetCurrentEntryIndex() + offset;
 }
 
+void NavigationControllerImpl::TakeScreenshot() {
+  static bool overscroll_enabled = CommandLine::ForCurrentProcess()->
+      HasSwitch(switches::kEnableOverscrollHistoryNavigation);
+  if (!overscroll_enabled)
+    return;
+
+  CHECK(web_contents_);

[Charlie Reis, 2012/12/21 23:22:07]

This CHECK isn't really necessary.  A NavigationController always has one, and
if we didn't, it would crash a few lines below anyway.

[sadrul, 2012/12/22 00:07:10]

Removed.

+
+  NavigationEntryImpl* active_entry =

[Charlie Reis, 2012/12/21 23:22:07]

Calling this active_entry is confusing.  Perhaps just entry?

[sadrul, 2012/12/22 00:07:10]

Done.

+      NavigationEntryImpl::FromNavigationEntry(GetLastCommittedEntry());
+  if (!active_entry)
+    return;
+
+  RenderViewHost* render_view_host = web_contents_->GetRenderViewHost();
+  content::RenderWidgetHostView* view = render_view_host->GetView();
+  if (!view)
+    return;
+
+  skia::PlatformBitmap* temp_bitmap = new skia::PlatformBitmap;
+  render_view_host->CopyFromBackingStore(gfx::Rect(),
+      view->GetViewBounds().size(),

[Charlie Reis, 2012/12/21 23:22:07]

What happens if the view size changes when you want to use it?

[sadrul, 2012/12/22 00:07:10]

Currently, we paint the screenshot in the original size, i.e. we do not try to
fit it into the new size (so there will be blanks if the size increases after
the navigation). An alternate approach would be to discard the screenshot in
such cases, or display the placeholder image instead.

+      base::Bind(&NavigationControllerImpl::TakingScreenshotComplete,
+                 base::Unretained(this),
+                 active_entry->GetUniqueID(),
+                 base::Owned(temp_bitmap)),
+      temp_bitmap);
+}
+
+void NavigationControllerImpl::TakingScreenshotComplete(
+    int unique_id,
+    skia::PlatformBitmap* bitmap,
+    bool success) {
+  NavigationEntryImpl* entry = NULL;
+  for (NavigationEntries::iterator i = entries_.begin();
+       i != entries_.end();
+       ++i) {
+    if ((*i)->GetUniqueID() == unique_id) {
+      entry = (*i).get();
+      break;
+    }
+  }
+
+  if (!entry) {
+    LOG(ERROR) << "Invalid entry with unique id: " << unique_id;
+    return;
+  }
+
+  if (!success) {

[Charlie Reis, 2012/12/21 23:22:07]

Should we just move this to the top and skip searching for the entry in this
case?  Not sure having the URL is that useful, and you could at least show the
ID.

[sadrul, 2012/12/22 00:07:10]

Sounds good. Done.

+    LOG(ERROR) << "Taking snapshot was unsuccessful for "
+               << entry->GetURL().spec();
+    return;
+  }
+
+  std::vector<unsigned char> data;
+  if (gfx::PNGCodec::EncodeBGRASkBitmap(bitmap->GetBitmap(), true, &data))
+    entry->SetScreenshotPNGData(data);
+}
+
 bool NavigationControllerImpl::CanGoBack() const {
   return entries_.size() > 1 && GetCurrentEntryIndex() > 0;
 }
@@ -690,6 +754,8 @@ void NavigationControllerImpl::DocumentLoadedInFrame() {
 bool NavigationControllerImpl::RendererDidNavigate(
     const ViewHostMsg_FrameNavigate_Params& params,
     LoadCommittedDetails* details) {
+  if (details->is_main_frame)
+    TakeScreenshot();

[Charlie Reis, 2012/12/21 23:22:07]

Are we certain this is going to show the previous page, and can't show anything
from page that has just committed?  The screenshot function looks asynchronous.

Also, please add a comment explaining why this exists here.

[sadrul, 2012/12/22 00:07:10]

Hm, good point. For the software compositing case, the data is copied from the
backing-store synchronously. So this should work properly.

For the compositing case, from what I understand, the new page should have a
different texture; so the worst that can happen is that the texture of the
current page will be destroyed, and taking the screenshot will fail. But the
screenshot should not include contents of the newly committed page in either
case.
Done.

 
   // Save the previous state before we clobber it.
   if (GetLastCommittedEntry()) {
@@ -1441,6 +1507,8 @@ void NavigationControllerImpl::PruneOldestEntryIfFull() {
 void NavigationControllerImpl::NavigateToPendingEntry(ReloadType reload_type) {
   needs_reload_ = false;
 
+  TakeScreenshot();

[Charlie Reis, 2012/12/21 23:22:07]

Why is this needed here?  This isn't called for all navigations (e.g.,
renderer-initiated ones), and it's redundant with the call in
RendererDidNavigate.

[sadrul, 2012/12/22 00:07:10]

Removed.

+
   // If we were navigating to a slow-to-commit page, and the user performs
   // a session history navigation to the last committed page, RenderViewHost
   // will force the throbber to start, but WebKit will essentially ignore the