Adds kind-of-live thumbnail generation for a potential tab switcher.

chrome/browser/tab_contents/thumbnail_generator.cc

+// Copyright (c) 2009 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.
+
+#include "chrome/browser/tab_contents/thumbnail_generator.h"
+
+#include <algorithm>
+
+#include "base/histogram.h"
+#include "base/time.h"
+#include "chrome/browser/renderer_host/backing_store.h"
+#include "chrome/browser/renderer_host/render_view_host.h"
+#include "chrome/common/notification_service.h"
+#include "chrome/common/property_bag.h"
+#include "skia/ext/image_operations.h"
+#include "skia/ext/platform_canvas.h"
+#include "third_party/skia/include/core/SkBitmap.h"
+
+// Overview
+// --------
+// This class provides current thumbnails for tabs. The simplest operation is
+// when a request for a thumbnail comes in, to grab the backing store and make
+// a smaller version of that.
+//
+// A complication happens because we don't always have nice backing stores for
+// all tabs (there is a cache of several tabs we'll keep backing stores for).
+// To get thumbnails for tabs with expired backing stores, we listen for
+// backing stores that are being thrown out, and generate thumbnails before
+// that happens. We attach them to the RenderWidgetHost via the property bag
+// so we can retrieve them later. When a tab has a live backing store again,
+// we throw away the thumbnail since it's now out-of-date.
+//
+// Another complication is performance. If the user brings up a tab switcher, we
+// don't want to get all 5 cached backing stores since it is a very large amount
+// of data. As a result, we generate thumbnails for tabs that are hidden even
+// if the backing store is still valid. This means we'll have to do a maximum
+// of generating thumbnails for the visible tabs at any point.
+//
+// The last performance consideration is when the user switches tabs quickly.
+// This can happen by doing Control-PageUp/Down or juct clicking quickly on
+// many different tabs (like when you're looking for one). We don't want to
+// slow this down by making thumbnails for each tab as it's hidden. Therefore,
+// we have a timer so that we don't invalidate thumbnails for tabs that are
+// only shown briefly (which would cause the thumbnail to be regenerated when
+// the tab is hidden).
+
+namespace {
+
+static const int kThumbnailWidth = 294;
+static const int kThumbnailHeight = 204;
+
+// Indicates the time that the RWH must be visible for us to update the
+// thumbnail on it. If the user holds down control enter, there will be a lot
+// of backing stores created and destroyed. WE don't want to interfere with
+// that.
+//
+// Any operation that happens within this time of being shown is ignored.
+// This means we won't throw the thumbnail away when the backing store is
+// painted in this time.
+static const int kVisibilitySlopMS = 3000;
+
+struct WidgetThumbnail {
+  SkBitmap thumbnail;
+
+  // Indicates the last time the RWH was shown and hidden.
+  base::TimeTicks last_shown;
+  base::TimeTicks last_hidden;
+};
+
+PropertyAccessor<WidgetThumbnail>* GetThumbnailAccessor() {
+  static PropertyAccessor<WidgetThumbnail> accessor;
+  return &accessor;
+}
+
+// Returns the existing WidgetThumbnail for a RVH, or creates a new one and
+// returns that if none exists.
+WidgetThumbnail* GetDataForHost(RenderWidgetHost* host) {
+  WidgetThumbnail* wt = GetThumbnailAccessor()->GetProperty(
+      host->property_bag());
+  if (wt)
+    return wt;
+
+  GetThumbnailAccessor()->SetProperty(host->property_bag(),
+                                      WidgetThumbnail());
+  return GetThumbnailAccessor()->GetProperty(host->property_bag());
+}
+
+// Creates a downsampled thumbnail for the given backing store. The returned
+// bitmap will be isNull if there was an error creating it.
+SkBitmap GetThumbnailForBackingStore(BackingStore* backing_store) {
+  SkBitmap result;
+
+  // TODO(brettw) write this for other platforms. If you enable this, be sure
+  // to also enable the unit tests for the same platform in
+  // thumbnail_generator_unittest.cc
+#if defined(OS_WIN)
+  // Get the bitmap as a Skia object so we can resample it. This is a large
+  // allocation and we can tolerate failure here, so give up if the allocation
+  // fails.
+  base::TimeTicks begin_compute_thumbnail = base::TimeTicks::Now();
+
+  skia::PlatformCanvas temp_canvas;
+  if (!temp_canvas.initialize(backing_store->size().width(),
+                              backing_store->size().height(), true))
+    return SkBitmap();
+  HDC temp_dc = temp_canvas.beginPlatformPaint();
+  BitBlt(temp_dc,
+         0, 0, backing_store->size().width(), backing_store->size().height(),
+         backing_store->hdc(), 0, 0, SRCCOPY);
+  temp_canvas.endPlatformPaint();
+
+  // Get the bitmap out of the canvas and resample it.
+  const SkBitmap& bmp = temp_canvas.getTopPlatformDevice().accessBitmap(false);
+  result = skia::ImageOperations::DownsampleByTwoUntilSize(
+      bmp,
+      kThumbnailWidth, kThumbnailHeight);
+  if (bmp.width() == result.width() && bmp.height() == result.height()) {
+    // This is a bit subtle. SkBitmaps are refcounted, but the magic ones in
+    // PlatformCanvas can't be ssigned to SkBitmap with proper refcounting.
+    // If the bitmap doesn't change, then the downsampler will return the input
+    // bitmap, which will be the reference to the weird PlatformCanvas one
+    // insetad of a regular one. To get a regular refcounted bitmap, we need to
+    // copy it.
+    bmp.copyTo(&result, SkBitmap::kARGB_8888_Config);
+  }
+
+  HISTOGRAM_TIMES("Thumbnail.ComputeOnDestroyMS",
+                  base::TimeTicks::Now() - begin_compute_thumbnail);
+#endif
+
+  return result;
+}
+
+}  // namespace
+
+ThumbnailGenerator::ThumbnailGenerator()
+    : no_timeout_(false) {
+  // Even though we deal in RenderWidgetHosts, we only care about its subclass,
+  // RenderViewHost when it is in a tab. We don't make thumbnails for
+  // RenderViewHosts that aren't in tabs, or RenderWidgetHosts that aren't
+  // views like select popups.
+  registrar_.Add(this, NotificationType::RENDER_VIEW_HOST_CREATED_FOR_TAB,
+                 NotificationService::AllSources());
+
+  registrar_.Add(this, NotificationType::RENDER_WIDGET_VISIBILITY_CHANGED,
+                 NotificationService::AllSources());
+  registrar_.Add(this, NotificationType::RENDER_WIDGET_HOST_DESTROYED,
+                 NotificationService::AllSources());
+}
+
+ThumbnailGenerator::~ThumbnailGenerator() {
+}
+
+SkBitmap ThumbnailGenerator::GetThumbnailForRenderer(
+    RenderWidgetHost* renderer) const {
+  // Return a cached one if we have it and it's still valid. This will only be
+  // valid when there used to be a backing store, but there isn't now.
+  WidgetThumbnail* wt = GetThumbnailAccessor()->GetProperty(
+      renderer->property_bag());
+  if (wt && !wt->thumbnail.isNull() &&
+      (no_timeout_ ||
+       base::TimeTicks::Now() -
+       base::TimeDelta::FromMilliseconds(kVisibilitySlopMS) < wt->last_shown))
+    return wt->thumbnail;
+
+  BackingStore* backing_store = renderer->GetBackingStore(false);
+  if (!backing_store)
+    return SkBitmap();
+
+  // Save this thumbnail in case we need to use it again soon. It will be
+  // invalidated on the next paint.
+  wt->thumbnail = GetThumbnailForBackingStore(backing_store);
+  return wt->thumbnail;
+}
+
+void ThumbnailGenerator::WidgetWillDestroyBackingStore(
+    RenderWidgetHost* widget,
+    BackingStore* backing_store) {
+  // Since the backing store is going away, we need to save it as a thumbnail.
+  WidgetThumbnail* wt = GetDataForHost(widget);
+
+  // If there is already a thumbnail on the RWH that's visible, it means that
+  // not enough time has elapsed since being shown, and we can ignore generating
+  // a new one.
+  if (!wt->thumbnail.isNull())
+    return;
+
+  // Save a scaled-down image of the page in case we're asked for the thumbnail
+  // when there is no RenderViewHost. If this fails, we don't want to overwrite
+  // an existing thumbnail.
+  SkBitmap new_thumbnail = GetThumbnailForBackingStore(backing_store);
+  if (!new_thumbnail.isNull())
+    wt->thumbnail = new_thumbnail;
+}
+
+void ThumbnailGenerator::WidgetDidUpdateBackingStore(
+    RenderWidgetHost* widget) {
+  // Clear the current thumbnail since it's no longer valid.
+  WidgetThumbnail* wt = GetThumbnailAccessor()->GetProperty(
+      widget->property_bag());
+  if (!wt)
+    return;  // Nothing to do.
+
+  // If this operation is within the time slop after being shown, keep the
+  // existing thumbnail.
+  if (no_timeout_ ||
+      base::TimeTicks::Now() -
+      base::TimeDelta::FromMilliseconds(kVisibilitySlopMS) < wt->last_shown)
+    return;  // TODO(brettw) schedule thumbnail generation for this renderer in
+             // case we don't get a paint for it after the time slop, but it's
+             // still visible.
+
+  // Clear the thumbnail, since it's now out of date.
+  wt->thumbnail = SkBitmap();
+}
+
+void ThumbnailGenerator::Observe(NotificationType type,
+                                 const NotificationSource& source,
+                                 const NotificationDetails& details) {
+  switch (type.value) {
+    case NotificationType::RENDER_VIEW_HOST_CREATED_FOR_TAB: {
+      // Install our observer for all new RVHs.
+      RenderViewHost* renderer = Details<RenderViewHost>(details).ptr();
+      renderer->set_painting_observer(this);
+      break;
+    }
+
+    case NotificationType::RENDER_WIDGET_VISIBILITY_CHANGED:
+      if (*Details<bool>(details).ptr())
+        WidgetShown(Source<RenderWidgetHost>(source).ptr());
+      else
+        WidgetHidden(Source<RenderWidgetHost>(source).ptr());
+      break;
+
+    case NotificationType::RENDER_WIDGET_HOST_DESTROYED:
+      WidgetDestroyed(Source<RenderWidgetHost>(source).ptr());
+      break;
+
+    default:
+      NOTREACHED();
+  }
+}
+
+void ThumbnailGenerator::WidgetShown(RenderWidgetHost* widget) {
+  WidgetThumbnail* wt = GetDataForHost(widget);
+  wt->last_shown = base::TimeTicks::Now();
+
+  // If there is no thumbnail (like we're displaying a background tab for the
+  // first time), then we don't have do to invalidate the existing one.
+  if (wt->thumbnail.isNull())
+    return;
+
+  std::vector<RenderWidgetHost*>::iterator found =
+      std::find(shown_hosts_.begin(), shown_hosts_.end(), widget);
+  if (found != shown_hosts_.end()) {
+    NOTREACHED() << "Showing a RWH we already think is shown";
+    shown_hosts_.erase(found);
+  }
+  shown_hosts_.push_back(widget);
+
+  // Keep the old thumbnail for a small amount of time after the tab has been
+  // shown. This is so in case it's hidden quickly again, we don't waste any
+  // work regenerating it.
+  if (timer_.IsRunning())
+    return;
+  timer_.Start(base::TimeDelta::FromMilliseconds(
+                   no_timeout_ ? 0 : kVisibilitySlopMS),
+               this, &ThumbnailGenerator::ShownDelayHandler);
+}
+
+void ThumbnailGenerator::WidgetHidden(RenderWidgetHost* widget) {
+  WidgetThumbnail* wt = GetDataForHost(widget);
+  wt->last_hidden = base::TimeTicks::Now();
+
+  // If the tab is on the list of ones to invalidate the thumbnail, we need to
+  // remove it.
+  EraseHostFromShownList(widget);
+
+  // There may still be a valid cached thumbnail on the RWH, so we don't need to
+  // make a new one.
+  if (!wt->thumbnail.isNull())
+    return;
+  wt->thumbnail = GetThumbnailForRenderer(widget);
+}
+
+void ThumbnailGenerator::WidgetDestroyed(RenderWidgetHost* widget) {
+  EraseHostFromShownList(widget);
+}
+
+void ThumbnailGenerator::ShownDelayHandler() {
+  base::TimeTicks threshold = base::TimeTicks::Now() -
+      base::TimeDelta::FromMilliseconds(kVisibilitySlopMS);
+
+  // Check the list of all pending RWHs (normally only one) to see if any of
+  // their times have expired.
+  for (size_t i = 0; i < shown_hosts_.size(); i++) {
+    WidgetThumbnail* wt = GetDataForHost(shown_hosts_[i]);
+    if (no_timeout_ || wt->last_shown <= threshold) {
+      // This thumbnail has expired, delete it.
+      wt->thumbnail = SkBitmap();
+      shown_hosts_.erase(shown_hosts_.begin() + i);
+      i--;
+    }
+  }
+
+  // We need to schedule another run if there are still items in the list to
+  // process. We use half the timeout for these re-runs to catch the items
+  // that were added since the timer was run the first time.
+  if (!shown_hosts_.empty()) {
+    DCHECK(!no_timeout_);
+    timer_.Start(base::TimeDelta::FromMilliseconds(kVisibilitySlopMS) / 2, this,
+                 &ThumbnailGenerator::ShownDelayHandler);
+  }
+}
+
+void ThumbnailGenerator::EraseHostFromShownList(RenderWidgetHost* widget) {
+  std::vector<RenderWidgetHost*>::iterator found =
+      std::find(shown_hosts_.begin(), shown_hosts_.end(), widget);
+  if (found != shown_hosts_.end())
+    shown_hosts_.erase(found);
+}