Debounce watching of thumbnails.

Instead of watching and unwatching the thumbnail associated with a hover
card every time the mouse moves on or off a tab (or something grabs
mouse focus) we now watch a handful of thumbnails of tabs we've recently
hovered.

Note that re-watching a tab with a valid thumbnail is mostly a no-op.

Bug: 1021332
Change-Id: I55646c89514a001052cd6d453ffe3a3b6e11a731
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1899388
Auto-Submit: Dana Fried <dfried@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Caroline Rising <corising@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712810}

chrome/browser/ui/views/tabs/tab_hover_card_bubble_view.cc

@@ -7,6 +7,7 @@
 #include <algorithm>
 #include <memory>
 
+#include "base/containers/mru_cache.h"
 #include "base/metrics/field_trial_params.h"
 #include "base/metrics/histogram_macros.h"
 #include "build/build_config.h"
@@ -308,6 +309,75 @@ class TabHoverCardBubbleView::FadeLabel : public views::Label {
   }
 };
 
+// Maintains a set of thumbnails to watch, ensuring the capture count on the
+// associated WebContents stays nonzero until a valid thumbnail has been
+// captured.
+class TabHoverCardBubbleView::ThumbnailWatcher {
+ public:
+  explicit ThumbnailWatcher(TabHoverCardBubbleView* hover_card)
+      : hover_card_(hover_card) {}
+  ~ThumbnailWatcher() = default;
+
+  // Begin watching the specified thumbnail image for updates. Ideally, should
+  // trigger the associated WebContents to load (if not loaded already) and
+  // retrieve a valid thumbnail. If too many thumbnails are being watched, the
+  // least-recently watched will be unwatched.
+  void Watch(scoped_refptr<ThumbnailImage> thumbnail_image) {
+    ThumbnailImage* const ptr = thumbnail_image.get();
+    auto it = recent_observers_.Get(ptr);
+    if (it == recent_observers_.end()) {
+      recent_observers_.Put(ptr, std::make_unique<ThumbnailObserver>(
+                                     this, std::move(thumbnail_image)));
+    }
+    ptr->RequestThumbnailImage();
+  }
+
+  // Returns the current (most recent) thumbnail being watched.
+  ThumbnailImage* current_image() const {
+    return recent_observers_.empty() ? nullptr
+                                     : recent_observers_.begin()->first;
+  }
+
+  void OnNewImage(const ThumbnailImage* thumbnail, gfx::ImageSkia image) {
+    DCHECK(!recent_observers_.empty());
+    if (recent_observers_.begin()->first == thumbnail)
+      hover_card_->OnThumbnailImageAvailable(std::move(image));
+  }
+
+ private:
+  // Actually does the work of watching a single thumbnail. Cleans itself up
+  // (including unregistering as an observer) on destruction.
+  class ThumbnailObserver : public ThumbnailImage::Observer {
+   public:
+    ThumbnailObserver(ThumbnailWatcher* thumbnail_watcher,
+                      scoped_refptr<ThumbnailImage> thumbnail_image)
+        : thumbnail_watcher_(thumbnail_watcher),
+          thumbnail_image_(std::move(thumbnail_image)) {
+      scoped_observer_.Add(thumbnail_image_.get());
+    }
+    ~ThumbnailObserver() override = default;
+
+    base::Optional<gfx::Size> GetThumbnailSizeHint() const override {
+      return TabStyle::GetPreviewImageSize();
+    }
+
+    void OnThumbnailImageAvailable(gfx::ImageSkia preview_image) override {
+      thumbnail_watcher_->OnNewImage(thumbnail_image_.get(),
+                                     std::move(preview_image));
+    }
+
+   private:
+    ThumbnailWatcher* const thumbnail_watcher_;
+    scoped_refptr<ThumbnailImage> thumbnail_image_;
+    ScopedObserver<ThumbnailImage, ThumbnailImage::Observer> scoped_observer_{
+        this};
+  };
+
+  TabHoverCardBubbleView* const hover_card_;
+  base::MRUCache<ThumbnailImage*, std::unique_ptr<ThumbnailObserver>>
+      recent_observers_{5};
+};
+
 TabHoverCardBubbleView::TabHoverCardBubbleView(Tab* tab)
     : BubbleDialogDelegateView(tab, views::BubbleBorder::TOP_LEFT) {
   // We'll do all of our own layout inside the bubble, so no need to inset this
@@ -403,6 +473,7 @@ TabHoverCardBubbleView::TabHoverCardBubbleView(Tab* tab)
       std::make_unique<WidgetSlideAnimationDelegate>(this);
   fade_animation_delegate_ =
       std::make_unique<WidgetFadeAnimationDelegate>(widget_);
+  thumbnail_watcher_ = std::make_unique<ThumbnailWatcher>(this);
 
   constexpr int kFootnoteVerticalMargin = 8;
   GetBubbleFrameView()->set_footnote_margins(
@@ -504,7 +575,6 @@ bool TabHoverCardBubbleView::IsVisible() {
 
 void TabHoverCardBubbleView::FadeOutToHide() {
   delayed_show_timer_.Stop();
-  RegisterToThumbnailImageUpdates(nullptr);
   if (!widget_->IsVisible())
     return;
   slide_animation_delegate_->StopAnimation();
@@ -665,7 +735,13 @@ void TabHoverCardBubbleView::UpdateCardContent(const Tab* tab) {
 
   // If the preview image feature is not enabled, |preview_image_| will be null.
   if (preview_image_ && preview_image_->GetVisible()) {
-    RegisterToThumbnailImageUpdates(tab->data().thumbnail);
+    auto thumbnail = tab->data().thumbnail;
+    if (!thumbnail) {
+      ClearPreviewImage();
+    } else if (thumbnail != thumbnail_watcher_->current_image()) {
+      waiting_for_decompress_ = true;
+      thumbnail_watcher_->Watch(thumbnail);
+    }
   }
 }
 
@@ -674,28 +750,6 @@ void TabHoverCardBubbleView::UpdateTextFade(double percent) {
   domain_fade_label_->SetFade(percent);
 }
 
-void TabHoverCardBubbleView::RegisterToThumbnailImageUpdates(
-    scoped_refptr<ThumbnailImage> thumbnail_image) {
-  if (thumbnail_image_ == thumbnail_image)
-    return;
-
-  if (thumbnail_image_) {
-    thumbnail_observer_.Remove(thumbnail_image_.get());
-    thumbnail_image_.reset();
-  }
-
-  if (thumbnail_image) {
-    if (!thumbnail_image->has_data())
-      ClearPreviewImage();
-    else
-      waiting_for_decompress_ = true;
-
-    thumbnail_image_ = thumbnail_image;
-    thumbnail_observer_.Add(thumbnail_image_.get());
-    thumbnail_image->RequestThumbnailImage();
-  }
-}
-
 void TabHoverCardBubbleView::ClearPreviewImage() {
   // Check the no-preview color and size to see if it needs to be
   // regenerated. DPI or theme change can cause a regeneration.
@@ -748,10 +802,6 @@ void TabHoverCardBubbleView::OnThumbnailImageAvailable(
   waiting_for_decompress_ = false;
 }
 
-base::Optional<gfx::Size> TabHoverCardBubbleView::GetThumbnailSizeHint() const {
-  return TabStyle::GetPreviewImageSize();
-}
-
 gfx::Size TabHoverCardBubbleView::CalculatePreferredSize() const {
   gfx::Size preferred_size = GetLayoutManager()->GetPreferredSize(this);
   preferred_size.set_width(TabStyle::GetPreviewImageSize().width());

chrome/browser/ui/views/tabs/tab_hover_card_bubble_view.h

@@ -7,12 +7,10 @@
 
 #include <memory>
 
-#include "base/memory/weak_ptr.h"
 #include "base/scoped_observer.h"
 #include "base/time/time.h"
 #include "base/timer/timer.h"
 #include "chrome/browser/ui/tabs/tab_utils.h"
-#include "chrome/browser/ui/thumbnails/thumbnail_image.h"
 #include "ui/views/bubble/bubble_dialog_delegate_view.h"
 
 namespace gfx {
@@ -28,8 +26,7 @@ class Widget;
 class Tab;
 
 // Dialog that displays an informational hover card containing page information.
-class TabHoverCardBubbleView : public views::BubbleDialogDelegateView,
-                               public ThumbnailImage::Observer {
+class TabHoverCardBubbleView : public views::BubbleDialogDelegateView {
  public:
   explicit TabHoverCardBubbleView(Tab* tab);
 
@@ -71,6 +68,7 @@ class TabHoverCardBubbleView : public views::BubbleDialogDelegateView,
   class WidgetFadeAnimationDelegate;
   class WidgetSlideAnimationDelegate;
   class FadeLabel;
+  class ThumbnailWatcher;
 
   // Get delay in milliseconds based on tab width.
   base::TimeDelta GetDelay(int tab_width) const;
@@ -83,9 +81,7 @@ class TabHoverCardBubbleView : public views::BubbleDialogDelegateView,
   // Update the text fade to the given percent, which should be between 0 and 1.
   void UpdateTextFade(double percent);
 
-  void RegisterToThumbnailImageUpdates(
-      scoped_refptr<ThumbnailImage> thumbnail_image);
-
+  void OnThumbnailImageAvailable(gfx::ImageSkia thumbnail_image);
   void ClearPreviewImage();
 
   // Called when a hover card lands on the tab it's supposed to be a preview
@@ -93,10 +89,6 @@ class TabHoverCardBubbleView : public views::BubbleDialogDelegateView,
   // animation completes.
   void OnHoverCardLanded();
 
-  // ThumbnailImage::Observer:
-  void OnThumbnailImageAvailable(gfx::ImageSkia thumbnail_image) override;
-  base::Optional<gfx::Size> GetThumbnailSizeHint() const override;
-
   // views::BubbleDialogDelegateView:
   gfx::Size CalculatePreferredSize() const override;
 
@@ -109,6 +101,7 @@ class TabHoverCardBubbleView : public views::BubbleDialogDelegateView,
   std::unique_ptr<WidgetFadeAnimationDelegate> fade_animation_delegate_;
   // Used to animate the tab hover card's movement between tabs.
   std::unique_ptr<WidgetSlideAnimationDelegate> slide_animation_delegate_;
+  std::unique_ptr<ThumbnailWatcher> thumbnail_watcher_;
 
   // Timestamp of the last time a hover card was visible, recorded before it is
   // hidden. This is used for metrics.
@@ -130,13 +123,8 @@ class TabHoverCardBubbleView : public views::BubbleDialogDelegateView,
   // Counter used to keep track of the number of tab hover cards seen before a
   // tab is selected by mouse press.
   size_t hover_cards_seen_count_ = 0;
-  scoped_refptr<ThumbnailImage> thumbnail_image_;
-  ScopedObserver<ThumbnailImage, ThumbnailImage::Observer> thumbnail_observer_{
-      this};
   bool waiting_for_decompress_ = false;
 
-  base::WeakPtrFactory<TabHoverCardBubbleView> weak_factory_{this};
-
   DISALLOW_COPY_AND_ASSIGN(TabHoverCardBubbleView);
 };