alternate ntp: implement Show/HideBars API to reduce jank when showing/hiding bars

chrome/browser/ui/search/search_model.cc

Index: chrome/browser/ui/search/search_model.cc
diff --git a/chrome/browser/ui/search/search_model.cc b/chrome/browser/ui/search/search_model.cc
index b1830e69a0dcc8e6d8c009e260bee829015927ff..6637b8fd1e41c3dc93ffc0fe05ee207ba9251261 100644
--- a/chrome/browser/ui/search/search_model.cc
+++ b/chrome/browser/ui/search/search_model.cc
@@ -16,19 +16,68 @@ SearchModel::SearchModel() {
 SearchModel::~SearchModel() {
 }
 
+// static.
+bool SearchModel::ShouldChangeTopBarsVisibility(const State& old_state,
+                                                const State& new_state) {
+  // If mode has changed, only change top bars visibility if new mode is not
+  // |SEARCH_SUGGESTIONS| or |SEARCH_RESULTS|.  Top bars visibility for
+  // these 2 modes is determined when the mode stays the same, and:
+  // - for |NTP/SERP| pages: by SearchBox API, or
+  // - for |DEFAULT| pages: by platform-specific implementation of
+  //   |InstantOverlayController| when it shows/hides the Instant overlay.
+  return old_state.mode != new_state.mode ?
+      !new_state.mode.is_search() : new_state.mode.is_search();
+}
+
+void SearchModel::SetState(const State& new_state) {
+  DCHECK(IsInstantExtendedAPIEnabled())
+      << "Please do not try to set the SearchModel mode without first "
+      << "checking if Search is enabled.";
+
+  if (state_ == new_state)
+    return;
+
+  const State old_state = state_;
+  state_ = new_state;
+
+  FOR_EACH_OBSERVER(SearchModelObserver, observers_,
+                    ModelChanged(old_state, state_));
+}
+
 void SearchModel::SetMode(const Mode& new_mode) {
   DCHECK(IsInstantExtendedAPIEnabled())
       << "Please do not try to set the SearchModel mode without first "
       << "checking if Search is enabled.";
 
-  if (mode_ == new_mode)
+  if (state_.mode == new_mode)
+    return;
+
+  const State old_state = state_;
+  state_.mode = new_mode;
+
+  // For |SEARCH_SUGGESTIONS| and |SEARCH_RESULTS| modes, SearchBox API will
+  // determine visibility of top bars via SetTopBarsVisible(); for other modes,
+  // top bars are always visible, if available.
+  if (!state_.mode.is_search())
+    state_.top_bars_visible = true;
+
+  FOR_EACH_OBSERVER(SearchModelObserver, observers_,
+                    ModelChanged(old_state, state_));
+}
+
+void SearchModel::SetTopBarsVisible(bool visible) {
+  DCHECK(IsInstantExtendedAPIEnabled())
+      << "Please do not try to set the SearchModel mode without first "
+      << "checking if Search is enabled.";
+
+  if (state_.top_bars_visible == visible)
     return;
 
-  const Mode old_mode = mode_;
-  mode_ = new_mode;
+  const State old_state = state_;
+  state_.top_bars_visible = visible;
 
   FOR_EACH_OBSERVER(SearchModelObserver, observers_,
-                    ModeChanged(old_mode, mode_));
+                    ModelChanged(old_state, state_));
 }
 
 void SearchModel::AddObserver(SearchModelObserver* observer) {