Add post-layout callback to the ScrollView

This change adds a new `post-layout` callback to the `ScrollView` to
allow the scroll view to invoke the callback at the end of every layout
operation. This is a useful feature to allow the user to explicitly
scroll the contents to a specific position after a layout and update the
scrollbars. It would allow e.g. to restore the position of the scrolled
view in a large content view when the contents is scrolled out of the
view.

Bug: b:413733359
Change-Id: I9e58136dbbedb3c128b31bba669db060900a87ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6498548
Auto-Submit: Leonid Baraz <lbaraz@chromium.org>
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: Keren Zhu <kerenzhu@chromium.org>
Commit-Queue: Leonid Baraz <lbaraz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454078}

ui/views/controls/scroll_view.cc

@@ -834,6 +834,15 @@ void ScrollView::Layout(PassKey) {
   if (contents_) {
     UpdateOverflowIndicatorVisibility(CurrentOffset());
   }
+
+  // If registered, run the post-layout callback. This is used to move the
+  // scroll view contents to the appropriate position that's different from the
+  // position assigned above.
+  if (post_layout_callback_) {
+    const bool layout_needed = needs_layout();
+    post_layout_callback_.Run(this);
+    CHECK_EQ(layout_needed, needs_layout());
+  }
 }
 
 bool ScrollView::OnKeyPressed(const ui::KeyEvent& event) {
@@ -1362,6 +1371,11 @@ void ScrollView::UpdateOverflowIndicatorVisibility(const gfx::PointF& offset) {
           offset.x() < horiz_sb_->GetMaxPosition() && draw_overflow_indicator_);
 }
 
+void ScrollView::RegisterPostLayoutCallback(
+    base::RepeatingCallback<void(ScrollView*)> post_layout_callback) {
+  post_layout_callback_ = post_layout_callback;
+}
+
 View* ScrollView::GetContentsViewportForTest() const {
   return contents_viewport_;
 }

ui/views/controls/scroll_view.h

@@ -242,6 +242,15 @@ class VIEWS_EXPORT ScrollView : public View, public ScrollBarController {
                          bool is_positive) override;
   void OnScrollEnded() override;
 
+  // Registers a callback to be called after the layout is complete. This
+  // callback can be used e.g. to scroll the view to the appropriate position
+  // in the contents by explicitly calling `ScrollToOffset` or `ScrollByOffset`
+  // and to update the scrollbars to reflect the new position.
+  // The callback should not trigger any new layouts on the scroll view,
+  // otherwise it will lead to a CHECK failure.
+  void RegisterPostLayoutCallback(
+      base::RepeatingCallback<void(ScrollView*)> post_layout_callback);
+
   bool is_scrolling() const {
     return horiz_sb_->is_scrolling() || vert_sb_->is_scrolling();
   }
@@ -399,6 +408,9 @@ class VIEWS_EXPORT ScrollView : public View, public ScrollBarController {
   // Scrolling callbacks.
   ScrollViewCallbackList on_contents_scrolled_;
   ScrollViewCallbackList on_contents_scroll_ended_;
+
+  // Post-layout callback.
+  base::RepeatingCallback<void(ScrollView*)> post_layout_callback_;
 };
 
 // When building with GCC this ensures that an instantiation of the

ui/views/controls/scroll_view_unittest.cc

@@ -17,6 +17,7 @@
 #include "base/task/single_thread_task_runner.h"
 #include "base/test/gtest_util.h"
 #include "base/test/icu_test_util.h"
+#include "base/test/mock_callback.h"
 #include "base/test/scoped_feature_list.h"
 #include "base/test/test_timeouts.h"
 #include "base/timer/timer.h"
@@ -563,6 +564,17 @@ TEST_F(ScrollViewTest, BoundedViewportSizedToFit) {
   EXPECT_EQ(96, contents->width());
 }
 
+// Verifies that the scroll view calls post-layout callback on every layout.
+TEST_F(ScrollViewTest, LayoutCallbackCalledOnEveryLayout) {
+  auto mock_post_layout_cb_ =
+      base::MockCallback<base::RepeatingCallback<void(ScrollView*)>>();
+  scroll_view_->RegisterPostLayoutCallback(mock_post_layout_cb_.Get());
+  EXPECT_CALL(mock_post_layout_cb_, Run(scroll_view_.get())).Times(1);
+  InstallContents();
+  ASSERT_FALSE(scroll_view_->GetContentsBounds().IsEmpty());
+  views::test::RunScheduledLayout(scroll_view_.get());
+}
+
 // Verifies that the vertical scrollbar does not unnecessarily appear for a
 // contents whose height always matches the height of the viewport.
 TEST_F(ScrollViewTest, VerticalScrollbarDoesNotAppearUnnecessarily) {