Notify DragController of the view drag start

This CL adds a views::DragController API that gets called when view
drag will start.

This CL should not bring any noticeable UI difference.

Bug: b/271490637
Change-Id: Ideedd9c11a2f344b4b485fdd9000c708f24aeccc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4327347
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1115834}

ui/views/BUILD.gn

@@ -1001,6 +1001,8 @@ source_set("test_support") {
     "test/menu_runner_test_api.h",
     "test/menu_test_utils.cc",
     "test/menu_test_utils.h",
+    "test/mock_drag_controller.cc",
+    "test/mock_drag_controller.h",
     "test/mock_input_event_activation_protector.cc",
     "test/mock_input_event_activation_protector.h",
     "test/native_widget_factory.cc",

ui/views/drag_controller.h

@@ -39,6 +39,9 @@ class VIEWS_EXPORT DragController {
                                    const gfx::Point& press_pt,
                                    const gfx::Point& p) = 0;
 
+  // Called when the drag on view will start.
+  virtual void OnWillStartDragForView(View* dragged_view) {}
+
  protected:
   virtual ~DragController() = default;
 };

ui/views/test/mock_drag_controller.cc

@@ -0,0 +1,33 @@
+// Copyright 2023 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/views/test/mock_drag_controller.h"
+
+#include "ui/base/dragdrop/drag_drop_types.h"
+#include "ui/base/dragdrop/os_exchange_data.h"
+
+namespace views {
+
+MockDragController::MockDragController() = default;
+
+MockDragController::~MockDragController() = default;
+
+void MockDragController::WriteDragDataForView(View* sender,
+                                              const gfx::Point& press_pt,
+                                              ui::OSExchangeData* data) {
+  data->SetString(u"test");
+}
+
+int MockDragController::GetDragOperationsForView(View* sender,
+                                                 const gfx::Point& p) {
+  return ui::DragDropTypes::DRAG_COPY;
+}
+
+bool MockDragController::CanStartDragForView(View* sender,
+                                             const gfx::Point& press_pt,
+                                             const gfx::Point& p) {
+  return true;
+}
+
+}  // namespace views

ui/views/test/mock_drag_controller.h

@@ -0,0 +1,44 @@
+// Copyright 2023 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UI_VIEWS_TEST_MOCK_DRAG_CONTROLLER_H_
+#define UI_VIEWS_TEST_MOCK_DRAG_CONTROLLER_H_
+
+#include "ui/views/drag_controller.h"
+
+#include "testing/gmock/include/gmock/gmock.h"
+
+namespace gfx {
+class Point;
+}  // namespace gfx
+
+namespace ui {
+class OSExchangeData;
+}  // namespace ui
+
+namespace views {
+class View;
+
+// A mocked drag controller for testing.
+class MockDragController : public DragController {
+ public:
+  MockDragController();
+  MockDragController(const MockDragController&) = delete;
+  MockDragController& operator=(const MockDragController&) = delete;
+  ~MockDragController() override;
+
+  // DragController:
+  void WriteDragDataForView(View* sender,
+                            const gfx::Point& press_pt,
+                            ui::OSExchangeData* data) override;
+  int GetDragOperationsForView(View* sender, const gfx::Point& p) override;
+  bool CanStartDragForView(View* sender,
+                           const gfx::Point& press_pt,
+                           const gfx::Point& p) override;
+  MOCK_METHOD(void, OnWillStartDragForView, (View * dragged_view), (override));
+};
+
+}  // namespace views
+
+#endif  // UI_VIEWS_TEST_MOCK_DRAG_CONTROLLER_H_

ui/views/widget/widget.cc

@@ -34,6 +34,7 @@
 #include "ui/events/event_utils.h"
 #include "ui/gfx/image/image_skia.h"
 #include "ui/views/controls/menu/menu_controller.h"
+#include "ui/views/drag_controller.h"
 #include "ui/views/event_monitor.h"
 #include "ui/views/focus/focus_manager.h"
 #include "ui/views/focus/focus_manager_factory.h"
@@ -1032,6 +1033,10 @@ void Widget::RunShellDrag(View* view,
   for (WidgetObserver& observer : observers_)
     observer.OnWidgetDragWillStart(this);
 
+  if (view && view->drag_controller()) {
+    view->drag_controller()->OnWillStartDragForView(view);
+  }
+
   WidgetDeletionObserver widget_deletion_observer(this);
   native_widget_->RunShellDrag(view, std::move(data), location, operation,
                                source);

ui/views/widget/widget_unittest.cc

@@ -43,6 +43,7 @@
 #include "ui/views/event_monitor.h"
 #include "ui/views/layout/fill_layout.h"
 #include "ui/views/style/platform_style.h"
+#include "ui/views/test/mock_drag_controller.h"
 #include "ui/views/test/native_widget_factory.h"
 #include "ui/views/test/test_views.h"
 #include "ui/views/test/test_widget_observer.h"
@@ -53,6 +54,7 @@
 #include "ui/views/widget/native_widget_delegate.h"
 #include "ui/views/widget/native_widget_private.h"
 #include "ui/views/widget/root_view.h"
+#include "ui/views/widget/unique_widget_ptr.h"
 #include "ui/views/widget/widget_deletion_observer.h"
 #include "ui/views/widget/widget_interactive_uitest_utils.h"
 #include "ui/views/widget/widget_removals_observer.h"
@@ -4304,6 +4306,37 @@ TEST_F(WidgetTest, ScrollGestureEventDispatch) {
   widget->Close();
 }
 
+// TODO(b/271490637): on Mac a drag controller should still be notified when
+// drag will start. Figure out how to write a unit test for Mac. Then remove
+// this build flag check.
+#if !BUILDFLAG(IS_MAC)
+
+// Verifies that the drag controller is notified when the view drag will start.
+TEST_F(WidgetTest, NotifyDragControllerWhenDragWillStart) {
+  // Create a widget whose contents view is draggable.
+  UniqueWidgetPtr widget(std::make_unique<Widget>());
+  Widget::InitParams params = CreateParams(Widget::InitParams::TYPE_POPUP);
+  params.bounds = gfx::Rect(/*width=*/650, /*height=*/650);
+  widget->Init(std::move(params));
+  widget->Show();
+  MockDragController mock_drag_controller;
+  views::View contents_view;
+  contents_view.set_drag_controller(&mock_drag_controller);
+  widget->SetContentsView(&contents_view);
+
+  // Expect the drag controller is notified of the drag start.
+  EXPECT_CALL(mock_drag_controller, OnWillStartDragForView(&contents_view));
+
+  // Drag-and-drop `contents_view` by mouse.
+  ui::test::EventGenerator generator(GetContext(), widget->GetNativeWindow());
+  generator.MoveMouseTo(contents_view.GetBoundsInScreen().CenterPoint());
+  generator.PressLeftButton();
+  generator.MoveMouseBy(/*x=*/200, /*y=*/0);
+  generator.ReleaseLeftButton();
+}
+
+#endif  // !BUILDFLAG(IS_MAC)
+
 // A class used in WidgetTest.GestureEventLocationWhileBubbling to verify
 // that when a gesture event bubbles up a View hierarchy, the location
 // of a gesture event seen by each View is in the local coordinate space