LibGUI: Let GModel specify the drag data type

GModel subclasses can now override drag_data_type() to specify which type
GAbstractView should set for drag data. The default implementation returns a
null string, which disables dragging from this widget.

Libraries/LibGUI/GAbstractView.cpp

@@ -191,7 +191,7 @@ void GAbstractView::mousedown_event(GMouseEvent& event)
         m_selection.clear();
     } else if (event.modifiers() & Mod_Ctrl) {
         m_selection.toggle(index);
-    } else if (event.button() == GMouseButton::Left) {
+    } else if (event.button() == GMouseButton::Left && !m_model->drag_data_type().is_null()) {
         // We might be starting a drag, so don't throw away other selected items yet.
         m_might_drag = true;
         m_selection.add(index);
@@ -219,6 +219,9 @@ void GAbstractView::mousemove_event(GMouseEvent& event)
     if (distance_travelled_squared <= drag_distance_threshold)
         return GScrollableWidget::mousemove_event(event);
 
+    auto data_type = m_model->drag_data_type();
+    ASSERT(!data_type.is_null());
+
     dbg() << "Initiate drag!";
     auto drag_operation = GDragOperation::construct();
 
@@ -248,7 +251,7 @@ void GAbstractView::mousemove_event(GMouseEvent& event)
 
     drag_operation->set_text(text_builder.to_string());
     drag_operation->set_bitmap(bitmap);
-    drag_operation->set_data("url-list", data_builder.to_string());
+    drag_operation->set_data(data_type, data_builder.to_string());
 
     auto outcome = drag_operation->exec();
 

Libraries/LibGUI/GFileSystemModel.h

@@ -117,6 +117,7 @@ public:
     virtual void update() override;
     virtual GModelIndex parent_index(const GModelIndex&) const override;
     virtual GModelIndex index(int row, int column = 0, const GModelIndex& parent = GModelIndex()) const override;
+    virtual StringView drag_data_type() const override { return "url-list"; }
 
     static String timestamp_string(time_t timestamp)
     {

Libraries/LibGUI/GModel.h

@@ -94,6 +94,8 @@ public:
     virtual GSortOrder sort_order() const { return GSortOrder::None; }
     virtual void set_key_column_and_sort_order(int, GSortOrder) {}
 
+    virtual StringView drag_data_type() const { return {}; }
+
     void register_view(Badge<GAbstractView>, GAbstractView&);
     void unregister_view(Badge<GAbstractView>, GAbstractView&);
 

Libraries/LibGUI/GSortingProxyModel.cpp

@@ -87,6 +87,11 @@ void GSortingProxyModel::update()
     target().update();
 }
 
+StringView GSortingProxyModel::drag_data_type() const
+{
+    return target().drag_data_type();
+}
+
 void GSortingProxyModel::set_key_column_and_sort_order(int column, GSortOrder sort_order)
 {
     if (column == m_key_column && sort_order == m_sort_order)

Libraries/LibGUI/GSortingProxyModel.h

@@ -40,6 +40,7 @@ public:
     virtual ColumnMetadata column_metadata(int) const override;
     virtual GVariant data(const GModelIndex&, Role = Role::Display) const override;
     virtual void update() override;
+    virtual StringView drag_data_type() const override;
 
     virtual int key_column() const override { return m_key_column; }
     virtual GSortOrder sort_order() const override { return m_sort_order; }