port of https://blog.gtk.org/2017/04/23/drag-and-drop-in-lists/ with scrolling and more advanced drag and drop

main.vala

using Gtk;

class DragListBox : ListBox {
    private ListBoxRow? hover_row;
    private ListBoxRow? drag_row;
    private bool top = false;
    private int hover_top;
    private int hover_bottom;
    private bool should_scroll = false;
    private bool scrolling = false;
    private bool scroll_up;
    
    private const int SCROLL_STEP_SIZE = 8;
    private const int SCROLL_DISTANCE = 30;
    private const int SCROLL_DELAY = 50;

    public Adjustment? vadjustment {
        public set {
            _vadjustment = value;
            if (_vadjustment == null) {
                should_scroll = false;
            }
        }
        public get {
            return _vadjustment;
        }
    }
    private Adjustment? _vadjustment;
    
    const TargetEntry[] entries = {
        { "GTK_LIST_BOX_ROW", Gtk.TargetFlags.SAME_APP, 0}
    };

    public DragListBox () {
        drag_dest_set (this, Gtk.DestDefaults.ALL, entries, Gdk.DragAction.MOVE);
    }
    
    void row_drag_begin (Widget widget, Gdk.DragContext context) {
        ListBoxRow row;
        Allocation alloc;
        Cairo.Surface surface;
        Cairo.Context cr;
        int x, y;
        DragListBox parent;
        
        row = (ListBoxRow) widget.get_ancestor (typeof (ListBoxRow));
        row.get_allocation (out alloc);
        surface = new Cairo.ImageSurface (Cairo.Format.ARGB32, alloc.width, alloc.height);
        cr = new Cairo.Context (surface);
        
        parent = row.get_parent () as DragListBox;
        if (parent != null)
            parent.drag_row = row;
        
        row.get_style_context ().add_class ("drag-icon");
        row.draw (cr);
        row.get_style_context ().remove_class ("drag-icon");
        
        widget.translate_coordinates (row, 0, 0, out x, out y);
        surface.set_device_offset (-x, -y);
        drag_set_icon_surface (context, surface);
    }
    
    public override bool drag_motion (
        Gdk.DragContext context, int x, int y, uint time_
    ) {
        if (y > hover_top || y < hover_bottom) {
            Allocation alloc;
            var row = get_row_at_y (y);
            bool old_top = top;
            
            row.get_allocation (out alloc);
            int hover_row_y = alloc.y;
            int hover_row_height = alloc.height;
            if (row != drag_row) {
                if (y < hover_row_y + hover_row_height/2) {
                    hover_top = hover_row_y;
                    hover_bottom = hover_top + hover_row_height/2;
                    row.get_style_context ().add_class ("drag-hover-top");
                    row.get_style_context ().remove_class ("drag-hover-bottom");
                    top = true;
                } else {
                    hover_top = hover_row_y + hover_row_height/2;
                    hover_bottom = hover_row_y + hover_row_height;
                    row.get_style_context ().add_class ("drag-hover-bottom");
                    row.get_style_context ().remove_class ("drag-hover-top");
                    top = false;
                }
            }
            
            if (hover_row != null && hover_row != row) {
                if (old_top) 
                    hover_row.get_style_context ().remove_class ("drag-hover-top");
                else
                    hover_row.get_style_context ().remove_class ("drag-hover-bottom");
            }
            
            hover_row = row;
        }
        
        check_scroll (y);
        if(should_scroll && !scrolling) {
            scrolling = true;
            Timeout.add (SCROLL_DELAY, scroll);
        }
        
        return true;
    }
    
    public override void drag_leave (Gdk.DragContext context, uint time_) {
        should_scroll = false;
    }
    
    void check_scroll (int y) {
        if (vadjustment == null) {
            return;
        } 
        double vadjustment_min = vadjustment.value;
        double vadjustment_max = vadjustment.page_size + vadjustment_min;
        double show_min = double.max(0, y - SCROLL_DISTANCE);
        double show_max = double.min(vadjustment.upper, y + SCROLL_DISTANCE);
        if(vadjustment_min > show_min) {
            should_scroll = true;
            scroll_up = true;
        } else if (vadjustment_max < show_max){
            should_scroll = true;
            scroll_up = false; 
        } else {
            should_scroll = false;
        }
    }
    
    bool scroll () {
        if (should_scroll) {
            if(scroll_up) {
                vadjustment.value -= SCROLL_STEP_SIZE;
            } else {
                vadjustment.value += SCROLL_STEP_SIZE; 
            }
        } else {
            scrolling = false;
        }
        return should_scroll;
    }
    
    void row_drag_data_get (
        Widget widget, Gdk.DragContext context, SelectionData selection_data, 
        uint info, uint time_
    ) {
        uchar[] data = new uchar[(sizeof (Widget))];
        ((Widget[])data)[0] = widget;
        selection_data.set (
            Gdk.Atom.intern_static_string ("GTK_LIST_BOX_ROW"), 32, data
        );
    }

    public override void drag_data_received (
        Gdk.DragContext context, int x, int y, 
        SelectionData selection_data, uint info, uint time_
    ) {
        Widget handle;
        ListBoxRow row;
        
        int index = 0;
        if (hover_row != null) {
            if (top) {
                index = hover_row.get_index () - 1;
                hover_row.get_style_context ().remove_class ("drag-hover-top");
            } else {
                index = hover_row.get_index ();
                hover_row.get_style_context ().remove_class ("drag-hover-bottom");
            }
            handle = ((Widget[])selection_data.get_data ())[0];
            row = (ListBoxRow) handle.get_ancestor (typeof (ListBoxRow));
            
            if (row != hover_row) {
                row.get_parent ().remove (row);
                insert (row, index);
            }
        }
        drag_row = null;
    }

    public ListBoxRow create_row (string text) {
        ListBoxRow row;
        EventBox handle;
        Box box;
        Label label;
        Image image;
          
        row = new ListBoxRow ();
          
        box = new Box (Orientation.HORIZONTAL, 10);
        box.margin_start = 10;
        box.margin_end = 10;
        row.add (box);
          
        handle = new EventBox ();
        image = new Image.from_icon_name ("view-list-symbolic", IconSize.MENU);
        handle.add (image);
        box.add (handle);
          
        label = new Gtk.Label (text);
        box.pack_end (label, true);
          
        drag_source_set (
            handle, Gdk.ModifierType.BUTTON1_MASK, entries, Gdk.DragAction.MOVE
        );
        handle.drag_begin.connect (row_drag_begin);
        handle.drag_data_get.connect (row_drag_data_get);
        
        return row;
    }
}

const string css =
  ".drag-icon { " +
  "  background: white; " +
  "  border: 1px solid black; " +
  "}" +
  ".drag-hover-top {" +
  "  background: linear-gradient(to bottom, rgba(0,0,0,0.65) 0%,rgba(0,0,0,0) 35%); " +
  "}" +
  ".drag-hover-bottom {" +
  "  background: linear-gradient(to bottom, rgba(0,0,0,0) 65%,rgba(0,0,0,0.65) 100%); " +
  "}";

int main (string[] args) {
    Window window; 
    DragListBox list;
    ScrolledWindow sw;
    ListBoxRow row;
    string text;
    CssProvider provider;
    
    Gtk.init (ref args);
    
    provider = new CssProvider ();
    try {
        provider.load_from_data (css);
    } catch (Error e) {
        warning (e.message);
    }
    
    Gtk.StyleContext.add_provider_for_screen (
        Gdk.Screen.get_default (), provider, STYLE_PROVIDER_PRIORITY_APPLICATION
    );
    
    window = new Window ();
    window.set_default_size (-1, 300);
    window.destroy.connect (Gtk.main_quit);
    
    sw = new ScrolledWindow (null, null);
    sw.hexpand = true;
    sw.set_policy (PolicyType.NEVER, PolicyType.ALWAYS);
    window.add (sw);
    
    list = new DragListBox ();
    list.set_selection_mode (SelectionMode.NONE);
    sw.add (list);
    
    for (int i = 0; i < 20; i++) {
        text = "Row %d".printf (i);
        row = list.create_row (text);
        list.insert (row, -1);
    }
    
    list.vadjustment = sw.vadjustment;
    
    window.show_all ();
    
    Gtk.main ();
    
    return 0;
}