[nautilus] dnd: Create paintable for DnD

[António Fernandes <antoniof src gnome org>]

commit 7ee588a9d34d08da065c3d1c45a2d289042a1529
Author: Corey Berla <corey berla me>
Date:   Wed Jun 15 14:51:18 2022 -0700

    dnd: Create paintable for DnD
    
    Get the icon (paintable) for the dragged items.  If a single item is
    being dragged, this is easy (just get the icon).  If multiple items
    are being dragged, used multiple icons (up to 10) with the
    appropropriate spacing (depending on number of icons).
    
    Very significant contribution by António Fernandes.

 src/nautilus-dnd.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++--
 src/nautilus-dnd.h |  3 ++
 2 files changed, 92 insertions(+), 2 deletions(-)
---
diff --git a/src/nautilus-dnd.c b/src/nautilus-dnd.c
index b43e758b8..c35698652 100644
--- a/src/nautilus-dnd.c
+++ b/src/nautilus-dnd.c
@@ -21,11 +21,11 @@
  *           Ettore Perazzoli <ettore gnu org>
  */
 
-#if 0 && NAUTILUS_DND_NEEDS_GTK4_REIMPLEMENTATION
-
 #include <config.h>
 #include "nautilus-dnd.h"
 
+#if 0 && NAUTILUS_DND_NEEDS_GTK4_REIMPLEMENTATION
+
 #include "nautilus-program-choosing.h"
 #include <eel/eel-glib-extensions.h>
 #include <eel/eel-string.h>
@@ -990,3 +990,90 @@ nautilus_drag_autoscroll_stop (NautilusDragInfo *drag_info)
 }
 
 #endif
+
+#define MAX_DRAWN_DRAG_ICONS 10
+#define NAUTILUS_DRAG_SURFACE_ICON_SIZE 64
+
+GdkPaintable *
+get_paintable_for_drag_selection (GList *selection,
+                                  int    scale)
+{
+    g_autoqueue (GdkPaintable) icons = g_queue_new ();
+    g_autoptr (GtkSnapshot) snapshot = gtk_snapshot_new ();
+    NautilusFileIconFlags flags;
+    GdkPaintable *icon;
+    guint n_icons;
+    guint icon_size = NAUTILUS_DRAG_SURFACE_ICON_SIZE;
+    float dy;
+    /* A wide shadow for the pile of icons gives a sense of floating. */
+    GskShadow stack_shadow = {.color = {0, 0, 0, .alpha = 0.15}, .dx = 0, .dy = 2, .radius = 10 };
+    /* A slight shadow swhich makes each icon in the stack look separate. */
+    GskShadow icon_shadow = {.color = {0, 0, 0, .alpha = 0.20}, .dx = 0, .dy = 1, .radius = 1 };
+    GskRoundedRect rounded_rect;
+
+    g_return_val_if_fail (NAUTILUS_IS_FILE (selection->data), NULL);
+
+    /* The selection list is reversed compared to what the user sees. Get the
+     * first items by starting from the end of the list. */
+    flags = (NAUTILUS_FILE_ICON_FLAGS_USE_THUMBNAILS | NAUTILUS_FILE_ICON_FLAGS_FORCE_THUMBNAIL_SIZE);
+    for (GList *l = g_list_last (selection);
+         l != NULL && g_queue_get_length (icons) <= MAX_DRAWN_DRAG_ICONS;
+         l = l->prev)
+    {
+        icon = nautilus_file_get_icon_paintable (l->data, icon_size, scale, flags);
+        g_queue_push_tail (icons, icon);
+    }
+
+    /* When there are 2 or 3 identical icons, we need to space them more,
+     * otherwise it would be hard to tell there is more than one icon at all.
+     * The more icons we have, the easier it is to notice multiple icons are
+     * stacked, and the more compact we want to be.
+     *
+     *  1 icon          2 icons         3 icons         4+ icons
+     *  .--------.      .--------.      .--------.      .--------.
+     *  |        |      |        |      |        |      |        |
+     *  |        |      |        |      |        |      |        |
+     *  |        |      |        |      |        |      |        |
+     *  |        |      |        |      |        |      |        |
+     *  '--------'      |--------|      |--------|      |--------|
+     *                  |        |      |        |      |--------|
+     *                  |        |      |--------|      |--------|
+     *                  '--------'      |        |      |--------|
+     *                                  '--------'      '--------'
+     */
+    n_icons = g_queue_get_length (icons);
+    dy = (n_icons == 2) ? 10 : (n_icons == 3) ? 6 : (n_icons >= 4) ? 4 : 0;
+
+    /* We want the first icon on top of every other. So we need to start drawing
+     * the stack from the bottom, that is, from the last icon. This requires us
+     * to jump to the last position and then move upwards one step at a time.
+     * Also, add 10px horizontal offset, for shadow, to workaround this GTK bug:
+     * https://gitlab.gnome.org/GNOME/gtk/-/issues/2341
+     */
+    gtk_snapshot_translate (snapshot, &GRAPHENE_POINT_INIT (10, dy * n_icons));
+    gtk_snapshot_push_shadow (snapshot, &stack_shadow, 1);
+    for (GList *l = g_queue_peek_tail_link (icons); l != NULL; l = l->prev)
+    {
+        double w = gdk_paintable_get_intrinsic_width (l->data);
+        double h = gdk_paintable_get_intrinsic_height (l->data);
+        /* Offsets needed to center thumbnails. Floored to keep images sharp. */
+        float x = floor ((icon_size - w) / 2);
+        float y = floor ((icon_size - h) / 2);
+        gsk_rounded_rect_init_from_rect (&rounded_rect, &GRAPHENE_RECT_INIT (0, 0, w, h), 6);
+
+        gtk_snapshot_translate (snapshot, &GRAPHENE_POINT_INIT (0, -dy));
+
+        gtk_snapshot_translate (snapshot, &GRAPHENE_POINT_INIT (x, y));
+        gtk_snapshot_push_shadow (snapshot, &icon_shadow, 1);
+        gtk_snapshot_push_rounded_clip (snapshot, &rounded_rect);
+
+        gdk_paintable_snapshot (l->data, snapshot, w, h);
+
+        gtk_snapshot_pop (snapshot); /* End of rounded clip */
+        gtk_snapshot_pop (snapshot); /* End of icon shadow */
+        gtk_snapshot_translate (snapshot, &GRAPHENE_POINT_INIT (-x, -y));
+    }
+    gtk_snapshot_pop (snapshot); /* End of stack shadow */
+
+    return gtk_snapshot_to_paintable (snapshot, NULL);
+}
diff --git a/src/nautilus-dnd.h b/src/nautilus-dnd.h
index cbe630ad5..a8fc416ff 100644
--- a/src/nautilus-dnd.h
+++ b/src/nautilus-dnd.h
@@ -143,3 +143,6 @@ NautilusDragInfo *          nautilus_drag_get_source_data                 (GdkDr
 
 GList *                     nautilus_drag_file_list_from_selection_list   (const GList                       
 *selection_list);
 #endif
+
+GdkPaintable *     get_paintable_for_drag_selection              (GList            *selection,
+                                                                  int               scale);