[balsa/gtk4: 221/311] balsa-index: Restore dragging selected messages

[Peter Bloomfield <peterb src gnome org>]

commit cdfbe1f1288a5afb83e3a1231184848c0f1e27df
Author: Peter Bloomfield <PeterBloomfield bellsouth net>
Date:   Thu Oct 22 13:03:31 2020 -0400

    balsa-index: Restore dragging selected messages
    
    so they can be dropped on a folder in BalsaMBList.

 src/balsa-index.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)
---
diff --git a/src/balsa-index.c b/src/balsa-index.c
index 893ca7a14..ec86c984f 100644
--- a/src/balsa-index.c
+++ b/src/balsa-index.c
@@ -320,6 +320,30 @@ bndx_key_pressed_cb(GtkEventControllerKey *controller,
     return FALSE;
 }
 
+/*
+ * Handler for the drag-source's "prepare" signal
+ */
+static GdkContentProvider *
+bndx_drag_source_prepare(GtkDragSource *source,
+                         gdouble        x,
+                         gdouble        y,
+                         gpointer       user_data)
+{
+    BalsaIndex *bindex = user_data;
+    GtkTreeView *tree_view = GTK_TREE_VIEW(bindex);
+    GtkTreeSelection *selection = gtk_tree_view_get_selection(tree_view);
+    GdkContentProvider *provider = NULL;
+
+    if (gtk_tree_selection_count_selected_rows(selection) > 0) {
+        GValue value;
+
+        g_value_init_from_instance(&value, bindex);
+        provider = gdk_content_provider_new_for_value(&value);
+    }
+
+    return provider;
+}
+
 /* BalsaIndex instance init method; no tree store is set on the tree
  * view--that's handled later, when the view is populated. */
 static void
@@ -505,6 +529,14 @@ balsa_index_init(BalsaIndex * index)
                            NULL);
     gtk_tree_view_set_enable_search(tree_view, FALSE);
 
+    {
+        GtkDragSource *drag_source;
+
+        drag_source = gtk_drag_source_new();
+        gtk_widget_add_controller(GTK_WIDGET(index), GTK_EVENT_CONTROLLER(drag_source));
+        g_signal_connect(drag_source, "prepare", G_CALLBACK(bndx_drag_source_prepare), index);
+    }
+
     balsa_index_set_column_widths(index);
 }