[california] Don't pass non-null Value messages holding null

[Jim Nelson <jnelson src gnome org>]

commit e9c5a195d6fe80c9a0a5065495f0aaa95073ad9a
Author: Jim Nelson <jim yorba org>
Date:   Fri Jun 27 13:46:49 2014 -0700

    Don't pass non-null Value messages holding null
    
    It's a pain for the Toolkit.Cards to have to check for non-null
    Value messages holding null references, so Toolkit.Deck will "strip"
    those messages out and simply pass null to the Card.
    
    This occurred because apparently either GSignal or some Vala-generated
    code was turning a null Value into a non-null Value holding null and
    the code on the other end of the signal wasn't expecting that.

 src/toolkit/toolkit-card.vala |    8 ++++++++
 src/toolkit/toolkit-deck.vala |   20 ++++++++++++++++++--
 2 files changed, 26 insertions(+), 2 deletions(-)
---
diff --git a/src/toolkit/toolkit-card.vala b/src/toolkit/toolkit-card.vala
index 158a8a3..ed7ef81 100644
--- a/src/toolkit/toolkit-card.vala
+++ b/src/toolkit/toolkit-card.vala
@@ -126,6 +126,14 @@ public interface Card : Gtk.Widget {
      * message may be null even if the Card expects one; generally this means { link jump_back}
      * or { link jump_home} was invoked, resulting in this Card being activated.
      *
+     * Due to some mechanism inside of GSignal or Vala, it's possible for a caller to pass null
+     * that gets translated into a Value object holding a null pointer.  Deck will watch for this
+     * situation and convert those Values into a null reference.  This means passing Value(null)
+     * as a message is impossible.
+     *
+     * In order for this null-checking to work, the message must be holding a pointer, Object, or
+     * a string.  Other types (including Vala-generated fundamental types!) are not safe-guarded.
+     *
      * This is called before dealing with { link default_widget} and { link initial_focus}, so
      * changes to those properties in this call, if need be.
      */
diff --git a/src/toolkit/toolkit-deck.vala b/src/toolkit/toolkit-deck.vala
index e5f8acc..c738acf 100644
--- a/src/toolkit/toolkit-deck.vala
+++ b/src/toolkit/toolkit-deck.vala
@@ -191,6 +191,22 @@ public class Deck : Gtk.Stack {
         }
     }
     
+    private Value? strip_null_value(Value? message) {
+        if (message == null)
+            return null;
+        
+        if (message.holds(typeof(string)))
+            return message.get_string() != null ? message : null;
+        
+        if (message.holds(typeof(Object)))
+            return message.get_object() != null ? message : null;
+        
+        if (message.holds(typeof(void*)))
+            return message.get_pointer() != null ? message : null;
+        
+        return message;
+    }
+    
     /**
      * Force the { link Deck} to jump to the { link home} { link Card}.
      *
@@ -207,7 +223,7 @@ public class Deck : Gtk.Stack {
         navigation_stack.clear();
         
         set_visible_child(home);
-        home.jumped_to(null, message);
+        home.jumped_to(null, strip_null_value(message));
     }
     
     private void on_jump_to_card(Card card, Card next, Value? message) {
@@ -226,7 +242,7 @@ public class Deck : Gtk.Stack {
         }
         
         set_visible_child(next);
-        next.jumped_to(card, message);
+        next.jumped_to(card, strip_null_value(message));
     }
     
     private void on_jump_to_card_by_name(Card card, string name, Value? message) {