Test returning invalid value from GetValue() in dataview sample

Add an item with unspecified year and update the custom model GetValue()
to not return anything in this case to check, and confirm, that all
implementations handle this properly by simply not showing anything in
the cell in this case.

samples/dataview/mymodels.cpp

@@ -40,6 +40,8 @@ MyMusicTreeModel::MyMusicTreeModel()
     m_pop = new MyMusicTreeModelNode( m_root, "Pop music" );
     m_pop->Append(
         new MyMusicTreeModelNode( m_pop, "You are not alone", "Michael Jackson", 1995 ) );
+    m_pop->Append(
+        new MyMusicTreeModelNode( m_pop, "Yesterday", "The Beatles", -1 /* not specified */ ) );
     m_pop->Append(
         new MyMusicTreeModelNode( m_pop, "Take a bow", "Madonna", 1994 ) );
     m_root->Append( m_pop );
@@ -193,7 +195,8 @@ void MyMusicTreeModel::GetValue( wxVariant &variant,
         variant = node->m_artist;
         break;
     case 2:
-        variant = (long) node->m_year;
+        if (node->m_year != -1)
+            variant = (long) node->m_year;
         break;
     case 3:
         variant = node->m_quality;
@@ -202,7 +205,9 @@ void MyMusicTreeModel::GetValue( wxVariant &variant,
         variant = 80L;  // all music is very 80% popular
         break;
     case 5:
-        if (GetYear(item) < 1900)
+        if (node->m_year == -1)
+            variant = "n/a";
+        else if (node->m_year < 1900)
             variant = "old";
         else
             variant = "new";
@@ -248,7 +253,16 @@ bool MyMusicTreeModel::IsEnabled( const wxDataViewItem &item,
     MyMusicTreeModelNode *node = (MyMusicTreeModelNode*) item.GetID();
 
     // disable Beethoven's ratings, his pieces can only be good
-    return !(col == 3 && node->m_artist.EndsWith("Beethoven"));
+    if (col == 3 && node->m_artist.EndsWith("Beethoven"))
+        return false;
+
+    // also disable editing the year when it's not specified, this doesn't work
+    // because the editor needs some initial value
+    if (col == 2 && node->m_year == -1)
+        return false;
+
+    // otherwise allow editing
+    return true;
 }
 
 wxDataViewItem MyMusicTreeModel::GetParent( const wxDataViewItem &item ) const

samples/dataview/mymodels.h

@@ -26,7 +26,7 @@ class MyMusicTreeModelNode
 public:
     MyMusicTreeModelNode( MyMusicTreeModelNode* parent,
                           const wxString &title, const wxString &artist,
-                          unsigned int year )
+                          int year )
     {
         m_parent = parent;