How to add a generic "All" value in GXT ComboBox - gwt

Question

I have a generic GXT3 ComboBox which display all available values for enums :
public static <T extends Enum<T>> ComboBox<T> buildEnumCombo(Class<T> t){
ListStore<T> listStore=new ListStore<T>(new EnumModelKeyProvider<T>());
for(T e:t.getEnumConstants()){
listStore.add(e);
}
ComboBox<T> combo= new ComboBox<T>(listStore, new EnumLabelProvider<T>());
combo.setTriggerAction(ComboBoxCell.TriggerAction.ALL);
return combo;
}
This combo works fine.
What I need : I would like a be able to add a "All" value.
I tried to add "null" in the store and customize the LabelProvider to display "All" for this particular case but it does not work as expected : the combo contains the expected line but it displays an empty text instead of "All" and the line does not have a correct size.
Here is my generic ModelKeyProvider for enums
public class EnumModelKeyProvider<T extends Enum> implements ModelKeyProvider<T> {
#Override
public String getKey(T item) {
if(item==null){
return null;
}else{
return item.name();
}
}
And my generic LabelProvider :
public class EnumLabelProvider<T extends Enum<T>> implements LabelProvider<T> {
#Override
public String getLabel(T item) {
if(item==null){
return "All";
}else{
return I18nEnum.i18nEnum(item);
}
}
}

Answer 1

Maybe not the solution you are looking for, but I solved this but simply setting the emptyText of the ComboBox to "All".

Answer 2

Try SimpleComboBox (tested on gxt 2.2.5)
private SimpleComboBox<String> createSimpleComboBox(){
SimpleComboBox<String> combo = new SimpleComboBox<String>();
combo.setTypeAhead(true);
combo.setTriggerAction(TriggerAction.ALL);
combo.setEditable(editable);
combo.setForceSelection(true);
combo.setTemplate(getComboTemplate());
return combo;
}
private native String getComboTemplate() /*-{
return [
'<tpl for=".">',
'<tpl if="value == \'\'">',
'<div class="x-combo-list-item" qtip="N/A" qtitle=""></BR></div>',
'</tpl>',
'<tpl if="value != \'\'">',
'<div class="x-combo-list-item" qtip="{value}" qtitle="">{value}</div>',
'</tpl>',
'</tpl>'
].join("");
}-*/;
public SimpleComboBox<String> buildComboBox(){
SimpleComboBox<String> combo = createSimpleComboBox();
combo.add("");
List<String> list = new ArrayList<String>();
for(T e:t.getEnumConstants()){
list.add(e.name());
}
combo.add(list);
return combo;
}