Using custom action you can create your own ribbon elements. In a post I have published a while ago I showed how to add a button to the ribbon to generate a QR-Code. But what if you want your button only to be enabled under certain circumstances. For example; if the user has sufficient privileges.

You can specify a JavaScript function in the EnabledScript element within the custom action which will be called by SharePoint if the availability of the button should be checked.

  
<CustomAction
    Id="AlainDeKlerk.SP2010.DocumentIdLink"
    Location="CommandUI.Ribbon"
    RegistrationId="101"
    RegistrationType="List">
    <CommandUIExtension>
      <CommandUIDefinitions>
        <CommandUIDefinition
          Location="Ribbon.Documents.Share.Controls._children">
          <Button Id="AlainDeKlerk.SP2010.DocumentIdLink"
            Command="CreateDocumentIdLink"
            Image16by16="/_layouts/images/URL16.GIF"
            Image32by32="/_layouts/images/URL32.GIF"
            LabelText="Get Document ID Link"
            TemplateAlias="o2" />
        </CommandUIDefinition>
      </CommandUIDefinitions>
      <CommandUIHandlers>
        <CommandUIHandler
          Command="CreateDocumentIdLink"
          CommandAction="javascript:AlainDeKlerk_CreateDocumentIdLink();"
          EnabledScript="javascript:AlainDeKlerk_IsAvailable_CreateDocumentIdLink();"
          />
      </CommandUIHandlers>
    </CommandUIExtension>
  </CustomAction>

As an example; I will demonstrate a EnabledScript that will enable the button of 4 conditions are met:

1. Exactly one item is selected
2. The selected item is not a folder
3. The current user has enough permissions to edit the selected item
4. The selected item does not has the word ‘confidential’ in its name

Conditions (1) and (2) are quite easy to check, however conditions (3) and (4) are a bit more difficult but can be accomplished by querying SharePoint using the Client Side Object Model. The JavaScript below can be used to enable or disable a ribbon button based on the  described conditions:

var __document_id_permissions = new Array()

function AlainDeKlerk_IsAvailable_CreateDocumentIdLink() {
    var items = SP.ListOperation.Selection.getSelectedItems();
    if (items.length == 1) {
        if (items[0].fsObjType == '0') {
            return CheckListItem(items[0]);
        }
    }
}

function CheckListItem(item) {
    if (__document_id_permissions[item.id] === undefined)
    {
        var context = SP.ClientContext.get_current();
        var list = context.get_web().get_lists().getById(SP.ListOperation.Selection.getSelectedList());

        this.listItem = list.getItemById(item.id);

        context.load(this.listItem, 'FileRef', 'EffectiveBasePermissions', "ID");
        context.executeQueryAsync(Function.createDelegate(this, this.onCheckListItemSucceeded),
                                  Function.createDelegate(this, this.onCheckListItemSucceededFailed));

        return false;
    }
    return __document_id_permissions[item.id];
}

function onCheckListItemSucceeded(sender, args) {

    var __current_user_has_permissions = this.listItem.get_effectiveBasePermissions().has(SP.PermissionKind.editListItems);
    var __document_is_confidential = this.listItem.get_item('FileRef').indexOf('confidential') >= 0;
    var __itemId = this.listItem.get_item('ID')

    __document_id_permissions[__itemId] = __current_user_has_permissions && !__document_is_confidential
    if (__document_id_permissions[__itemId]) {
        RefreshCommandUI();
    }
}

function onCheckListItemSucceededFailed(sender, args) {
    alert('request failed ' + args.get_message() + 'n' + args.get_stackTrace());
}