Hi Roman,

At this point not setting dataSource is not supported scenario. But I investigated this and logged an issue in github, so that we could support loadOnDemand capabilities into an Angular TreeGrid.

You can see the issue here https://github.com/IgniteUI/igniteui-angular2/issues/146

Once we got it solved you could verify your scenario. In my opinion when there is no data source the treegrid should not throw an error "Cannot read property 'length' of undefined"

You can specify requestDataCallback  and use your custom request to get the data and the additional parameters you want to send.

You could specify this callback into the rowExpanding event.

For example:

rowExpanding: function() {

if (ui.owner.dataSource.settings.treeDS.requestDataCallback === null ||
 ui.owner.dataSource.settings.treeDS.requestDataCallback === undefined) {
 ui.owner.dataSource.settings.treeDS.requestDataCallback = function (record, expand, callbackArgs) {
  if (!record) {
   return;
  }
  var opts, me = ui.owner.dataSource, url, path, params, func, s = ui.owner.dataSource.settings.treeDS,
   args = {
    record: record,
    callbackArgs: callbackArgs,
    expand: expand
   };
  path = ui.owner.dataSource.getPathBy(record);
  params = ui.owner.dataSource._encodeUrl();
  params.expand = expand;
  url = s.dataSourceUrl + "?" + ui.owner.dataSource._encodeUrlPath(path, record[ s.propertyDataLevel ]);
  opts = {
   type: "GET",
   url: url,
   data: params,
   success: function (data, textStatus, jqXHR) {
    var func = s.requestDataErrorCallback,
     noCancel = true;
    if ($.type(func) === "function") {
     noCancel = func(args, data, textStatus, jqXHR);
    }
    if (noCancel) {
     me._requestDataSuccess(args, data, textStatus, jqXHR);
    }
   },
   error: function (jqXHR, textStatus, errorThrown) {
    var func = s.requestDataErrorCallback;
    if ($.type(func) === "function") {
     func(args, jqXHR, textStatus, errorThrown);
    }
   }
  };
  $.ajax(opts);
 }
}

}

You can add your custom parameters to the params variable

No you can assign the customEncodeUrlFunc into the rendered event, to make sure the data source is already created:

rendered: function (e, ui) {
 var ds = ui.owner.dataSource;
 ui.owner.dataSource.settings.treeDS.customEncodeUrlFunc = function(record, expand){
  var dsUrl = ds.settings.treeDS.dataSourceUrl;
  var path = ds.getPathBy(record);
  return dsUrl + "?" + "path=" + path + "&depth=" + record[ds.settings.treeDS.propertyDataLevel] + "&additionalparam=foo";
 }
}