The OnRequesting client-side event occurs when data is requested form the web service.
The event handler receives two parameters:
sender - an instance of the datasource;
args -containing the following methods:
get_modelName()- returns the model for which the event is fired;
get_options() - returns the parameters object that will be sent to the service.
set_filterExpression() - sets filters for the request;
set_pageIndex() - sets the page index;
set_pageSize() - sets the page size;