SAP DESIGN STUDIO 1.6: how to read input datasource to create custom datasource

Hi Gurus, I created a custom datasource filled by a standard datasource:

My issue is how to identify where the data are stored into my custom datasource for example:

I thought that "this.data()" into my routine, contained my input datasource "DS_2" but it's empty.

this.data = function(value) {     if (value === undefined) {          return "";     }else {          if (value) {               this.buffer = value;               if (!this.buffer.formattedData) {                    this.buffer.formattedData = this.buffer.data;               }               this.fireUpdate(false);          }          return this;     }
};

I need to read input datasorce to create my custom datasource. Any idea?

Thanks

Patrizia