function (cb, context) {
        var zlevelList = this._zlevelList;
        var z;
        var i;
        for (i = 0; i < zlevelList.length; i++) {
            z = zlevelList[i];
            cb.call(context, this._layers[z], z);
        }
    }
function (cb, context) {
        var zlevelList = this._zlevelList;
        var z;
        var i;
        for (i = 0; i < zlevelList.length; i++) {
            z = zlevelList[i];
            cb.call(context, this._layers[z], z);
        }
    }
function ( cb , context ) { var zlevelList = this . _zlevelList ; var z ; var i ; for ( i = 0 ; i < zlevelList . length ; i ++ ) { z = zlevelList [ i ] ; cb . call ( context , this . _layers [ z ] , z ) ; } }
Iterate each layer
Iterate each layer
call the callback for each layer