How to use the awkward1.layout.VirtualArray function in awkward1

dtype = interpretation.type
            length = len(self)
        parameters = {
            "__doc__": self._tree[branch_name].title.decode("ascii"),
            "__record__": "NanoColumn",
        }
        # use hint to resolve platform-dependent format
        formhint = awkward1.forms.Form.fromjson('"%s"' % dtype)
        form = awkward1.forms.NumpyForm(
            [], formhint.itemsize, formhint.format, parameters=parameters
        )
        generator = awkward1.layout.ArrayGenerator(
            self.reader, (branch_name, parameters), {}, form=form, length=length,
        )
        source = "file"
        return awkward1.layout.VirtualArray(
            generator,
            self._cache,
            "/".join([self._keyprefix, source, branch_name]),
            parameters=parameters,
        )

n = len(indexers)
            out = numpy.empty(n * len(indexers[0]), dtype="int64")
            for i, idx in enumerate(indexers):
                out[i::n] = idx
            offsets = numpy.arange(0, len(out) + 1, n, dtype=numpy.int64)
            return awkward1.layout.ListOffsetArray64(
                awkward1.layout.Index64(offsets), awkward1.layout.NumpyArray(out),
            )

        form = awkward1.forms.Form.fromjson(
            '{"class": "ListOffsetArray64", "offsets": "i64", "content": "int64"}'
        )
        generator = awkward1.layout.ArrayGenerator(
            nestedindex, (), {}, form=form, length=indexers[0].generator.length,
        )
        return awkward1.layout.VirtualArray(
            generator,
            self._cache,
            cache_key="/".join([self._keyprefix, "nestedindex", name]),
        )

gidx = awkward1.Array(
                awkward1.layout.ListOffsetArray32(
                    awkward1.layout.Index32(source_offsets), index.generator(),
                )
            )
            gidx = gidx.mask[gidx >= 0] + target_offsets[:-1]
            return awkward1.fill_none(awkward1.flatten(gidx), -1)

        generator = awkward1.layout.ArrayGenerator(
            globalindex,
            (),
            {},
            form=awkward1.forms.Form.fromjson('"int64"'),
            length=index.generator.length,
        )
        return awkward1.layout.VirtualArray(
            generator,
            index.cache,
            "/".join([self._keyprefix, "local2global", name]),
            index.identities,
            index.parameters,
        )

def _with_length(array: awkward1.layout.VirtualArray, length: int):
    return awkward1.layout.VirtualArray(
        array.generator.with_length(length),
        array.cache,
        array.cache_key,
        array.identities,
        array.parameters,
    )

"__array__": mixin,
                    "events_key": self._keyprefix,
                    "collection_name": name,
                }
                form = awkward1.forms.ListOffsetForm(
                    "i32", content.form, parameters=params
                )
                generator = awkward1.layout.ArrayGenerator(
                    self._listarray,
                    (offsets, content, params),
                    {},
                    form=form,
                    length=len(self),
                )
                source = "runtime"
                return awkward1.layout.VirtualArray(
                    generator,
                    self._cache,
                    cache_key="/".join([self._keyprefix, source, name]),
                    parameters=params,
                )
            elif name in arrays:
                # singleton
                return arrays[name]
            else:
                # simple collection
                content = {
                    k[len(name) + 1 :]: arrays[k]
                    for k in arrays
                    if k.startswith(name + "_")
                }
                return awkward1.layout.RecordArray(