How to use the deltachat.capi.ffi function in deltachat

CFFI only allows us to set one global event handler, so this one
    looks up the correct event handler for the given context.
    """
    try:
        callback = _DC_CALLBACK_MAP.get(ctx, lambda *a: 0)
    except AttributeError:
        # we are in a deep in GC-free/interpreter shutdown land
        # nothing much better to do here than:
        return 0

    # the following code relates to the deltachat/_build.py's helper
    # function which provides us signature info of an event call
    evt_name = get_dc_event_name(evt)
    event_sig_types = capi.lib.dc_get_event_signature_types(evt)
    if data1 and event_sig_types & 1:
        data1 = ffi.string(ffi.cast('char*', data1)).decode("utf8")
    if data2 and event_sig_types & 2:
        try:
            data2 = ffi.string(ffi.cast('char*', data2)).decode("utf8")
        except UnicodeDecodeError:
            # XXX ignoring this error is not quite correct but for now
            # i don't want to hunt down encoding problems in the c lib
            data2 = ffi.string(ffi.cast('char*', data2))

    try:
        ret = callback(ctx, evt_name, data1, data2)
        if event_sig_types & 4:
            return ffi.cast('uintptr_t', ret)
        elif event_sig_types & 8:
            return ffi.cast('int', ret)
    except:  # noqa
        raise