# `TargetCustom`

class pyvips.vtargetcustom.TargetCustom

  An output target you can connect action signals to to implement behaviour.

  on_write(handler)

    Attach a write handler.

    The interface is exactly as io.write(). The handler is given a bytes-like object to write, and should return the number of bytes written.

  on_read(handler)

    Attach a read handler.

    The interface is exactly as io.read(). The handler is given a number of bytes to fetch, and should return a bytes-like object containing up to that number of bytes. If there is no more data available, it should return None.

    Read handlers are optional for targets. If you do not set one, your target will be treated as unreadable and libvips will be unable to write some file types (just TIFF, as of the time of writing).

  on_seek(handler)

    Attach a seek handler.

    The interface is the same as io.seek(), so the handler is passed parameters for offset and whence with the same meanings.

    However, the handler MUST return the new seek position. A simple way to do this is to call io.tell() and return that result.

    Seek handlers are optional. If you do not set one, your target will be treated as unseekable and libvips will be unable to write some file types (just TIFF, as of the time of writing).

  on_end(handler)

    Attach an end handler.

    This optional handler is called at the end of write. It should do any cleaning up necessary, and return 0 on success and -1 on error.

  on_finish(handler)

    Attach a finish handler.

    For libvips 8.13 and later, this method is deprecated in favour of `on_end()`.