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Connects an I/O handler. 
 
 is equivalent to: Glib::signal_io().connect(SigC::slot(&io_handler), fd, Glib::IO_IN | Glib::IO_HUP);
Parameters:
  
    | slot | A slot to call when polling fd results in an event that matches condition. The event will be passed as a parameter to slot. If io_handler returns falsethe signal is disconnected. |  | fd | The file descriptor (or a HANDLEon Win32 systems) to watch. |  | condition | The conditions to watch for. |  | priority | The priority of the new event source. | 
 Returns:A connection handle, which can be used to disconnect the handler. 
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