跨类的实例共享

Question

I have this python script which has a listener class and a main class. A serial stream is created in the main class and a listener instance is created in the main class. The whole purpose of the listener is to send a message on the serial port when a listened property has changed. The problem is that the listener doesn't have access to the output stream created in the main class. The listener does an abrupt return when trying to execute the outputStream.write statement How can I give the listener access to the output stream?

import purejavacomm
import java.beansSensorListener

class MyListener(java.beans.PropertyChangeListener):
  def propertyChange(self, event):
    if (< some property has changed >) :
        self.outputStream.write(message) # send notice on serial port
    return


class MainClass(jmri.jmrit.automat.AbstractAutomaton) :
    def __init__(self) :
        self.portID = purejavacomm.CommPortIdentifier.getPortIdentifier("COM3")
        self.port = self.portID.open("SerialCom", 50)
        self.outputStream = self.port.getOutputStream()
        return

    def init(self) :
        myListener = MyListener()
        deviceList = devices.getNamedBeanSet()

        for device in deviceList :
            device.addPropertyChangeListener(myListener)
        return

a = MainClass()
a.start();

Answer 1

It seems that really MyListener should either have a stream passed into the constructor or the callback (using lambda expression).

First, which is probably the cleanest way, would look like this:

def init(self) :
        myListener = MyListener(self.outputStream)

Second version: device.addPropertyChangeListener(lambda x: myListener(x, self.outputStream)) Both need changes in the MyListener class to either constructor, or definition of propertyChange .

If you are set of sharing an instance of here are two options.

If you can modify arguments to MyListener constructor you pass in instance of MainClass to it:

def init(self) :
        myListener = MyListener(self)

If not, a little bit more hacky way of doing this is by making MainClass a singelton (see example here ), and then calling it's constructor within MyListener