Pattern Mediator

#!/usr/bin/env python3
# Copyright © 2012-13 Qtrac Ltd. All rights reserved.
# This program or module is free software: you can redistribute it
# and/or modify it under the terms of the GNU General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version. It is provided for
# educational purposes and is distributed in the hope that it will be
# useful, but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
# General Public License for more details.

from Qtrac import coroutine


def main():
    form = Form()
    test_user_interaction_with(form)


class Form:

    def __init__(self):
        self.create_widgets()
        self.create_mediator()


    def create_widgets(self):
        self.nameText = Text()
        self.emailText = Text()
        self.okButton = Button("OK")
        self.cancelButton = Button("Cancel")


    def create_mediator(self):
        self.mediator = self._update_ui_mediator(self._clicked_mediator())
        for widget in (self.nameText, self.emailText, self.okButton,
                self.cancelButton):
            widget.mediator = self.mediator
        self.mediator.send(None)


    @coroutine
    def _update_ui_mediator(self, successor=None):
        while True:
            widget = (yield)
            self.okButton.enabled = (bool(self.nameText.text) and
                                     bool(self.emailText.text))
            if successor is not None:
                successor.send(widget)


    @coroutine
    def _clicked_mediator(self, successor=None):
        while True:
            widget = (yield)
            if widget == self.okButton:
                print("OK")
            elif widget == self.cancelButton:
                print("Cancel")
            elif successor is not None:
                successor.send(widget)


class Mediated:

    def __init__(self):
        self.mediator = None


    def on_change(self):
        if self.mediator is not None:
            self.mediator.send(self)


class Button(Mediated):

    def __init__(self, text=""):
        super().__init__()
        self.enabled = True
        self.text = text


    def click(self):
        if self.enabled:
            self.on_change()


    def __str__(self):
        return "Button({!r}) {}".format(self.text,
                "enabled" if self.enabled else "disabled")


class Text(Mediated):

    def __init__(self, text=""):
        super().__init__()
        self.__text = text


    @property
    def text(self):
        return self.__text


    @text.setter
    def text(self, text):
        if self.text != text:
            self.__text = text
            self.on_change()


    def __str__(self):
        return "Text({!r})".format(self.text)


def test_user_interaction_with(form):
    form.okButton.click()           # Ignored because it is disabled
    print(form.okButton.enabled)    # False
    form.nameText.text = "Fred"
    print(form.okButton.enabled)    # False
    form.emailText.text = "fred@bloggers.com"
    print(form.okButton.enabled)    # True
    form.okButton.click()           # OK
    form.emailText.text = ""
    print(form.okButton.enabled)    # False
    form.cancelButton.click()       # Cancel


if __name__ == "__main__":
    main()