Mediator¶

The Mediator pattern is similar to the [Observer] (observer.md) pattern in that it describes how to execute and handle a request. This pattern is worth using when in the application we find many components that somehow communicate with each other. When the relationships between them become complicated, for example:

no_mediator

its use is worth considering.

The mediator is an additional object inserted between all these components, which instead of communicating directly with each other, communicating only with it:

mediator

This approach reduces the amount of dependencies between objects.

Example¶

The example simulates simple dependencies between graphic components. Updating one of them (e.g. by selecting the user) results in updating some of them. All communication is through one Mediator:

class Component:
    def send_request(self, context=None):
        pass

class ActionAppliedMessage(Component):
    def __init__(self):
        self._mediator = None

    def set_mediator(self, mediator):
        self._mediator = mediator

    def display_info(self):
        print("Action was applied successfully")

    def send_request(self, context=None):
        self._mediator.send_info(self, "ActionAppliedMessage")

class SelectOptions(Component):
    def __init__(self):
        self._mediator = None

    def set_mediator(self, mediator):
        self._mediator = mediator

    def display_options(self):
        print("Options are: Save, Load, Restart")

    def choose_save(self):
        print("Status was saved")

    def choose_load(self):
        print("Loading previous data")

    def choose_restart(self):
        print("Status is restarting")

    def hide_options(self):
        print("Hiding options")

    def send_request(self, context=None):
        self._mediator.send_info(self, context if context else "displayOptions")

class WarningMessage(Component):
    def __init__(self):
        self._mediator = None

    def set_mediator(self, mediator):
        self._mediator = mediator

    def show_warning_message(self):
        print("Are you sure?")

    def hide_warning(self):
        print("")

    def send_request(self, context=None):
        self._mediator.send_info(self, "WarningMessage")

class Mediator:
    def send_info(self, requester, context):
        pass

class UserActionMediator(Mediator):
    def __init__(self, action_applied_message, select_options, warning_message):
        self._action_applied_message = action_applied_message
        self._select_options = select_options
        self._warning_message = warning_message
        self._action_applied_message.set_mediator(self)
        self._select_options.set_mediator(self)
        self._warning_message.set_mediator(self)

    def send_info(self, requester, context):
        if requester == self._action_applied_message:
            self._action_applied_message.display_info()
            self._warning_message.hide_warning()
            self._select_options.hide_options()
        elif requester == self._select_options:
            if context == "load":
                self._select_options.choose_load()
                self._action_applied_message.display_info()
            elif context == "restart":
                self._select_options.choose_restart()
                self._warning_message.show_warning_message()
            elif context == "save":
                self._select_options.choose_save()
                self._action_applied_message.display_info()
            elif context == "displayOptions":
                self._select_options.display_options()
        elif requester == self._warning_message:
            if context == "hide":
                self._warning_message.hide_warning()
            else:
                self._warning_message.show_warning_message()

def main():
    action_applied_message = ActionAppliedMessage()
    select_options = SelectOptions()
    warning_message = WarningMessage()

    mediator = UserActionMediator(action_applied_message, select_options, warning_message)

    select_options.send_request()
    select_options.send_request("load")
    select_options.send_request("save")
    select_options.send_request("restart")
    select_options.send_request("hide")


if __name__ == '__main__':
    main()

Sample output:
  Options are: Save, Load, Restart
  Loading previous data
  Action was applied successfully
  Status was saved
  Action was applied successfully
  Status is restarting
  Are you sure?