Controllers and Mocks
1 Controllers and Mocks

Controllers and Mocks

CS 3500, Summer I 2020

Recall: The MVC Architecture

  • Controller: takes input from the user and decides what to do

  • View: knows how to display the interface to the user

  • Model: the domain information that the program manipulates


  • Glue of the system

  • Controls how and when the model is used

  • Controls what must be shown to the view and when (in some cases)

  • Controls what action must be taken when user interacts with system (in some cases)

  • Controls the sequence of operations in an application(in some cases)

Model, View and Controller relative to each other

  • Model: Offers actual operations

    • Oblivious to when they are needed and how to present results
  • View: Presents results

    • Oblivious to how they are produced or what to do next
  • Controller: Delegates to model and view

    • Oblivious to how model completes the operations or how specifically the view shows output
Ignorance is bliss

An example: simple games

  • “Controls” the game

  • Uses the model and the view in specific sequence

    • Determines when to ask for user input
    • Determines when to use which operation of model
    • Determines when to show output to the view
  • The sequence of the game is determined by the controller, not the user

How to handle user input?

Synchronous Controllers

  • Determines the sequence of operations of the system

  • Determines when and how the system interacts with something external

  • Suitable for pre-baked “rules-based” applications, like games

  • Challenges:

    • How to truly separate controller from view so each one is replaceable?

    • What to do when system is reactive in nature (to user input)?

      • User interaction decides program behavior, not controller

      • Examples: Microsoft Word, IntelliJ

Asynchronous Controllers

  • Controller gets control when external input is given

  • Controller methods are called as a response to external input

  • Such methods in the controller are called callbacks

  • GUI programs typically work this way

    • User clicks a button, moves the mouse ⇒ a controller method is called

    • In OO GUIs, callbacks are often wrapped in classes called listeners

  • Look at MVCExample code

Who is the puppeteer?

  • Who creates the controller, model, view objects and sets the program in motion?

  • For standalone Java program: the main method

public class MainRunner {
   public static void main(String[] args) {
     IModel theModel = makeAModel();
     IView aView = makeAView();
     IController theController = makeController(theModel, aView);

Simple input and output

  • Printing to console System.out.print

  • Reading from keyboard Scanner

  • How to we test this (using a tester, not a human)?


  • A component that can “replace” an actual component

    • For testing purposes

    • As a placeholder for simultaneous development (also called a stub)

  • Design that facilitates replacement promotes mocking

    • Good encapsulation ⇒ fixed role ⇒ easier to replace

    • Design by interface ⇒ Not dependent on specific implementation

    • Refer to components using interfaces ⇒ work with any implementation of that interface

    • Mock implements actual interface, but does so simply or to enable testing

  • Use mocks to

    • Test components that otherwise require I/O

    • Emulate a component that is being concurrently designed/implemented