State Machine

A state machine, also known as a finite state machine (FSM), is a mathematical model that describes the behavior of a system by defining a set of states, transitions between those states, and actions associated with those transitions. In the context of web development, state machines are commonly used to model user interface interactions and to manage application state.

Purpose

State machines provide a powerful and flexible approach to modeling complex behavior. By breaking a system down into discrete states and defining how it transitions between those states, a state machine can capture a wide range of behavior without becoming overly complex or difficult to reason about.

In the context of web development, state machines are particularly useful for modeling user interfaces. A user interface typically has a large number of possible states, ranging from simple things like “the user has not yet clicked a button” to more complex states like “the user has filled out a form and submitted it”. By modeling these states as part of a state machine, developers can more easily reason about how the interface should behave in each state and how to handle user interactions that cause state transitions.

State machines are also useful for managing application state. In many web applications, the state of the application is constantly changing as the user interacts with it. By modeling this state as a state machine, developers can more easily reason about how the application should behave in different situations and how to handle complex interactions between different parts of the application.

Usage

There are a number of different ways to implement state machines in web development, depending on the specific requirements of the project. However, most state machines share some common features:

States

At the core of any state machine is a set of states that describe the possible behavior of the system. In the context of web development, these states might represent different parts of a user interface or different states of an application.

Each state is defined by a set of properties that describe the behavior of the system in that state. These might include things like which user interface elements are visible, which buttons are enabled, or which data is currently loaded.

Transitions

In addition to states, a state machine also defines a set of transitions between those states. Transitions are triggered by events or actions that cause the system to move from one state to another.

For example, in a user interface, a transition might be triggered by a user clicking a button or entering data into a form. In an application, a transition might be triggered by a network request completing or a data source changing.

Each transition is defined by a set of properties that describe the conditions under which the transition can occur. These might include things like whether a certain form field is valid, whether a network request has completed successfully, or whether a user is logged in.

Actions

Finally, a state machine also defines a set of actions that are associated with each transition. These actions describe what should happen when the system moves from one state to another.

In a user interface, actions might include things like updating the contents of a form field, changing which buttons are visible, or displaying an error message. In an application, actions might include things like updating the UI to reflect new data, triggering a network request, or saving data to a local database.

Example

To see how a state machine might be used in practice, let’s consider a simple example: a login form. The login form has two possible states: “not logged in” and “logged in”. When the user is not logged in, they see a form with two fields (username and password) and a “login” button. When they are logged in, they see a message that says “You are logged in as [username]” and a “logout” button.

To model this behavior as a state machine, we might define the following states:

const states = [
  { name: 'notLoggedIn', onEntry: showLoginForm },
  { name: 'loggedIn', onEntry: showLoggedInMessage },
];

In this example, each state is represented as an object with a name property that describes the name of the state (in this case, “notLoggedIn” and “loggedIn”) and an onEntry property that describes what should happen when the system enters that state.

We might then define the following transitions:

const transitions = [
  { from: 'notLoggedIn', to: 'loggedIn', action: login, condition: isLoggedIn },
  { from: 'loggedIn', to: 'notLoggedIn', action: logout, condition: isNotLoggedIn },
];

In this example, each transition is represented as an object with a from property that describes the state the system is transitioning from, a to property that describes the state the system is transitioning to, an action property that describes what should happen when the transition occurs, and a condition property that describes the conditions under which the transition can occur.

We might then define the following actions:

function showLoginForm() {
  // show the login form
}

function showLoggedInMessage() {
  // show the logged in message
}

function login() {
  // submit the login form
}

function logout() {
  // log the user out
}

function isLoggedIn() {
  // check whether the user is logged in
}

function isNotLoggedIn() {
  // check whether the user is not logged in
}

In this example, each action is represented as a function that describes what should happen when the associated transition occurs. We also have two condition functions (isLoggedIn and isNotLoggedIn) that are used to determine whether a transition should occur.

With this state machine defined, we can now reason about how the login form should behave in each state and how to handle user interactions that cause state transitions. For example, when the user clicks the “login” button, we might trigger the login action, which would check whether the form fields are valid and then transition the system from the “notLoggedIn” state to the “loggedIn” state if the login is successful.