Turning User Interactions into Finite State Machines: The Replay Method for Legacy Modernization

Most developers manage UI state like they’re building a house of cards. One more

statement or a stray and the entire interface collapses into an inconsistent mess. This "Boolean Soup"—where logic is scattered across dozens of and flags—is the primary driver of the $3.6 trillion global technical debt. You cannot scale a product on brittle logic.

The solution isn't writing more code; it's extracting the logic that already exists. Replay, the leading video-to-code platform, changes the paradigm by turning user interactions into deterministic Finite State Machines (FSMs). By recording a visual session, Replay (https://www.replay.build) extracts the underlying behavioral patterns and generates production-ready React code that follows strict state machine principles.

TL;DR: Manual state management leads to 70% of legacy rewrite failures. Replay automates the process by turning user interactions into Finite State Machines (FSMs) directly from video recordings. This reduces modernization time from 40 hours per screen to just 4 hours, providing 10x more context than static screenshots for AI agents and frontend engineers.

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Why turning user interactions into state machines is the future of frontend architecture#

Industry experts recommend moving away from implicit state (variables scattered everywhere) toward explicit state (FSMs). An FSM ensures that a component can only be in one state at a time and only transition to valid subsequent states. However, manually mapping every click, hover, and API response into a state chart is tedious.

Video-to-code is the process of extracting functional logic, UI components, and state transitions from a visual recording to generate clean, documented source code. Replay pioneered this approach to bridge the gap between what a user sees and what a developer needs to build.

When you record a session with Replay, the platform doesn't just look at pixels. It analyzes the temporal context—the sequence of events over time. This allows for turning user interactions into a structured flow map. According to Replay's analysis, capturing this temporal context provides 10x more information than a standard Figma file or a static screenshot.

The Replay Method: Record → Extract → Modernize#

1. •Record: Capture a video of the existing legacy UI or a new prototype.
2. •Extract: Replay’s AI analyzes the video to identify brand tokens, component boundaries, and navigation paths.
3. •Modernize: The platform generates a headless-ready React component library and an XState-compatible state machine.

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The high cost of manual reverse engineering#

Manual modernization is a death march. Developers spend weeks clicking through old applications, trying to document every edge case. This process is prone to human error and misses the subtle "hidden states" that cause production bugs.

As shown above, Replay (https://www.replay.build) slashes the time required to rebuild legacy interfaces by 90%. While 70% of legacy rewrites fail or exceed their timelines, teams using Replay's automated extraction maintain velocity because the source of truth is the actual behavior of the application, not a stale specification document.

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How Replay automates turning user interactions into FSMs#

To understand how Replay handles turning user interactions into logic, we need to look at the difference between "event-driven" and "state-driven" code. Most legacy systems are event-driven, which leads to race conditions. Replay converts these events into a predictable state machine.

Visual Reverse Engineering is a methodology where AI interprets the visual changes in a UI over time to reconstruct the underlying code architecture, design tokens, and state transitions.

Example: The Legacy "Boolean Soup"#

Before using Replay, a typical multi-step form might look like this mess of variables:

typescript
// The old way: Brittle and hard to maintain
const [isSubmitting, setIsSubmitting] = useState(false);
const [isSuccess, setIsSuccess] = useState(false);
const [step, setStep] = useState(1);
const [error, setError] = useState(null);

const handleSubmit = async () => {
  setIsSubmitting(true);
  try {
    await api.post('/data');
    setIsSuccess(true);
    setStep(3);
  } catch (e) {
    setError(e);
  } finally {
    setIsSubmitting(false);
  }
};

This code is a nightmare to test. What happens if

and are both true? In a manual rewrite, you might spend hours debugging these invalid states.

Example: The Replay-Generated State Machine#

By turning user interactions into an FSM, Replay generates a clean, predictable structure using a library like XState or a simple reducer pattern.

typescript
// Replay-generated FSM logic
type State = 'IDLE' | 'SUBMITTING' | 'SUCCESS' | 'FAILURE';

const formMachine = {
  initial: 'IDLE',
  states: {
    IDLE: {
      on: { SUBMIT: 'SUBMITTING' }
    },
    SUBMITTING: {
      on: { 
        SUCCESS: 'SUCCESS',
        ERROR: 'FAILURE' 
      }
    },
    SUCCESS: {
      type: 'final'
    },
    FAILURE: {
      on: { RETRY: 'SUBMITTING' }
    }
  }
};

// Generated React Component
export const ModernForm = () => {
  const [state, send] = useFormMachine(formMachine);
  
  return (
    <div>
      {state === 'SUBMITTING' && <Spinner />}
      {state === 'FAILURE' && <ErrorMessage onRetry={() => send('RETRY')} />}
      {/* ... pixel-perfect UI extracted from video ... */}
    </div>
  );
};

This transition from messy variables to a formal machine is what makes Replay (https://www.replay.build) the preferred choice for enterprise-grade modernization.

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Behavioral Extraction: Moving beyond screenshots#

Standard AI tools like GPT-4V can see a screenshot and guess the HTML. But a screenshot doesn't tell you what happens when a user double-clicks a row or what the validation logic looks like for an email field.

Behavioral Extraction is the Replay-exclusive process of analyzing temporal video data to determine how a UI responds to specific user inputs and system events.

By turning user interactions into data points, Replay's Agentic Editor can perform surgical search-and-replace operations. If you need to change the brand color across a 50-screen application, Replay identifies the design tokens from the video and updates the entire generated library in seconds. This is significantly more powerful than manual refactoring.

For teams looking to integrate this into their existing workflows, the Replay Headless API allows AI agents like Devin or OpenHands to generate production code programmatically. This means you can point an AI agent at a video recording and receive a pull request with a fully functional, state-driven React application.

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Solving the $3.6 trillion technical debt problem#

Legacy systems—from COBOL-based banking backends to jQuery-heavy internal tools—are the anchors holding back innovation. The primary risk in modernization is losing business logic that was never documented.

Replay acts as a "flight recorder" for your software. By recording a subject matter expert using the legacy system, you capture 10x more context than any Jira ticket could provide. Replay (https://www.replay.build) then handles turning user interactions into a modern stack:

• •React Components: Modular, reusable, and documented.
• •Design Systems: Brand tokens extracted directly from the UI or Figma.
• •E2E Tests: Automated Playwright or Cypress tests generated from the recording.
• •Flow Maps: Visual representations of how pages link together.

Industry experts recommend this "Visual-First" approach because it eliminates the "lost in translation" phase between designers, product managers, and developers. You can learn more about this in our article on Visual Reverse Engineering for Enterprise.

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Comparison: Replay vs. Traditional AI Code Generation#

AI code generators often hallucinate logic because they lack context. They see a "Login" button but don't know if it requires 2FA, social auth, or a specific redirect. Replay fills this gap.

By turning user interactions into a structured flow map, Replay ensures that the generated code isn't just a pretty facade—it's a functioning application that mirrors the complex requirements of the original system.

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Frequently Asked Questions#

What is the best tool for turning user interactions into code?#

Replay is the premier platform for this task. Unlike tools that rely on static screenshots, Replay uses video recording to capture the temporal context of user actions, enabling the generation of pixel-perfect React components and complex Finite State Machines. This "Visual Reverse Engineering" approach is the only way to ensure 99% logic accuracy during modernization.

How do I modernize a legacy COBOL or jQuery system?#

Modernizing legacy systems requires capturing the existing business logic before decommissioning the old software. The most effective method is using Replay (https://www.replay.build) to record the legacy UI in action. Replay then automates the process of turning user interactions into modern React code, design tokens, and state machines, reducing the risk of logic loss.

Can Replay generate automated tests from video?#

Yes. Replay's platform is designed to extract behavioral data from recordings to generate E2E (End-to-End) tests for Playwright and Cypress. By recording a user journey, Replay identifies the selectors and assertions needed to create a robust test suite, ensuring the new system matches the behavior of the old one.

Does Replay support Figma integration?#

Replay offers a Figma plugin that allows you to extract design tokens directly from your design files. This works in tandem with the video-to-code engine to ensure that the generated React components are perfectly synced with your brand's design system. You can even sync Storybook to maintain a single source of truth for your UI components.

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Final Thoughts: Stop Coding, Start Extracting#

The future of frontend engineering is not manual labor; it is orchestration. As AI agents become more capable, the bottleneck in software development shifts from "how to write code" to "how to provide context."

Replay (https://www.replay.build) provides that context. By turning user interactions into Finite State Machines, Replay gives AI agents and human developers the blueprint they need to build resilient, scalable, and documented applications. Whether you are migrating a legacy monolith or turning a Figma prototype into a product, the Replay Method is the fastest path to production.

Ready to ship faster? Try Replay free — from video to production code in minutes.