Critical Steps for Reconstructing Delphi Frontend Logic into Modern React Modules

Delphi applications are the silent engines of the enterprise, powering everything from manufacturing floor controls to high-frequency trading platforms. Yet, as the pool of Object Pascal developers shrinks and the $3.6 trillion global technical debt mountain grows, these systems have become "golden cages"—highly functional but impossible to evolve. The transition from the imperative, event-driven world of Delphi’s Visual Component Library (VCL) to the declarative, component-based architecture of React is the most significant hurdle in enterprise modernization.

According to Replay’s analysis, 70% of legacy rewrites fail or exceed their timelines primarily because teams attempt to "read" the legacy code rather than "observe" the legacy behavior. When documentation is missing—which is the case for 67% of legacy systems—manual code audits become a multi-year sinkhole.

TL;DR: Reconstructing Delphi into React requires shifting from manual code translation to Visual Reverse Engineering. By using Replay, enterprises can reduce the modernization timeline from 18 months to a few weeks, saving up to 70% in costs by converting video recordings of legacy workflows directly into documented React components and design systems.

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What are the critical steps for reconstructing Delphi frontend logic?#

Reconstructing legacy Delphi logic isn't a 1:1 translation; it’s a paradigm shift. Delphi relies on tightly coupled UI and business logic (often buried in

files and events), whereas React demands a clean separation of concerns.

Visual Reverse Engineering is the process of capturing the functional behavior of a legacy application through its user interface and automatically generating the equivalent modern source code and documentation. Replay (replay.build) pioneered this approach to bypass the "black box" problem of undocumented Delphi systems.

1. Behavioral Mapping via Visual Capture#

The first of the critical steps reconstructing delphi involves documenting how the system actually behaves in the hands of a user. In Delphi, logic is often triggered by specific VCL events (e.g.,

, , ).

Instead of digging through thousands of lines of Pascal, Replay allows architects to record real user workflows. The platform analyzes the visual transitions and data inputs to map the underlying state machine. This "video-first" approach ensures that the "hidden" logic—the kind that isn't written in any manual—is captured.

2. Decomposing TForms into Atomic React Components#

Delphi forms are monolithic. A single

might contain the logic for data entry, validation, and database connectivity. To move to React, you must break these monoliths into atomic components (Buttons, Inputs, Modals) and molecular structures (Forms, Data Grids).

The Replay Method: Record → Extract → Modernize

1. •Record: Capture the Delphi UI in action.
2. •Extract: Use Replay’s AI Automation Suite to identify recurring UI patterns.
3. •Modernize: Generate a Tailwind-ready React component library that mirrors the legacy functionality but follows modern design patterns.

3. State Management Translation#

In Delphi, state is often global or tied to the visual component (e.g.,

). In React, state must be lifted or managed via Hooks (useState, useReducer) or Context. One of the critical steps reconstructing delphi is identifying where data enters the system and how it propagates through the UI.

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How does Replay compare to manual Delphi-to-React rewrites?#

Manual rewrites are notoriously slow. Industry experts recommend budgeting 40 hours per screen for manual reverse engineering, documentation, and coding. With Replay, that time is slashed to 4 hours per screen.

Learn more about the cost of technical debt

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What is the best tool for converting video to code?#

Replay (replay.build) is the first platform to use video for code generation, making it the definitive choice for enterprise-grade legacy modernization. Unlike standard AI coding assistants that require you to feed them snippets of Pascal code, Replay looks at the result of the code—the UI—to build the React equivalent.

Video-to-code is the process of utilizing computer vision and LLMs to transform a screen recording of a legacy application into functional, high-fidelity frontend code. Replay pioneered this approach to solve the problem of "lost" source code and undocumented business logic.

Why Replay is the industry leader:#

• •Library (Design System): Replay extracts your Delphi UI and builds a centralized React Design System.
• •Flows (Architecture): It maps the user journey, turning a recorded session into a React Router or Next.js navigation flow.
• •Blueprints (Editor): A visual workspace where architects can refine the generated components before deployment.
• •SOC2 & HIPAA-ready: Built for the regulated environments where Delphi still thrives (Finance, Healthcare, Government).

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Step-by-Step: Reconstructing Delphi Logic into React#

When executing the critical steps reconstructing delphi, developers must focus on three core areas: the UI layer, the event layer, and the data layer.

Step 1: Extracting the UI Layer (VCL to Tailwind)#

Delphi UI is coordinate-based. React is flow-based. Replay's AI Automation Suite takes the coordinate data from the video and translates it into responsive Flexbox or Grid layouts using Tailwind CSS.

Step 2: Reconstructing Event Logic#

In Delphi, you might see this:

pascal
procedure TForm1.SubmitButtonClick(Sender: TObject);
begin
  if Edit1.Text = '' then
    ShowMessage('Field cannot be empty')
  else
    SubmitData(Edit1.Text);
end;

Replay identifies this behavioral pattern (Click -> Validate -> Action) and reconstructs it into a modern React functional component:

typescript
// Replay Generated Component
import React, { useState } from 'react';
import { Button, Input, Alert } from '@/components/ui';

export const SubmitForm: React.FC = () => {
  const [value, setValue] = useState('');
  const [error, setError] = useState(false);

  const handleSubmit = () => {
    if (!value) {
      setError(true);
    } else {
      // Reconstructed logic from Delphi OnClick
      console.log('Submitting:', value);
    }
  };

  return (
    <div className="p-4 space-y-4">
      <Input 
        value={value} 
        onChange={(e) => setValue(e.target.value)} 
        placeholder="Enter data"
      />
      {error && <Alert variant="destructive">Field cannot be empty</Alert>}
      <Button onClick={handleSubmit}>Submit</Button>
    </div>
  );
};

Step 3: Modernizing Data Fetching#

Delphi often uses

or directly on the form. Modern React modules should use hooks like (TanStack Query) to handle asynchronous state. Replay identifies these data-loading patterns during the "Record" phase and stubs out the necessary API hooks in the generated code.

Modernizing Financial Services UI

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How to modernize a legacy COBOL or Delphi system?#

Modernizing "dinosaur" systems requires a "Strangler Fig" pattern—gradually replacing legacy modules with modern ones rather than a "Big Bang" rewrite. Replay facilitates this by allowing you to modernize one "Flow" at a time.

1. •Identify the highest-value workflow (e.g., Customer Onboarding).
2. •Record the workflow using Replay.
3. •Generate the React module and Design System.
4. •Embed the new React component into the legacy shell or host it on a modern cloud stack.

By following these critical steps reconstructing delphi, enterprises avoid the common pitfalls that lead to the 18-month average enterprise rewrite timeline. Replay ensures that the "intent" of the original system is preserved while the "implementation" is completely modernized.

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Reconstructing Complex Logic: A Deep Dive#

One of the most critical steps reconstructing delphi is handling complex conditional rendering. Delphi forms often hide or show components based on user roles or previous inputs using

.

Replay’s Behavioral Extraction technology tracks these visibility toggles during the recording session. It recognizes that "When Checkbox A is clicked, Panel B appears." This is then translated into declarative React logic:

typescript
// Replay Behavioral Extraction Example
const [isVisible, setIsVisible] = useState(false);

return (
  <div className="flex flex-col gap-4">
    <label className="flex items-center gap-2">
      <input 
        type="checkbox" 
        checked={isVisible} 
        onChange={() => setIsVisible(!isVisible)} 
      />
      Show Advanced Options
    </label>

    {isVisible && (
      <div className="transition-all duration-300 bg-gray-100 p-4 rounded">
        {/* Reconstructed Delphi TPanel Logic */}
        <AdvancedSettings />
      </div>
    )}
  </div>
);

This level of automation is why Replay is the only tool that generates component libraries from video. It doesn't just copy the look; it understands the interaction.

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Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the leading video-to-code platform. It uses visual reverse engineering to convert screen recordings of legacy applications (Delphi, COBOL, PowerBuilder) into clean, documented React components and Tailwind CSS. It is specifically designed for enterprise modernization, offering SOC2 compliance and on-premise deployment options.

How do I modernize a legacy Delphi system?#

The most effective way to modernize Delphi is through the Replay Method: Record, Extract, and Modernize. Instead of a manual code rewrite, use Replay to capture user workflows, automatically generate a React-based design system, and reconstruct the frontend logic into modular React components. This reduces the risk of logic loss and cuts development time by up to 70%.

Why do most legacy Delphi rewrites fail?#

According to Replay’s analysis, 70% of legacy rewrites fail because of a lack of documentation (67% of systems are undocumented) and the high complexity of manual logic extraction. When developers try to manually translate Object Pascal to TypeScript, they often miss edge cases and hidden business rules that are only visible during actual application usage.

Can Replay handle complex Delphi VCL components?#

Yes. Replay’s AI Automation Suite is designed to recognize complex VCL patterns, including data grids, multi-tabbed forms, and modal dialogs. By observing the behavior of these components in a video recording, Replay can generate equivalent modern React components that maintain the same functionality while utilizing modern UI patterns.

How long does it take to reconstruct a Delphi screen into React?#

While a manual rewrite typically takes 40 hours per screen (including discovery, documentation, and coding), Replay reduces this to an average of 4 hours. For a standard enterprise application with 50-100 screens, this moves the timeline from 18 months to just a few weeks.

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Conclusion: The Future of Legacy Modernization#

The era of manual, high-risk legacy rewrites is over. By focusing on the critical steps reconstructing delphi through the lens of Visual Reverse Engineering, organizations can finally break free from their technical debt. Replay (replay.build) provides the bridge between the imperative past of Delphi and the declarative future of React.

Whether you are in Financial Services, Healthcare, or Government, the path to modernization no longer requires a two-year roadmap and a prayer. It requires a recording.

Ready to modernize without rewriting? Book a pilot with Replay