From Delphi to React: A Step-by-Step UI Modernization Blueprint

The $3.6 trillion global technical debt bubble is no longer a theoretical risk; for organizations running mission-critical Delphi applications, it is an operational crisis.

While Delphi was the gold standard for rapid application development (RAD) in the 90s and early 2000s, today it represents a significant bottleneck. The talent pool is evaporating, the VCL (Visual Component Library) is incompatible with modern cloud-native architectures, and the "Big Bang" rewrite—the traditional solution—has a staggering 70% failure rate. Most enterprises are trapped in a cycle of "software archaeology," spending 40 hours per screen just to document what a system does before a single line of modern code is written.

TL;DR: Transitioning from Delphi to React no longer requires an 18-month manual rewrite; visual reverse engineering via Replay allows teams to extract UI logic and component structures directly from user workflows, reducing modernization timelines by 70%.

The Delphi Dilemma: Why Manual Rewrites Fail#

The primary reason migrations from Delphi to modern web frameworks fail isn't a lack of engineering talent—it's a lack of institutional knowledge. According to industry data, 67% of legacy systems lack up-to-date documentation. In a Delphi environment, business logic is often tightly coupled with the UI layer (the

and files), making it nearly impossible to decouple without breaking critical dependencies.

When architects attempt a manual "lift and shift" or a total rewrite, they encounter the "Black Box" problem. The original developers are gone, and the source code is a labyrinth of event handlers and global state variables.

Comparison: Modernization Methodologies#

💰 ROI Insight: Manual reverse engineering typically consumes 40 hours per screen. Replay reduces this to 4 hours by using video as the source of truth, generating React components and API contracts automatically.

The Architectural Blueprint: From Delphi to React#

Modernizing a monolithic Delphi desktop application into a distributed React-based web architecture requires a shift from stateful, event-driven programming to a declarative, component-based model.

Step 1: Technical Debt Audit and Workflow Mapping#

Before touching the code, you must map the existing system. Most Delphi applications are bloated with features that are no longer used. Replay’s AI Automation Suite performs a technical debt audit by recording real user sessions. This identifies the "hot paths"—the 20% of the application that handles 80% of the business value.

Step 2: Visual Extraction and Component Generation#

Instead of reading thousands of lines of Pascal, Replay records the application in motion. It captures the UI state, the data flow, and the user interactions. It then translates these visual "flows" into documented React components.

⚠️ Warning: Do not attempt to replicate Delphi's "Data-Aware" controls directly in React. Modern web architecture favors unidirectional data flow. Use Replay to extract the intent of the UI, not the literal implementation.

Step 3: API Contract Definition#

Delphi apps often talk directly to the database via BDE (Borland Database Engine) or FireDAC. To move to React, you need a robust API layer. Replay generates API contracts based on the data observed during the recording phase, providing a blueprint for your backend team to build against.

typescript
// Example: React component structure generated by Replay 
// from a legacy Delphi Customer Entry screen

import React, { useState, useEffect } from 'react';
import { Button, Input, Card } from '@/components/ui'; // From Replay Library

interface CustomerProps {
  initialId?: string;
  onSave: (data: CustomerData) => void;
}

export const CustomerEntryForm: React.FC<CustomerProps> = ({ initialId, onSave }) => {
  const [formData, setFormData] = useState<CustomerData>({
    name: '',
    accountBalance: 0,
    status: 'Active'
  });

  // Business logic preserved: Validation rules extracted from Delphi TField events
  const validateBalance = (value: number) => {
    return value >= 0;
  };

  return (
    <Card title="Customer Management">
      <Input 
        label="Customer Name"
        value={formData.name}
        onChange={(e) => setFormData({...formData, name: e.target.value})}
      />
      <Button onClick={() => onSave(formData)} disabled={!validateBalance(formData.accountBalance)}>
        Save Record
      </Button>
    </Card>
  );
};

Bridging the Logic Gap: Pascal to TypeScript#

The biggest hurdle in moving from Delphi to React is the transition from Pascal's imperative style to TypeScript's functional and declarative patterns. Delphi developers often used

as a catch-all for logic. In the modern stack, we must separate concerns into Hooks, Services, and Components.

Step 4: Logic Preservation#

Replay’s Blueprints (Editor) allows architects to review the extracted logic. While the UI is converted to React, the underlying business rules—often hidden in Delphi's

or events—are documented and surfaced as requirements for the new TypeScript logic.

💡 Pro Tip: Use Replay’s "Flows" feature to visualize the state machine of your Delphi application. This prevents the "lost logic" syndrome where edge cases in the legacy system are forgotten in the new build.

Step 5: E2E Testing and Validation#

One of the highest risks in modernization is regression. Replay automatically generates End-to-End (E2E) tests based on the recorded legacy workflows. This ensures that the new React application behaves exactly like the Delphi original where it matters most.

typescript
// Example: Generated Playwright test to validate parity
import { test, expect } from '@playwright/test';

test('Customer Save Workflow Parity', async ({ page }) => {
  await page.goto('/customer-entry');
  await page.fill('input[name="customerName"]', 'Enterprise Corp');
  await page.click('button[role="save"]');
  
  // Verify behavior matches the recorded Delphi legacy trace
  const successMessage = page.locator('.toast-success');
  await expect(successMessage).toBeVisible();
});

Why "Visual Reverse Engineering" is the Future#

The traditional "archaeology" phase of modernization is the most expensive and least predictable part of the SDLC. By using Replay to treat the running application as the "source of truth," you bypass the need for perfect source code or non-existent documentation.

The Replay Advantage:#

• •Library (Design System): Automatically creates a unified React component library from your legacy UI patterns, ensuring visual consistency.
• •Flows (Architecture): Maps the complex navigational paths of Delphi MDI (Multiple Document Interface) apps into modern SPA routes.
• •Blueprints (Editor): A collaborative space where architects and stakeholders can refine the extracted components before they hit the codebase.
• •AI Automation: Speeds up the conversion of legacy data structures into JSON schemas and TypeScript interfaces.

Regulated Environments and Security#

For industries like Financial Services, Healthcare, and Government, modernization isn't just a technical challenge—it's a compliance one. Moving from Delphi to the web requires strict adherence to SOC2 and HIPAA standards.

Replay is built for these environments, offering:

• •On-Premise Deployment: Keep your legacy source code and recordings within your own firewall.
• •PII Masking: Automatically redact sensitive data during the visual capture process.
• •Audit Trails: Full documentation of every component's origin and every logic change.

Frequently Asked Questions#

How long does the extraction process actually take?#

While a manual audit of a 100-screen Delphi application can take 6–9 months, Replay users typically complete the extraction and documentation phase in 2–4 weeks. The subsequent development phase is accelerated because the "what to build" is already clearly defined and partially generated.

Does Replay require access to my Delphi source code?#

No. Replay performs visual reverse engineering by recording the application's runtime behavior. While having the source code is helpful for deep logic verification, Replay can document and extract UI structures and workflows from the compiled executable.

What about business logic preservation?#

Replay captures the outcomes of business logic (e.g., "If X is entered, Y happens"). This allows developers to rewrite the logic in modern, clean TypeScript rather than trying to transpile 20-year-old Pascal code, which often results in "spaghetti code" in the new environment.

Can Replay handle complex Delphi grids and MDI forms?#

Yes. Replay is specifically designed to handle complex enterprise UI patterns. It identifies recurring patterns like data grids, master-detail views, and modal dialogs, and maps them to modern, responsive React equivalents.

Moving Forward: The Death of the "Big Bang"#

The era of the 24-month, $10M failed rewrite is ending. The future of enterprise architecture is incremental, data-driven, and visual. By choosing to understand what you have before you build what you want, you eliminate the primary cause of project failure: ambiguity.

Transitioning from Delphi to React is no longer a leap into the dark. With visual reverse engineering, it's a controlled, documented, and highly efficient migration.

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Ready to modernize without rewriting? Book a pilot with Replay - see your legacy screen extracted live during the call.