Delphi UI Reconstruction: From Legacy Windows Apps to Web-Native React in 12 Months

Every enterprise has a "closet of shame"—a cluster of mission-critical Delphi applications that haven't been touched in a decade because the original developers retired and the documentation is non-existent. These VCL-based monoliths often hold the keys to core business logic in financial services, manufacturing, and healthcare, yet they are the primary anchors contributing to the $3.6 trillion global technical debt.

The traditional path for delphi reconstruction from legacy environments is a suicide mission. Industry data shows that 70% of legacy rewrites fail or significantly exceed their timelines. When you're dealing with 20-year-old Pascal code where business logic is inextricably linked to the UI layer, a manual "rip and replace" strategy usually results in an 18-to-24-month death march that ends in a product that doesn't even match the original's functionality.

There is a better way. By leveraging Visual Reverse Engineering, organizations can now execute a complete delphi reconstruction from legacy desktop environments to web-native React architectures in under 12 months, saving an average of 70% in development time.

TL;DR: Manual Delphi migrations fail because of undocumented logic and UI-coupled code. Replay accelerates this by using Visual Reverse Engineering to convert video recordings of legacy workflows into documented React components and Design Systems. This reduces the per-screen reconstruction time from 40 hours to just 4 hours, enabling a full enterprise transition in 12 months rather than years.

The Delphi Dilemma: Why Manual Reconstruction Fails#

Delphi was the gold standard for rapid application development (RAD) in the 90s and early 2000s. However, its greatest strength—the tight coupling of the UI (DFM files) and the logic (PAS files)—is now its greatest liability.

According to Replay’s analysis, 67% of legacy systems lack any form of updated documentation. When you attempt a delphi reconstruction from legacy codebases manually, your developers spend 80% of their time "archaeologizing"—trying to figure out what a specific button does under five different conditional states—rather than actually writing new code.

Video-to-code is the process of recording a user interacting with a legacy application and using AI-driven visual analysis to generate the corresponding front-end code, component architecture, and state transitions.

The Cost of Manual vs. Replay-Accelerated Reconstruction#

Phase 1: Visual Inventory and Design System Extraction (Months 1-3)#

The first step in a successful delphi reconstruction from legacy systems isn't writing code; it's capturing reality. In a Delphi app, the UI is often a cluttered mess of nested panels and non-standard grids.

Using Replay, architects record subject matter experts (SMEs) performing standard workflows. Replay’s "Library" feature then analyzes these recordings to identify recurring UI patterns. Instead of manually mapping every TButton and TStringGrid, Replay identifies the intent of the component.

Building the Modern Component Library#

Industry experts recommend establishing a "Single Source of Truth" before a single line of React is written. Replay converts the visual artifacts from your Delphi app into a clean, themed Tailwind or CSS-in-JS design system.

typescript
// Example: A generated React component from a legacy Delphi TGroupBox
import React from 'react';
import { Card, Text, Stack } from '@/components/ui';

interface LegacyDataPanelProps {
  title: string;
  recordCount: number;
  onRefresh: () => void;
}

export const LegacyDataPanel: React.FC<LegacyDataPanelProps> = ({ 
  title, 
  recordCount, 
  onRefresh 
}) => {
  return (
    <Card className="p-4 border-l-4 border-blue-600 shadow-sm">
      <Stack direction="row" align="center" justify="between">
        <Text variant="h3" className="font-semibold text-slate-800">
          {title}
        </Text>
        <div className="badge badge-secondary">
          {recordCount} Records
        </div>
      </Stack>
      <div className="mt-4 border-t pt-4">
        {/* Replay identified this as a dynamic data grid placeholder */}
        <button 
          onClick={onRefresh}
          className="px-4 py-2 bg-blue-600 text-white rounded hover:bg-blue-700 transition"
        >
          Synchronize Data
        </button>
      </div>
    </Card>
  );
};

Phase 2: Mapping Logic and "Flows" (Months 4-6)#

One of the hardest parts of delphi reconstruction from legacy software is capturing the "hidden" logic—the modal popups that only appear when a specific checkbox is clicked while the user is in "Admin Mode."

Replay’s "Flows" feature maps these state transitions automatically. By recording the video of the user journey, Replay creates a visual architecture map. This ensures that the React reconstruction doesn't just look like the old app, but behaves like it, respecting the complex business rules buried in the original Pascal event handlers.

For more on how this works, check out our guide on Visual Reverse Engineering.

Phase 3: The Blueprint and AI Automation Suite (Months 7-9)#

Once the components and flows are captured, the Replay Blueprints editor allows architects to refine the generated code. This is where the delphi reconstruction from legacy moves from a "UI clone" to a "modernized application."

According to Replay's analysis, the AI Automation Suite can handle the heavy lifting of boilerplate generation, such as:

1. •Form validation logic (converting Delphi's
   text

   OnExit
   events to Formik or React Hook Form).
2. •Data fetching layers (mapping legacy DBExpress calls to modern REST or GraphQL hooks).
3. •Accessibility (A11y) compliance, which legacy Delphi apps almost universally lack.

Bridging the Data Gap#

Modernizing the UI is useless if it can't talk to the legacy database. During this phase, we wrap the existing Delphi backend (or its underlying SQL database) in a modern API layer.

typescript
// Replay-generated hook for data fetching, replacing legacy TClientDataSet
import { useQuery } from '@tanstack/react-query';
import { apiClient } from '@/lib/api';

export const useInventoryData = (warehouseId: string) => {
  return useQuery({
    queryKey: ['inventory', warehouseId],
    queryFn: async () => {
      const { data } = await apiClient.get(`/legacy-proxy/inventory/${warehouseId}`);
      return data;
    },
    // Implementing stale-while-revalidate to mimic Delphi's local caching
    staleTime: 1000 * 60 * 5, 
  });
};

Phase 4: Hardening for Regulated Environments (Months 10-12)#

For industries like Financial Services and Healthcare, delphi reconstruction from legacy isn't just about code—it's about compliance. Legacy apps often run on-premise behind thick firewalls.

Replay is built for these environments, offering SOC2 compliance, HIPAA-ready workflows, and even On-Premise deployment options. During the final three months, the focus shifts to:

• •Security Auditing: Ensuring the new React frontend doesn't expose data vulnerabilities.
• •User Acceptance Testing (UAT): Comparing the Replay-generated flows against the original Delphi recordings to ensure 100% parity.
• •Performance Optimization: Moving from the heavy footprint of a Windows executable to a lightweight, lazy-loaded React SPA.

To understand the broader implications of these shifts, read our deep dive into Legacy UI Modernization.

Overcoming the "Document Gap"#

The biggest hurdle in any delphi reconstruction from legacy project is the lack of institutional knowledge. The developers who wrote the original VCL components in 2004 are gone. The "Video-to-code" approach bypasses the need for source code analysis in the initial stages. By focusing on the observable behavior of the application, Replay creates a living documentation of the system as it exists today, not as it was intended to be two decades ago.

Why 12 Months is the Sweet Spot#

Many enterprise leaders are skeptical of a 12-month timeline for a complete delphi reconstruction from legacy systems. However, when you remove the 40-hour-per-screen manual development bottleneck, the math changes.

If an application has 100 core screens:

• •Manual: 100 screens x 40 hours = 4,000 hours (roughly 2 years for a small team).
• •Replay-Accelerated: 100 screens x 4 hours = 400 hours (roughly 3 months for the same team).

This leaves 9 months for complex API integration, security hardening, and organizational change management—the things that actually matter for a successful launch.

Frequently Asked Questions#

Can Replay handle custom third-party VCL components?#

Yes. Unlike static code analyzers that struggle with proprietary or third-party Delphi libraries (like DevExpress or TMS), Replay uses visual analysis. If the component renders on the screen and a user can interact with it, Replay can capture its behavior and reconstruct it in React, regardless of the underlying legacy technology.

Do we need the original Delphi source code to use Replay?#

No. While having the source code is helpful for backend logic migration, Replay’s visual reverse engineering works by observing the running application. This makes it ideal for delphi reconstruction from legacy projects where the source code is lost, corrupted, or too messy to be useful.

Is the generated React code maintainable?#

Absolutely. Replay doesn't produce "spaghetti code" or machine-readable obfuscation. It produces clean, documented TypeScript and React components based on your specific Design System. The goal is to provide a foundation that your modern web team can own and extend immediately.

How does Replay ensure the new app matches the old one's logic?#

By using "Flows," Replay captures the sequence of events. If clicking "Calculate" in the Delphi app triggers a specific series of modal updates and data changes, those are documented as requirements in the Replay Blueprint. This ensures that the delphi reconstruction from legacy isn't just a visual facelift, but a functional replication.

Can we deploy the modernized app on-premise?#

Yes. We understand that many Delphi applications exist in highly regulated industries. Replay offers flexible deployment models, including on-premise installations, to ensure that your sensitive data and proprietary workflows never leave your secure environment.

Ready to modernize without rewriting? Book a pilot with Replay