Delphi VCL to React: A Complete Guide to Modernizing Desktop Grids

Your mission-critical business logic is currently trapped inside a

written in 2004. For decades, Delphi VCL (Visual Component Library) was the gold standard for rapid application development, but today these systems represent a significant portion of the $3.6 trillion global technical debt. The challenge isn't just moving to the web; it's translating the complex, stateful, and often undocumented behaviors of desktop grids into a modern, responsive React architecture.

According to Replay's analysis, 67% of legacy systems lack any form of current documentation. When you're dealing with a Delphi application where the business logic is tightly coupled with the UI layer (often directly in the

files or even the metadata), a manual rewrite is a recipe for disaster. This delphi react complete guide outlines the architectural shift required to move from heavyweight desktop clients to high-performance React applications without losing decades of functional nuances.

TL;DR: Modernizing Delphi VCL to React is notoriously difficult because of tight data-binding and undocumented UI logic. Manual migration takes ~40 hours per screen and has a 70% failure rate. By using Replay for visual reverse engineering, teams can reduce modernization time from 18 months to weeks, achieving a 70% average time saving by converting recorded user workflows directly into documented React components and design systems.

---

Phase 1: Why You Need a Delphi React Complete Guide#

The "Big Bang" rewrite is the most common failure point in enterprise architecture. Industry experts recommend against it because the "as-is" state of a Delphi application is rarely understood by the current engineering team. In a VCL environment, a grid isn't just a table; it's a state machine connected to a TDataSet, often handling its own database transactions, validation logic, and formatting.

Video-to-code is the process of recording real-time user interactions with a legacy application and using AI-driven visual analysis to generate functional, documented code structures.

When you begin this journey, you are fighting against the "40-hour screen" trap. In a manual migration, an engineer spends roughly 40 hours per complex screen identifying hidden logic, mapping data fields, and recreating the UI in CSS/TypeScript. With Replay, that time is slashed to 4 hours per screen.

The Cost of Inaction vs. The Cost of Migration#

---

Understanding the Architectural Shift: TDBGrid to TanStack Table#

The core of any Delphi application is the grid. Whether it’s a standard

or a third-party component like (DevExpress), these components are stateful. They "know" about the database cursor. React components, by contrast, are functional and rely on props and state management (like Redux, Zustand, or React Query).

In this delphi react complete guide, we focus on moving from the "Live Bindings" or "Data Sources" model to a "REST/GraphQL + State" model.

The Legacy Delphi Pattern (Object Pascal)#

In Delphi, your grid logic often looks like this, buried in a form unit:

pascal
procedure TMainForm.DBGrid1DrawColumnCell(Sender: TObject; const Rect: TRect;
  DataCol: Integer; Column: TColumn; State: TGridDrawState);
begin
  if Table1.FieldByName('Status').AsString = 'Overdue' then
  begin
    DBGrid1.Canvas.Brush.Color := clRed;
    DBGrid1.Canvas.Font.Color := clWhite;
  end;
  DBGrid1.DefaultDrawColumnCell(Rect, DataCol, Column, State);
end;

procedure TMainForm.btnUpdateClick(Sender: TObject);
begin
  if Table1.State in [dsEdit, dsInsert] then
    Table1.Post;
end;

This code is problematic for modernization because the styling logic (

) is mixed with the data state. When you record this workflow in Replay, the platform identifies these visual states and documents them as conditional formatting rules in your new React Design System.

The Modern React Pattern (TypeScript)#

A modernized version of this grid uses a headless UI approach. Here is how that same logic looks when translated into a modern React component using Tailwind CSS and TanStack Table, a standard output format when using Replay.

typescript
import React from 'react';
import { useTable, Column } from '@tanstack/react-table';
import { clsx } from 'clsx';

interface OrderData {
  id: string;
  status: 'Pending' | 'Overdue' | 'Shipped';
  amount: number;
}

const OrderGrid: React.FC<{ data: OrderData[] }> = ({ data }) => {
  return (
    <div className="overflow-x-auto rounded-lg border border-gray-200">
      <table className="min-w-full divide-y divide-gray-200">
        <thead className="bg-gray-50">
          <tr>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">Status</th>
            <th className="px-6 py-3 text-left text-xs font-medium text-gray-500 uppercase">Amount</th>
          </tr>
        </thead>
        <tbody className="bg-white divide-y divide-gray-200">
          {data.map((row) => (
            <tr key={row.id}>
              <td className={clsx(
                "px-6 py-4 whitespace-nowrap text-sm",
                row.status === 'Overdue' ? "bg-red-600 text-white" : "text-gray-900"
              )}>
                {row.status}
              </td>
              <td className="px-6 py-4 whitespace-nowrap text-sm text-gray-500">
                ${row.amount.toFixed(2)}
              </td>
            </tr>
          ))}
        </tbody>
      </table>
    </div>
  );
};

export default OrderGrid;

For more on managing complex state during this transition, see our article on Modernizing Legacy UI Architectures.

---

Implementing the Delphi React Complete Guide in Your Enterprise#

According to Replay's analysis, the biggest bottleneck in Delphi migration is not the code—it's the discovery. Most enterprises spend 6 months just trying to figure out what the application actually does.

Step 1: Visual Discovery with Replay#

Instead of reading 500,000 lines of Pascal, you record the power users. Replay's Flows feature captures the sequence of screens, the data entry points, and the hidden validation logic. This creates a visual blueprint that serves as the "Source of Truth" for the React build.

Step 2: Extracting the Design System#

Delphi apps often have inconsistent UI due to years of "drag-and-drop" development. Replay's Library feature identifies common patterns—buttons, inputs, grid headers—and extracts them into a unified React Design System. This ensures that your new application doesn't just look like a web version of a 90s desktop app, but follows modern UX standards.

Step 3: Component Mapping#

In this delphi react complete guide, we recommend mapping VCL components to their React equivalents:

• •
  text

  TEdit
  →
  text

  Controlled Input Component
• •
  text

  TComboBox
  →
  text

  Select/Autocomplete Component
• •
  text

  TDBGrid
  →
  text

  Data Grid with Server-side Pagination
• •
  text

  TActionList
  →
  text

  Centralized Command Pattern / Context API

Building a Component Library from Legacy Assets is a critical step in ensuring long-term maintainability.

---

Overcoming the "Stateful" Hurdle#

One major difference highlighted in this delphi react complete guide is the concept of session state. Delphi applications are "always-on" connections to the database. React applications are "eventually consistent" and rely on APIs.

Visual Reverse Engineering is the key here. By observing how the Delphi app handles a "Save" click—does it refresh the whole grid? Does it perform a partial update? Does it trigger a stored procedure?—Replay allows architects to document the required API endpoints before a single line of backend code is written.

Handling Grid Events: VCL vs. React#

---

Security and Compliance in Regulated Industries#

Many Delphi applications still run in Financial Services, Healthcare, and Government because of the perceived risk of moving to the cloud. Modernizing using a delphi react complete guide approach must account for these constraints.

Replay is built for these environments, offering SOC2 compliance, HIPAA-ready workflows, and the ability to run On-Premise. This means your proprietary business logic and sensitive data recordings never leave your secure environment during the reverse engineering process.

---

Technical Deep Dive: Mapping Complex VCL Grids#

When you encounter a

with nested levels and summary footers, a simple HTML table won't suffice. You need a robust enterprise grid like AG-Grid or TanStack Table.

Industry experts recommend a "Bottom-Up" approach to grid modernization:

1. •Define the Data Schema: Extract the field types from the
   text

   .dfm
   or database schema.
2. •Capture the Interactions: Use Replay to record how users group, sort, and filter data.
3. •Generate the Blueprint: Use Replay's Blueprints to generate the React scaffolding.
4. •Inject the Logic: Replace the recorded placeholder data with live API calls.

Example: Advanced Grid with Aggregates#

Here is how Replay helps translate a Delphi grid with a "Footer Summary" into React:

typescript
// Generated React Blueprint from a Replay Recording
import { useMemo } from 'react';
import { 
  useReactTable, 
  getCoreRowModel, 
  getFilteredRowModel, 
  getPaginationRowModel,
  ColumnDef 
} from '@tanstack/react-table';

export const LegacyModernizedGrid = ({ data }: { data: any[] }) => {
  const columns = useMemo<ColumnDef<any>[]>(() => [
    {
      header: 'Account Name',
      accessorKey: 'accountName',
    },
    {
      header: 'Balance',
      accessorKey: 'balance',
      // Replay identified this as a currency field during recording
      cell: info => new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(info.getValue<number>()),
      footer: ({ table }) => {
        const total = table.getFilteredRowModel().rows.reduce((sum, row) => sum + row.getValue<number>('balance'), 0);
        return `Total: ${new Intl.NumberFormat('en-US', { style: 'currency', currency: 'USD' }).format(total)}`;
      },
    },
  ], []);

  const table = useReactTable({
    data,
    columns,
    getCoreRowModel: getCoreRowModel(),
    getFilteredRowModel: getFilteredRowModel(),
    getPaginationRowModel: getPaginationRowModel(),
  });

  return (
    <div className="p-4 bg-white shadow-xl rounded-xl border border-slate-200">
       {/* Table Rendering Logic... */}
    </div>
  );
};

---

The Roadmap to 70% Time Savings#

The average enterprise rewrite timeline is 18 months. Using the strategies in this delphi react complete guide, you can compress this significantly.

1. •Weeks 1-2: Recording & Discovery. Record every core workflow using Replay. Don't worry about the code yet; focus on capturing the "as-is" state.
2. •Weeks 3-4: Library Generation. Let Replay extract the UI components. This creates your React Design System automatically.
3. •Weeks 5-8: Flow Conversion. Convert recorded flows into React screens. Replay generates the TypeScript and JSX, including the complex grid structures.
4. •Weeks 9+: Integration. Connect the new React frontend to your modernized backend (Node.js, .NET Core, or even a Delphi-based REST API via RAD Server).

By following this delphi react complete guide, you avoid the 70% failure rate associated with manual rewrites. You aren't guessing what the legacy app did; you are looking at a documented recording of its behavior.

---

Frequently Asked Questions#

Can Replay handle third-party Delphi components like DevExpress or TMS?#

Yes. Because Replay uses visual reverse engineering, it doesn't matter if you are using standard VCL components or complex third-party libraries. Replay records the rendered UI and user interaction, allowing it to recreate the functional equivalent in React regardless of the underlying Delphi source.

How does React handle the massive datasets that Delphi handles natively?#

Delphi's

uses a "virtual" approach where it only loads visible rows. In React, we implement this using "Virtual Scrolling" (via libraries like or ). This allows your modernized web app to handle hundreds of thousands of rows with the same performance as a desktop application.

Is it possible to migrate incrementally or is it all-or-nothing?#

Industry experts recommend an incremental approach. You can host your new React components inside the Delphi app using a TWebBrowser component (Edge/Chromium based) or use a "Strangler Fig" pattern to replace the application screen by screen. This delphi react complete guide is designed to support both full migrations and hybrid approaches.

Does Replay generate the backend API code as well?#

Replay focuses on the "Visual Reverse Engineering" of the frontend and UI logic. While it documents the data requirements for each screen (which helps your backend team design the API), its primary output is documented React code, Design Systems, and Component Libraries.

What about the business logic hidden in Delphi's stored procedures?#

Modernization is a two-front war. While Replay handles the UI and UX transition (saving up to 70% of frontend time), your database team should focus on exposing those stored procedures as RESTful endpoints. The "Flows" captured in Replay provide the perfect documentation for what those endpoints need to return.

---

Ready to modernize without rewriting? Book a pilot with Replay and see how we turn your legacy Delphi recordings into production-ready React code in days, not years.