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Visual Component Library
Visual Component Library
Visual Component Library
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Visual Component Library (VCL)
Original authorBorland
DeveloperEmbarcadero Technologies
Initial release1995; 30 years ago (1995)
Operating systemCross-platform (Microsoft Windows, macOS, iOS, Android)
TypeGUI
LicenseCommercial
Websitewww.embarcadero.com/kr/products/rad-studio
docwiki.embarcadero.com/RADStudio/XE6/en/VCL_Overview

The Visual Component Library (VCL) is a visual component-based object-oriented framework for developing the user interface of Microsoft Windows applications. It is written in Object Pascal.

History

[edit]

The VCL was developed by Borland for use in, and is tightly integrated with, its Delphi and C++Builder RAD tools.

In 1995 Borland released Delphi, its first release of an Object Pascal IDE and language. Up until that point, Borland's Turbo Pascal for DOS and Windows was largely a procedural language, with minimal object-oriented features, and building UI frameworks with the language required using frameworks like Turbo Vision and Object Windows Library. OWL, a similar framework to MFC, required writing code to create UI objects.

A key aim of the VCL combined with the Delphi language was to change the requirements of building a user interface. (For context, the Delphi variant of Pascal had a number of innovative object-oriented features, such as properties and runtime type information, inspired by Modula and Smalltalk.) At the time, much UI code work required creating classes inheriting from other classes, and customized objects were often not reusable (for example, a button that performs a specific action cannot be reused in a different application.)[1] UI code was also complicated, forcing the programmer to understand and use the Windows API, manage GDI resources, etc.[2] Finally, a visual user interface arguably should be designed visually, and yet most tools to do so - at the time, mainly Visual Basic - did so in terms of the designer outputting code, creating a fragile, un-manually-editable situation - a problem that still persists today with many UI frameworks, particularly C++-based ones such as Qt.[3]

The combination of the Delphi language and the VCL framework written in that language addressed these by:

  • A streaming framework, allowing an object and subobjects to be streamed to text or binary format - TComponent, the root class of the VCL framework
  • A form designer that saved to a stream, saving the description of the objects not the code required to create them, with objects capable of self-creation when streaming the description back in
  • Language features that allowed instances of a class to be customised without subclassing, in two ways: first, through the use of properties, allowing instances of an object to have its own fields (such as a caption) easily changed through streaming; second, by allowing events - method pointers called at specific places in code - to be handled by another object; that is, the method pointers were attached to an object instance.[4] In other words, this creates "custom behaviours through delegation instead of inheritance".[5]
  • Controls that wrapped native Windows controls, in framework that managed resources[6]

The combination of these resulted in a visual designer that streamed a description; a form (window) that constructed itself from that description, including creating all the other components on that form; and code being written in one place (methods on the form) that could be called by UI elements in response to certain actions, such as a button being clicked calling an OnClick event handler implemented in the form code. At the time this was revolutionary, while today it is an approach used by several other frameworks, such as WinForms and Cocoa.

This also allowed new components - new visual or non-visual classes - to be written easily. The VCL is Windows-based and its implementations of common controls are wrappers of the Windows API, thus is close to the ground and fully native. New controls can be implemented from scratch or can use existing Windows controls.

With the release of C++ Builder, the Delphi compiler could emit C++ header files for compiled Delphi-language units, and so the flexibility of the streaming, visually designed UI framework became available for C++. VCL components can in fact be written in C++, but ultimately inherit from Delphi ancestors, since the C++Builder compiler and linker can consume object and header files produced by Delphi. This was one of the first examples of cross-language compatibility, something unseen until .Net.

Today the VCL includes several hundred visual and non-visual components, usable in both the Delphi and C++ languages.

Technology

[edit]

VCL forms a class hierarchy with a common ancestor, the TComponent class (which inherits from TObject, the root class in Delphi Object Pascal). This is a common approach shared by the Java programming language, Smalltalk, C# and many other object-oriented programming languages.

VCL components cover Windows controls like windows (TForm class), controls (e.g., TButton, TCheckBox, TLabel classes) as well as database access (e.g., ADO or IBX components) or Internet connections (Indy components). The component approach allows programmers to extend the VCL with many visual and non-visual additional components. There is a large number of free and commercial component packages. Among them are JEDI, TMS, Developer Express, Mitov Software, Raize Software, TurboPower, IOComp, SDL, DA-SOFT Technologies and many others.

While not all VCL components are thread-safe, VCL supports multi-threading too. One example is the built in multi-threading support of the OpenWire VCL library.

VCL provides native support for PME (properties, methods and events) model at the .NET level.[citation needed]

Much of the .NET design, especially WinForms, is modeled after the VCL. One of the main architects of the first Delphi versions, Anders Hejlsberg, was hired by Microsoft and became one of the main architects of .NET. This became the subject of a lawsuit: at the time (1996), "Borland alleged that Microsoft had hired 34 Borland employees over the past 30 months in order to steal Borland trade secrets. Borland also claimed that Microsoft offered and delivered expensive lures to Borland workers. In two cases, incentives topping $1 million were involved."[7] According to Zarko Gajic, the bonus was three million dollars in Anders' case.[7] Many Delphi developers find C# quite familiar, due to the design similarities.[8]

[edit]

A cross-platform equivalent of the VCL, called CLX (Component Library for Cross Platform), was later developed for use in Delphi, C++Builder and Kylix in 2000–2001. However, it was abandoned.

A second cross-platform framework, FireMonkey, was integrated into Delphi and C++Builder XE2 in 2011. FireMonkey is a vector-based WPF-like framework for UIs on Windows, OSX, iOS and Android.

The Lazarus project has a portable (*nix, OS/X, Win32/64+wince) equivalent called LCL, which was already working when Kylix and CLX emerged. The project maintained its course and ignored Kylix (and later FMX).

OpenOffice.org and thus LibreOffice contain an unrelated graphic library called Visual Class Library (VCL).[9][10]

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Visual Component Library

View on Grokipedia
from Grokipedia
The Visual Component Library (VCL) is a comprehensive framework of visual and nonvisual components designed for rapid development of native Windows desktop applications, primarily within Embarcadero's RAD Studio IDE using Object Pascal (Delphi) or C++ (C++Builder). It encapsulates Windows API functionalities into reusable, object-oriented controls that enable developers to build user interfaces through drag-and-drop design tools, while supporting advanced features like data binding and multi-tier architectures. Originally introduced by in February 1995 as a core element of 1, the VCL revolutionized Windows application development by providing visual two-way tools, a (RTL), and data-aware components that simplified database integration without requiring manual code for event handling or property synchronization. This library succeeded Borland's earlier Object Windows Library (OWL), which was deprecated in 1997 in favor of the more versatile VCL to support broader (RAD) paradigms. Over its nearly three-decade evolution, the VCL has been maintained and enhanced by (following their 2008 acquisition of Borland's CodeGear division), with ongoing updates in RAD Studio versions to ensure compatibility with modern Windows platforms, including Windows 11. Key strengths of the VCL include its extensive palette of over 100 standard and extended controls—such as buttons, edit boxes, menus, grids, and ribbons—that extend native Windows UI elements with additional properties, events, and behaviors for richer interactivity. It incorporates specialized frameworks like DBExpress for database connectivity, dbGo for ADO-based data access, Indy for TCP/IP networking, and DataSnap for building scalable multi-tier applications, allowing seamless integration of databases, web services, and COM/OLE components. The library's action-based architecture centralizes UI logic, reducing code duplication, while its nonvisual components handle tasks like threading, encryption, and XML processing in the background. Despite the rise of cross-platform alternatives like FireMonkey (FMX), the VCL remains the preferred choice for high-performance, Windows-specific applications due to its deep optimization for the operating system's native rendering and theming.

Overview

Definition and Purpose

The Visual Component Library (VCL) is a visual component-based object-oriented framework developed by and now maintained by , designed for creating user interfaces in Microsoft Windows applications primarily using the Delphi programming language or . It provides a comprehensive set of visual and nonvisual components that encapsulate complex functionality, allowing developers to build graphical user interfaces without directly managing low-level system calls. The primary purpose of VCL is to enable (RAD) by supporting drag-and-drop placement of components within an , which abstracts the intricacies of the while ensuring a native Windows for applications. This approach promotes productivity by focusing on high-level design and logic rather than for UI elements. VCL's scope is centered on desktop Windows applications, with full support for both 32-bit and 64-bit architectures, and it emphasizes an model where user interactions trigger predefined events handled by custom code. Introduced as a core element of Delphi 1.0 in February 1995, VCL was created to simplify GUI development in contrast to the verbose and error-prone process of coding directly against the Win32 API.

Key Features

The Visual Component Library (VCL) facilitates of user interfaces through its integrated visual designer within the RAD Studio IDE, enabling developers to drag-and-drop components onto forms, preview layouts in real-time, and edit properties interactively without writing code. This approach streamlines the development process by allowing immediate visual feedback and iterative design adjustments, significantly reducing the time required to build and refine Windows applications compared to manual coding. VCL offers an extensive class library comprising over 100 built-in components, covering essential UI elements such as forms, dialogs, data-aware controls for database integration, and multimedia handlers like image viewers and media players. These components encapsulate complex functionality, providing ready-to-use solutions for common tasks and promoting code reusability across projects. Additionally, VCL ensures native integration with Windows, automatically managing themes for consistent appearance, DPI scaling for high-resolution displays, and accessibility features such as high-contrast modes in line with Windows accessibility guidelines. A core strength of VCL lies in its streaming system, which serializes component states—including , events, and hierarchies—into human-readable, text-based DFM ( Form) files for persistent storage during design time and runtime loading. This mechanism supports seamless and migration of UI designs without proprietary binary formats. Furthermore, VCL's object-oriented supports and encapsulation, permitting developers to derive custom components from base classes like TControl, which provides foundational such as positioning and , thereby enabling modular extensions while adhering to principles of reusability and .

History

Origins and Development

The Visual Component Library (VCL) was created by Borland International in the early as an integral part of the Delphi project, which aimed to extend the capabilities of for on Windows. The project was led by , who had previously designed , Borland's groundbreaking compiler first released in 1983. Development of the VCL began with an initial draft document dated May 24, 1993, outlining a framework of reusable components to simplify Windows application building. The primary motivation behind the VCL was to offer Pascal developers a high-level over the low-level , reducing the complexity of manual message handling, , and graphical interface coding. Inspired by the rapid adoption of Microsoft's , which popularized visual programming for Windows but lacked strong typing and full object-oriented support, sought to create a more robust alternative tailored to Object Pascal's strengths in and . The VCL's design emphasized components with properties for state management, methods for actions, and events for user-defined responses, enabling developers to build applications without deep knowledge of Windows internals or advanced OOP concepts. VCL was first released in February 1995 as part of 1.0, targeting 16-bit and environments, and included over 75 pre-built visual and non-visual components for forms, controls, graphics, and database access. Early development faced challenges in adapting —a language originally suited for procedural and console-based programming—for visual, event-driven paradigms, including the creation of the Object Repository to facilitate the reuse and sharing of custom components across projects. This repository allowed developers to store and instantiate forms, dialogs, and components visually, streamlining in the IDE. In 2008, acquired Borland's CodeGear division, which encompassed and the VCL, for approximately $23 million, enabling continued of the library within the RAD Studio suite.

Evolution and Major Versions

The Visual Component Library (VCL) has evolved significantly since its , adapting to advancements in Windows operating systems and developer needs through successive releases. Early enhancements focused on broadening compatibility and introducing foundational UI features, while later versions emphasized modern aesthetics, performance, and cross-platform integration. In 2.0, released in by , VCL gained support for both 16-bit and 32-bit architectures, enabling applications to target and NT more effectively. This version also introduced multiple document interface (MDI) forms for complex application layouts and for inter-application communication, marking a shift toward more robust visual development. Delphi 5, launched in 1999, expanded VCL's capabilities with dedicated Internet components for web integration and improved data-aware controls that streamlined database-driven UIs. These additions facilitated easier connectivity to emerging web technologies while enhancing the responsiveness of form-based interactions. By Delphi 2006, under Borland's Developer Studio, VCL integrated with .NET through Delphi for .NET, allowing developers to build hybrid Win32 and managed applications with shared component logic. This release also incorporated layout enhancements like alignment and anchoring improvements, though full language features such as generics and anonymous methods arrived in subsequent versions like Delphi 2009. The XE series, starting in 2010 under (following the 2007 transition from to CodeGear and acquisition by Embarcadero in 2008), introduced VCL styles in XE2 for native theming, enabling customizable visual appearances without custom drawing. Later entries like XE3 added for touch-enabled devices and initial high-DPI awareness to ensure crisp rendering on high-resolution displays. In iterations such as RAD Studio 11 Alexandria (2021), VCL received targeted updates for compatibility, including support for rounded corners, new system styles, and Microsoft Edge browser integration via TEdgeBrowser. These enhancements, combined with improved high-DPI handling and design-time style previews, allow VCL applications to align seamlessly with modern Windows UI paradigms. The evolution continued with RAD Studio 12 Athens (2023) and its updates through 12.3 (March 2025), which added further VCL improvements such as enhanced style support, better integration, and performance optimizations for high-DPI and modern theming. Additionally, open-source efforts like the Visual Component Library (JVCL) project have contributed hundreds of extensions to VCL since the early 2000s, fostering community-driven innovations in component functionality.

Architecture

Object-Oriented Design

The Visual Component Library (VCL) employs principles to provide a structured framework for building Windows applications, with TComponent serving as the foundational root class for all components. Derived from TPersistent and ultimately TObject, TComponent encapsulates essential behaviors such as design-time integration with the IDE, persistence through streaming, and support for COM interfaces on Windows. This base class introduces key elements including published properties for runtime and design-time access, methods for operational logic, and events for handling notifications, enabling a unified model across visual and non-visual components. Polymorphism in VCL is facilitated through virtual methods, allowing derived classes to override base implementations for customized behavior while maintaining consistent interfaces. For instance, the Create constructor and Destroy destructor are declared as virtual, permitting subclasses to extend initialization and cleanup processes without altering the calling code. This design promotes extensibility, as developers can create specialized components by inheriting from TComponent or its descendants and redefining these methods to suit specific needs. Encapsulation is achieved primarily through properties, which abstract internal data representation and provide controlled access via getter and setter methods, thereby hiding implementation details from users. A representative example is the Caption property in controls like TButton, where changes to the displayed text are managed internally without exposing the underlying storage mechanism. This approach ensures data integrity and allows for future modifications to the internal logic without impacting dependent code. VCL incorporates established to enhance modularity and reusability. The observer pattern is implemented via the event system, where components act as subjects that notify registered observers through event handlers; for example, the OnClick event in TControl allows arbitrary procedures to respond to user interactions in a manner. Similarly, the governs container controls, enabling hierarchical composition where parent components like TPanel aggregate and manage child controls as a unified entity, facilitating complex UI structures through recursive nesting. Memory management in VCL relies on an ownership hierarchy rooted in TComponent, where each component can own others, automatically handling their destruction to prevent memory leaks. When a component is instantiated with an owner—typically passed via the Create constructor—the owner assumes responsibility for freeing the owned component and its subtree upon its own destruction, establishing a tree-like structure that mirrors the application's component relationships. This mechanism, confined to TComponent descendants, ensures efficient resource cleanup without manual intervention, though developers must explicitly manage non-component objects.

Component Hierarchy and Model

The Visual Component Library (VCL) organizes its components through a structured hierarchy that facilitates reusability and extensibility in applications. At the foundation lies the TObject class, which provides basic object-oriented capabilities such as construction, destruction, and dynamic typing. Descending from TObject is TPersistent, which introduces mechanisms for streaming object states and property assignment, enabling persistence of component data. The TComponent class extends TPersistent to form the base for all VCL components, adding support for design-time editing, naming, and ownership hierarchies that integrate seamlessly with the IDE. Visual components further inherit from TControl, a descendant of TComponent that imparts runtime visibility, positioning, and basic event handling for user interactions like mouse movements. For controls requiring window handles and child containment, such as buttons or edit fields, the hierarchy progresses to TWinControl, which builds on TControl by incorporating Windows API integration for focus management and message processing. Specialized classes like TButton (inheriting from TWinControl via intermediaries) or TEdit exemplify this tree, inheriting shared behaviors while adding domain-specific functionality, such as caption rendering or text input validation. This layered inheritance ensures that common traits, like property persistence, propagate efficiently across the library. The VCL component model revolves around a triad of properties, methods, and events, which collectively define interaction patterns and enable . Properties encapsulate data access, often with getters and setters for validation; methods perform operations like rendering or data manipulation; and events allow external code to respond to state changes, such as user clicks. In class declarations, the published section exposes these elements to the IDE's Object Inspector and form designer, making them editable at design time without requiring code inspection. This model promotes encapsulation while supporting , as developers can configure behaviors visually before runtime. Central to the model is the streaming mechanism, which serializes component states for storage and restoration, primarily through files in text or binary formats. Derived from TPersistent, streaming reads and writes published properties to streams, allowing forms to persist layouts and configurations across sessions; for instance, a form's DFM file captures positions and values of child components. To integrate custom components into the IDE palette, developers invoke RegisterComponents in an initialization section, associating class types with visual categories for drag-and-drop usage. This process ensures seamless design-time persistence without manual intervention. Ownership relationships, managed via the Owner property in TComponent, establish parent-child links that govern component lifecycles and . An owning component, typically a form or container, automatically destroys its owned children upon its own destruction, preventing memory leaks; it also handles streaming of owned components' properties during save and load operations. This model decouples (via Parent in controls) from lifetime management, allowing non-visual components like timers to be owned without spatial positioning. By default, forms own all dropped components, streamlining application deployment. For extensibility, TCustomControl serves as the primary base class for creating owner-drawn visual components, inheriting from TWinControl to provide a blank for custom painting via the Paint event and Windows GDI calls. Unlike standard controls with predefined appearances, TCustomControl requires manual implementation of drawing logic, enabling bespoke visuals like custom graphs or widgets while retaining VCL's event and property infrastructure. Developers typically derive public-facing classes from TCustomControl to add published properties, ensuring IDE compatibility and of windowed behaviors.

Core Components

Visual Controls

Visual controls in the Visual Component Library (VCL) form the foundation for creating graphical user interfaces in and applications, providing reusable components that handle rendering, user interaction, and layout on Windows platforms. These controls inherit from the base class TControl, which defines essential properties such as Align for positioning within a parent container, Font for text appearance, and Color for background or foreground customization. By encapsulating elements, VCL visual controls enable developers to build responsive interfaces without low-level programming. Standard controls offer basic interaction elements essential for most applications. The TButton control represents a for user actions like confirming inputs, supporting properties such as Align to dock it to form edges, Font to style its caption, and Color for visual distinction, though caption color changes require variants like TBitBtn. TEdit provides single-line text input, wrapping the Windows edit control and inheriting Align, Font, and Color for seamless integration into forms. For item selection, TListBox displays a scrollable list of strings, allowing multi-select modes and customization via the same core properties to match application themes. Menus are handled by TMainMenu, which creates a with drop-down items for navigation, also leveraging Align, Font, and Color for consistent appearance. Container controls facilitate layout management by grouping and organizing other components. TForm serves as the primary window container, holding child controls and supporting properties like Align for full-screen or docked layouts, while enabling focus navigation among its contents. TPanel acts as a sub-container for grouping related controls, such as buttons or labels, with inherited properties for alignment and styling, and methods like FlipChildren for bidirectional text support. To handle content overflow, TScrollBox creates a scrollable region within a form or panel, preventing unnecessary window scrolling and allowing nested controls like buttons to be viewed via horizontal or vertical scrollbars. Advanced visual controls extend capabilities for and dynamic updates. TPaintBox offers a for custom drawing, where developers handle the OnPaint event to render shapes or images manually, inheriting standard properties for integration. TImage displays static such as bitmaps, icons, or JPEGs via its Picture property, supporting stretch or center modes within bounded areas. VCL supports theming through styles, introduced in Delphi XE2, which allow custom skins to alter the appearance of controls' parts and states, such as button hover effects or scrollbar colors. These styles, stored in .vsf files, can be switched at runtime using TStyleManager for dynamic theming without recompilation. Responsiveness is inherent in VCL visual controls via built-in event handling for mouse actions (e.g., OnClick, OnMouseMove) and keyboard inputs (e.g., OnKeyDown), with TControl managing focus through methods like SetFocus to ensure accessible navigation. This framework supports non-visual components for supplementary event processing, enhancing overall interface interactivity.

Non-Visual Components

Non-visual components in the Visual Component Library (VCL) provide essential functionality for application logic, , and system interactions without rendering visible elements at runtime. These components, of TComponent, are designed for tasks such as database connectivity, network communication, and timed operations, enabling developers to build robust backend processes in and applications. Placed on forms or data modules at design time, they appear as icons in the IDE but operate invisibly during execution, integrating seamlessly with visual controls through events and properties. For data access, VCL includes the core TDataSource component, which connects datasets to visual controls for data-aware applications. Database connectivity is facilitated by frameworks such as dbExpress (e.g., TSQLQuery for SQL execution and parameterized queries) or the recommended FireDAC (e.g., TFDQuery for versatile data manipulation across multiple databases). These abstract low-level interactions, promoting rapid development with support for ODBC, native drivers, and relational databases. Networking capabilities are supported by components such as TClientSocket and TServerSocket for basic TCP/IP socket programming, alongside integration with the Indy library for advanced protocols. TClientSocket manages client-side connections, handling asynchronous read and write operations via events like OnRead and OnConnect, suitable for simple client-server architectures. TServerSocket enables server-side listening on specified ports, processing incoming connections through an OnClientConnect event. The Indy library, bundled with Delphi, extends this with components like TIdTCPClient for client-side TCP interactions and TIdTCPServer for robust server implementations, supporting protocols including HTTP, FTP, and SMTP with built-in threading for concurrent operations. System utilities encompass components for event timing, file I/O, and . TTimer generates periodic events based on an Interval property, invoking an OnTimer event handler without blocking the main thread, ideal for animations, polling, or scheduled tasks in Windows applications. relies on the RTL's Random function, often wrapped in utility classes, but VCL does not provide a dedicated TRandom component; developers typically use .SysUtils.Random for seeding and generating pseudo-random values in algorithms or simulations. Validation and formatting utilities include non-visual aspects integrated into broader components, though core masking is handled visually; for example, input validation logic can be implemented via event handlers on associated controls. File integration is abstracted by TOpenDialog and TSaveDialog, which invoke standard Windows common dialogs for user file selection without persistent UI presence. TOpenDialog prompts for file opening, filtering by extensions via the Filter property and returning the selected path through the FileName property upon successful Execute call. Similarly, TSaveDialog handles save operations, with options for overwrite confirmation and default extension handling, streamlining file I/O in applications. These components ensure cross-version compatibility with Windows APIs while allowing customization through properties like Options for behaviors such as ofPathMustExist.

Development Practices

Integration with RAD Studio IDE

The Visual Component Library (VCL) is deeply integrated into the RAD Studio (IDE), enabling through visual design tools for both and . The Form Designer serves as the primary interface for building user interfaces, where developers can visual components, such as buttons and labels, from the Tool Palette onto a form. The Tool Palette, located in the lower-right section of the IDE, organizes components into categorized pages like Standard and Data Controls, facilitating intuitive placement of both visual and non-visual elements. Once placed, components are configured using the Object Inspector, which displays and allows editing of published properties and events in real-time during design. For instance, selecting a component highlights its properties, such as alignment or caption, enabling immediate visual feedback on the form without manual coding. Non-visual components, like timers or data sources, appear attached to the form at design time and can be toggled for visibility via IDE options, ensuring a clutter-free workspace. The IDE automates code generation to streamline event handling and component declarations. Double-clicking a component or an event in the Object Inspector creates a corresponding procedure in the source file (e.g., .pas for or .cpp/.h for ), inserting boilerplate for handlers like button clicks. Form layouts are stored in binary or text-based .dfm files, which are automatically synchronized with the source units, allowing seamless switching between design and views. As of RAD Studio 13 Florence (September 2025), VCL development benefits from AI-powered Smart CodeInsight features, providing intelligent , explanations, and integration with local or large models (LLMs) to enhance . VCL applications occurs within the same IDE environment, supporting breakpoints directly on event handlers and live evaluation of variables during runtime. Developers can set breakpoints in the Code Editor, step through execution, and monitor values using the Watches window, which updates in real-time as the application runs. This integration allows for efficient testing of component interactions without external tools. VCL projects are managed through the Projects window, which organizes multi-form applications as a tree of source and .dfm file pairs under a central project node. New forms are added via the context menu, automatically generating the paired files, while the handles compilation, building, and navigation across units in complex projects. For deployment, the compilers in RAD Studio employ linking optimizations, such as smart linking in Delphi, to include only referenced VCL components and symbols in the final executable, reducing file size by eliminating unused code. This process ensures efficient distribution, with options to further optimize by linking against runtime packages if needed. Custom components can also be registered to the Tool Palette for drag-and-drop use, extending standard VCL workflows.

Customization and Extension

Developers can extend the Visual Component Library (VCL) by creating custom components, which typically involves deriving from base classes such as TCustomControl for controls requiring custom drawing. TCustomControl serves as a foundational class for such visual elements, providing the necessary windowed control infrastructure while allowing subclasses to handle their own rendering. To implement custom visuals, developers override the method, which is responsible for drawing the control's surface onto its canvas; this method is invoked during window repaints and enables precise control over appearance using GDI or other graphics APIs. For enhancing the design-time experience, custom property editors can be implemented to handle complex property types in the Object Inspector. These editors allow developers to associate specialized editing interfaces—such as modal dialogs or dropdown lists—with published properties of a component. Registration occurs using the RegisterPropertyEditor function from the design-time interfaces (DesignIntf unit in Delphi or equivalent in C++), which links the editor class to a specific property type, component class, or filter criteria, ensuring the custom editor appears only when appropriate during form design. This approach supports intuitive editing of non-standard data types, like collections or enumerated sets, without altering the core VCL behavior. Custom components and extensions are often distributed through runtime packages, compiled as .bpl files that enable and code sharing across applications. These packages encapsulate component units, allowing them to be loaded at runtime via the LoadPackage function or statically linked during build, which reduces executable size and promotes reusability. For deployment, .bpl files must reside in the system path or application directory, and applications built with runtime packages require matching versions to avoid compatibility issues. This mechanism is particularly useful for sharing VCL extensions without recompiling host applications. Visual customization of VCL applications can be achieved through and theming via the TStyleManager class in the Vcl.Themes unit. Developers apply custom visual styles by loading .vst files—either from resources, files, or streams—and setting them as the active style using TStyleManager.Engine.SetStyle, which propagates the theme across all compatible controls. This includes modifying colors, fonts, borders, and animations for a modernized appearance, with support for high-DPI scaling in recent versions. Custom styles can be created using tools like Bitmap Style Designer and registered programmatically for runtime switching. Adhering to best practices ensures robust custom VCL components. To prevent memory leaks, avoid circular ownership references between components, such as mutual parent-child assignments, by using weak references or explicit cleanup in ; this aligns with package-level guidelines to eliminate circular dependencies. Constructors should be declared as virtual when overriding in subclasses to support polymorphic instantiation via class references. For , since VCL is inherently single-threaded and UI-bound, test components by marshaling all VCL accesses through the main thread using methods like Synchronize in TThread, preventing exceptions from concurrent modifications.

Cross-Platform Alternatives

(FMX) serves as Embarcadero's official cross-platform successor to the Visual Component Library (VCL), enabling developers to build native applications for multiple operating systems including Windows, macOS, , Android, and . Unlike VCL, which is limited to Windows, FMX leverages GPU acceleration through frameworks such as on Windows and or Metal on other platforms, supporting hardware-accelerated 2D and for visually rich interfaces. A key architectural difference lies in FMX's use of a scene graph model for layout and compositing, which contrasts with VCL's traditional control hierarchy based on Windows API wrappers and GDI rendering. This approach facilitates platform-agnostic code sharing and flexible animations, but it may sacrifice some native Windows look-and-feel fidelity compared to VCL's tight integration with Windows controls. For migrating VCL applications to FMX, third-party tools such as Mida Converter automate the refactoring of forms and components to the cross-platform framework. Beyond Embarcadero's ecosystem, Qt provides a robust C++-based alternative for cross-platform user interfaces, supporting desktop (Windows, macOS, ), mobile (, Android), and embedded systems with a unified for GUI development. For developers remaining on Windows but seeking a modern .NET-based option, Windows Presentation Foundation (WPF) offers vector-based, resolution-independent UIs with support for data binding and styling, though it remains Windows-exclusive. Developers typically select VCL for high-performance, native Windows applications where platform specificity is prioritized, while opting for FMX when multi-platform deployment is required to minimize code duplication. Qt suits broader C++ projects needing extensive customization across diverse environments, and WPF fits .NET workflows focused on advanced Windows desktop experiences.

Third-Party Libraries

The JEDI Visual Component Library (JVCL) is an open-source project offering approximately 600 visual and non-visual components for extending and applications, with compatibility starting from version 6 and supporting newer releases up to the latest RAD Studio editions. Released under the , JVCL builds upon the standard VCL by providing enhancements to core controls, including advanced grid components for data display and reporting tools for document generation, enabling developers to incorporate complex features without custom coding. These components are categorized into groups such as enhanced standard controls, visual elements, and data-aware utilities, fostering reuse in both freeware and commercial projects. Commercial alternatives like the TMS VCL UI Pack deliver over 600 specialized components tailored for feature-rich Windows desktop development in Delphi and C++Builder, with version 13.5 released in April 2025 including improvements in emoji support and other UI enhancements. This suite emphasizes modern UI elements, including advanced charting libraries for data visualization, planning and scheduling components such as versatile planners for agenda management, and web view controls leveraging Edge Chromium for seamless and integration. Similarly, DevExpress VCL extends the framework with more than 230 professional-grade controls, focusing on data grids for tabular data handling, ribbon bars mimicking interfaces, and docking systems for flexible layout management, all optimized for high-performance applications, with further accessibility improvements planned for v25.2 in December 2025. These libraries prioritize Office-like aesthetics and productivity tools, reducing development time for business-oriented software. Integration of such third-party libraries into RAD Studio is streamlined through the GetIt Package Manager, where developers can search for packages like JVCL or TMS components, review licenses, and install them directly within the IDE, with automatic compilation for supported platforms. For legacy environments or custom configurations, manual installation requires compiling the library and registering the resulting .bpl packages via the IDE's . Community contributions further amplify VCL's ecosystem, as seen in projects like the Konopka Signature VCL Controls (previously Raize Components), which supply over 200 additional interface elements to fill gaps in standard VCL, such as improved buttons, dialogs, and panels with high-DPI and styling support; version 8 was released in May 2025 with enhanced designers and compatibility updates. This collective impact has sustained VCL's relevance by enabling rapid prototyping of polished, extensible user interfaces.

References

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