Delphi custom TPanel component

Creating a Custom TPanel Component in Delphi

Learn how to extend Delphi's standard TPanel component to create reusable, custom UI elements with enhanced functionality and visual appeal.

Delphi's VCL (Visual Component Library) provides a rich set of components for building Windows applications. While TPanel is a fundamental container, often developers need specialized panels with custom drawing, event handling, or aggregated controls. This article guides you through the process of creating your own custom TPanel component, enhancing its capabilities, and integrating it seamlessly into the Delphi IDE.

Understanding Custom Component Development

Custom component development in Delphi involves inheriting from an existing VCL component and overriding or adding new properties, methods, and events. For a custom TPanel, you'll typically inherit from TPanel or TCustomPanel. Inheriting from TCustomPanel gives you more control over the component's behavior by starting with a more basic implementation, while TPanel provides default behaviors like BevelInner and BevelOuter.

classDiagram
    VCL.Controls.TControl <|-- VCL.Controls.TWinControl
    VCL.Controls.TWinControl <|-- VCL.ExtCtrls.TCustomPanel
    VCL.ExtCtrls.TCustomPanel <|-- VCL.ExtCtrls.TPanel
    VCL.ExtCtrls.TPanel <|-- MyCustomPanel
    MyCustomPanel : +property MyCustomProperty: Integer
    MyCustomPanel : +procedure MyCustomMethod()
    MyCustomPanel : +event OnMyCustomEvent

Inheritance hierarchy for a custom TPanel component

The diagram above illustrates the typical inheritance chain for a custom TPanel. Your custom component, MyCustomPanel, will inherit from TPanel (or TCustomPanel), gaining all its base functionalities while allowing you to add your unique features. Key aspects of customization include:

• Properties: Add new properties to control appearance or behavior.
• Methods: Implement new methods for specific actions.
• Events: Define custom events to allow users to respond to specific interactions.
• Drawing: Override the Paint method for custom visual rendering.

Implementing a Basic Custom Panel

Let's create a simple custom panel that changes its background color when the mouse hovers over it. This demonstrates overriding events and basic property management. We'll call our component THoverPanel.

unit HoverPanel;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.ExtCtrls;

type
  THoverPanel = class(TPanel)
  private
    FHoverColor: TColor;
    FNormalColor: TColor;
    FIsHovering: Boolean;
    procedure SetHoverColor(const Value: TColor);
    procedure SetNormalColor(const Value: TColor);
    procedure CM_MouseLeave(var Message: TMessage);
    procedure CM_MouseEnter(var Message: TMessage);
    procedure UpdateColor;
  protected
    procedure Paint;
    procedure MouseEnter;
    procedure MouseLeave;
    procedure WndProc(var Message: TMessage);
  public
    constructor Create(AOwner: TComponent);
  published
    property HoverColor: TColor read FHoverColor write SetHoverColor default clHighlight;
    property NormalColor: TColor read FNormalColor write SetNormalColor default clBtnFace;
  end;

procedure Register;

implementation

procedure Register;
begin
  RegisterComponents('Custom Panels', [THoverPanel]);
end;

constructor THoverPanel.Create(AOwner: TComponent);
begin
  inherited Create(AOwner);
  FHoverColor := clHighlight;
  FNormalColor := clBtnFace;
  Color := FNormalColor;
  FIsHovering := False;
  ControlStyle := ControlStyle + [csOpaque]; // Ensure panel draws its own background
end;

procedure THoverPanel.SetHoverColor(const Value: TColor);
begin
  if FHoverColor <> Value then
  begin
    FHoverColor := Value;
    UpdateColor;
  end;
end;

procedure THoverPanel.SetNormalColor(const Value: TColor);
begin
  if FNormalColor <> Value then
  begin
    FNormalColor := Value;
    UpdateColor;
  end;
end;

procedure THoverPanel.UpdateColor;
begin
  if FIsHovering then
    Color := FHoverColor
  else
    Color := FNormalColor;
end;

procedure THoverPanel.MouseEnter;
begin
  inherited MouseEnter;
  FIsHovering := True;
  UpdateColor;
end;

procedure THoverPanel.MouseLeave;
begin
  inherited MouseLeave;
  FIsHovering := False;
  UpdateColor;
end;

procedure THoverPanel.Paint;
begin
  inherited Paint;
  // Custom drawing can go here if needed, after inherited Paint
end;

procedure THoverPanel.WndProc(var Message: TMessage);
begin
  case Message.Msg of
    CM_MOUSEENTER: CM_MouseEnter(Message);
    CM_MOUSELEAVE: CM_MouseLeave(Message);
  else
    inherited WndProc(Message);
  end;
end;

procedure THoverPanel.CM_MouseEnter(var Message: TMessage);
begin
  MouseEnter;
end;

procedure THoverPanel.CM_MouseLeave(var Message: TMessage);
begin
  MouseLeave;
end;

end.

Registering and Using Your Component

After creating the unit file (HoverPanel.pas), you need to register it with the Delphi IDE. This makes your component available in the Tool Palette. The Register procedure in the unit handles this. Once registered, you can install it into a design-time package.

1. Create a New Package

In Delphi, go to File -> New -> Other..., then select Delphi Projects -> Package. Save the package (e.g., MyCustomComponents.dpk).

2. Add Your Unit to the Package

Right-click on Contains in the Project Manager for your new package, select Add..., and then browse to and select your HoverPanel.pas unit file.

3. Compile the Package

Right-click on the package in the Project Manager and select Compile.

4. Install the Package

Right-click on the package again and select Install. You should see a confirmation message that THoverPanel has been registered. Your new component will now appear in the 'Custom Panels' category of the Tool Palette.

The THoverPanel component visible in the Delphi Tool Palette after successful installation.

Now you can drag and drop THoverPanel onto your forms just like any other standard VCL component. You'll find its HoverColor and NormalColor properties in the Object Inspector, allowing you to customize its appearance at design time.