# How To Write Lazarus Component

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This is a guide on how to build components.

## Step 1: Create the package

• On the Lazarus IDE menu, click Package > New Package to run the Package Manager.

• A Save file dialog will appear. Chose a folder and a file name and press save. If the IDE prompts for using lowercase filenames press 'yes'.
• Congratulations: You have just created your first package!

## Step 2: Create the unit

You can create a new unit or use an existing file. Both options are described below.

### Create a new unit

• Use the Add button > New component.

• Choose a component like TComboBox.
• Choose customcontrol1.pas as Unit File Name and CustomControl1 as Unit Name.
• You can now give to the component a icon and determine on which palette the component should appear later in the Lazarus-IDE.
• Click Create New Component.
unit CustomControl1;

{$mode objfpc}{$H+}

interface

uses
Classes, SysUtils, LResources, Forms, Controls, Graphics, Dialogs, StdCtrls;

type
TCustomControl1 = class(TComboBox)
private
{ Private declarations }
protected
{ Protected declarations }
public
{ Public declarations }
published
{ Published declarations }
end;

procedure Register;

implementation

procedure Register;
begin
RegisterComponents('Standard',[TCustomControl1]);
end;

end.
• Install the package by clicking the Use -> Install button at the top of the package editor.

• Now the IDE will ask you, if the IDE should be rebuilt. Say yes.

• Restart Lazarus and see your new component in the component palette. Congratulations: you have just installed your first package with your first package component.

Note: If you do not see your new component in the component palette, it is most likely that you are not running the re-compiled version of Lazarus. You can set where Lazarus builds to using Tools -> Options -> Files -> Lazarus directory. Instead of calling Lazarus directly, you also can use startlazarus, which starts the newly created Lazarus, for example the Lazarus executable in the ~/.lazarus directory, if you don't have write access to the directory Lazarus was installed into.

• Click the Add button, go to the Add Files tab. At the Unit file name, browse to your existing file. Click Add files to package. If the package manager complains that the unit is not in the unit path, click yes to add the directory to the unit path.
• Click the Add button again, go to the Add Files tab, browse to the .lrs file and click OK (See Step 3 about creating this icon file).
• Click the Add button again, go to the New Requirement tab. In the Package name select LCL and click OK.

The end result should look like this:

• Click under the Files tree in the Package Manager. In the File Properties, make sure Register unit is checked.
• Click the Options button. Select the IDE Integration tab. In Package Type, make sure Designtime and Runtime is selected.
• Click the Compile button to check to see that the files compile without errors.
• Click the Install button. Lazarus will rebuild and restart automatically.

The component is created and ready to be used:

## Step 3: Create icons for the package

You should create 24x24 pixel size PNG files as icons.

FPC is capable of using standard resource files .rc or compiled resources .res since fpc version 2.6. See Lazarus_Resources#FPC_resources

The .lrs: see below.

### Using the Lazarus Image Editor

You can use Lazarus Image Editor to create the icons in .lrs format.

### Using lazres

lazres is usually located in the Lazarus tools directory.

You may need to compile lazres at first use. Simply open the lazres.lpi in the IDE and click run > build in the menu.

Create lrs file

To create the lrs file run:

~/lazarus/tools/lazres samplepackage.lrs TMyCom.png

Where

• samplepackage is the name of your package
• TMyCom is the name of your component! The name of the image must match the name of your component!

You can add more than one image to the lrs file by appending the image file name at the end. Eg. ~/lazarus/tools/lazres samplepackage.lrs TMyCom.png TMyOtherCom.png ...

Sample

Following is a sample of the resulting samplepackage.lrs file.

LazarusResources.Add('TMyCom','PNG',[
#137'PNG'#13#10#26#10#0#0#0#13'IHDR'#0#0#0#24#0#0#0#24#8#2#0#0#0'o'#21#170#175
+#0#0#0#4'gAMA'#0#0#177#143#11#252'a'#5#0#0#0'|IDAT8O'#237#212#209#10#192' '#8
+#5'P'#247#231#251's'#215#138#133#164#166'\'#220#195''#209'c'#157'L'#173#131
+#153#169'd4'#168'dP'#137'r_'#235'5'#136'@Zmk'#16'd9'#144#176#232#164'1'#247
+'I'#8#160'IL'#206'C'#179#144#12#199#140'.'#134#244#141'~'#168#247#209'S~;'#29
+'V+'#196#201'^'#10#15#150'?'#255#18#227#206'NZ>42'#181#159#226#144#15'@'#201
+#148#168'e'#224'7f<@4'#130'u_YD'#23#213#131#134'Q]'#158#188#135#0#0#0#0'IEND'
+#174'B'#130
]);

Make sure to include your resource file in your new component source by adding the following at the bottom your component's unit and including "LResources" in the uses clause

initialization
{$I samplepackage.lrs} Create res file lazres can also create a compiled resource file, just specify a file with the .res extension, e.g.: ~/lazarus/tools/lazres samplepackage.res TMyCom.png In this case, instead of including the lrs file in the initialization section, just include the resource file anywhere in your component source file  {$R samplepackage.res}

### Using glazres

GLazRes is the graphical version of lazres that can assemble files into a Lazarus resource file (.lrs). It can be found in the tools directory of a Lazarus installation.

## Recompiling Packages

You need to rebuild the package every time you make changes to the mycom.pas file. To rebuild the package, open the samplepackage.lpk file in the Package Manager and click the Install button.

## Removing Packages

• To remove installed components: on the IDE menu, click Package > Configure installed packages. The following image shows the Installed Packages tool.


• Select the package you want to uninstall and click Uninstall selection.

If something goes wrong with a package (e.g. package directory is deleted without first uninstalling it), Lazarus may not allow you to uninstall packages. To fix the problem, at the IDE menu click Tools > Build Lazarus. Lazarus will rebuild all packages and restart. You should now be able to uninstall problematic packages.

## Enhancing mycom.pas

• The code in mycom.pas above gives you the basics on what you need to create a component. The following is an enhanced version with some tips on how to write procedures and events for components.
• The OnChange2 shows how to create events
• The OnSample shows how to create custom events
• MyText and MyText2 shows different ways to write properties.
• You can use TComboBox instead of TCustomComboBox as the base class, which publishes all properties as TComboBox.
• If TCustomComboBox is used as the base class, you'll notice a lot of properties and events will be missing in the IDE Object Inspector. To add those properties and events, just copy and paste the properties as listed below // properties from TComboBox. These list of properties can be got from the TComboBox declaration in the StdCtrls unit. Omit any property which you want to handle yourself.
unit mycom;

{$mode objfpc}{$H+}

interface

uses
Classes, SysUtils, StdCtrls, Forms, Dialogs,
LCLType,LCLIntf,lresources,LCLProc;

type

TSampleEvent = procedure(MyText: String) of Object;

TMyCom = class (TCustomComboBox)
private
FMyText: String;
FOnChange2: TNotifyEvent;
FOnSample: TSampleEvent;
public
constructor Create(TheOwner: TComponent); override;
procedure CreateWnd; override;
procedure Change; override;
protected
function GetMyText2: String;
procedure SetMyText2(MyText: String);
published
property MyText: String read FMyText write FMyText;
property MyText2: String read GetMyText2 write SetMyText2;
property OnChange2: TNotifyEvent read FOnChange2 write FOnChange2;
property OnSample: TSampleEvent read FOnSample write FOnSample;

// properties from TComboBox
property Align;
property Anchors;
property ArrowKeysTraverseList;
property AutoComplete;
property AutoCompleteText;
property AutoDropDown;
property AutoSelect;
property AutoSize;
property BidiMode;
property BorderSpacing;
property CharCase;
property Color;
property Ctl3D;
property Constraints;
property DragCursor;
property DragMode;
property DropDownCount;
property Enabled;
property Font;
property ItemHeight;
property ItemIndex;
property Items;
property ItemWidth;
property MaxLength;
property OnChange;
property OnChangeBounds;
property OnClick;
property OnCloseUp;
property OnContextPopup;
property OnDblClick;
property OnDragDrop;
property OnDragOver;
property OnDrawItem;
property OnEndDrag;
property OnDropDown;
property OnEditingDone;
property OnEnter;
property OnExit;
property OnGetItems;
property OnKeyDown;
property OnKeyPress;
property OnKeyUp;
property OnMeasureItem;
property OnMouseDown;
property OnMouseMove;
property OnMouseUp;
property OnStartDrag;
property OnSelect;
property OnUTF8KeyPress;
property ParentBidiMode;
property ParentColor;
property ParentCtl3D;
property ParentFont;
property ParentShowHint;
property ShowHint;
property Sorted;
property Style;
property TabOrder;
property TabStop;
property Text;
property Visible;
end;

procedure Register;

implementation

procedure Register;
begin
RegisterComponents('Sample',[TMyCom]);
end;

constructor TMyCom.Create(TheOwner: TComponent);
begin
inherited Create(TheOwner);
Self.Style := csDropDownList;
end;

procedure TMyCom.CreateWnd;
begin
inherited CreateWnd;
Items.Assign(Screen.Fonts);
end;

procedure TMyCom.Change;
begin
inherited;
if Assigned(FOnChange2) then FOnChange2(Self);
if Assigned(FOnSample) then FOnSample(FMyText);
end;

function TMyCom.GetMyText2: String;
begin
Result:=FMyText;
end;

procedure TMyCom.SetMyText2(MyText: String);
begin
FMyText:=MyText;
end;

initialization
{\$I samplepackage.lrs}

end.

Also you will notice that some undeclared and possibly unwanted elements exist in the IDE Object Inspector. To remove those of them, which you do not need, you can redeclare them in the Published section as simple variables. For example:

Published
...
property Height: Integer;
property Width:  Integer;
...

## Using embedded (visual) components

It's possible to use standard components embedded in your own components (look for example at TLabeledEdit or TButtonPanel).

Let's say you want to create a custom panel with a TLabel on it. With the steps described above the base package and source files can be created. Now do the following to add a TLabel to the component:

• Add a private attribute for the label component (FEmbeddedLabel: TLabel;).
• Create the label in the component's (overridden) constructor (FEmbeddedLabel := TLabel.Create(self); )
• Set the parent of the component (FEmbeddedLabel.Parent := self;)
• If the component to be embedded is not a 'subcomponent' by default (like TBoundLabel, TPanelBitBtn etc) then add the call to SetSubComponent. This is necessary for the IDE so it knows that it has to store the properties of the embedded component as well. TLabel is not a subcomponent by default so the call to the method must be added (FEmbeddedLabel.SetSubComponent(true);).

To sum it up you would get something like this (only the essential parts are shown):

TEnhancedPanel = class(TCustomControl)
private
{ The new attribute for the embedded label }
FEmbeddedLabel: TLabel;

public
{ The constructor must be overriden so the label can be created }
constructor Create(AOwner: TComponent); override;

published
{ Make the label visible in the IDE }
end;

implementation

constructor TEnhancedPanel.Create(AOwner: TComponent);
begin
inherited Create(AOwner);

// Set default width and height
with GetControlClassDefaultSize do
SetInitialBounds(0, 0, CX, CY);

FEmbeddedLabel := TLabel.Create(Self); // Add the embedded label
FEmbeddedLabel.Parent := self;         // Show the label in the panel
FEmbeddedLabel.SetSubComponent(true);  // Tell the IDE to store the modified properties
FLabel.Name := 'EmbeddedLabel';
FLabel.Caption := 'Howdy World!';

// Make sure the embedded label can not be selected/deleted within the IDE
FLabel.ControlStyle := FLabel.ControlStyle - [csNoDesignSelectable];

// Set other properties if necessary
//...

end;

## Using custom paint procedure

You can always subclass a component inside your program. For example, this implements a custom Paint procedure to a TLabel:

type
TMyLabel = class(TLabel)
protected
procedure Paint; override;
end;
{...}
implementation
{...}
procedure TMyLabel.Paint;
begin
// your code to implement Paint, for example
Canvas.TextOut(0,0,Caption);
end;

Now you can create a MyLabel inside your program, at run time, with that overridden Paint procedure instead of the standard one.

For most components, and for most methods, it would be recommendable to call inherited procedure inside it:

procedure TMyLabel.Paint;
begin

inherited Paint;   /////////////////////

// your code to implement Paint, for example
Canvas.TextOut(0,0,Caption);
end;

However, inherited behavior is not desirable in this case, since the second writing action would overlap the first (inherited) one.

## Integrating the component with the IDE

### Property editors

Property editors provide custom dialogs to edit properties in the object inspector. For most properties, like strings, string lists, images, enumerated types and others, there are already default property editors, but if a custom component has a special kind of property it may require a custom dialog to edit the property.

Each property editor is a class, which should descend from TPropertyEditor or one of its descendents and implement methods from this base class. They should be registered in the 'Register' procedure by using the function RegisterPropertyEditor from the unit PropEdits. It is a standard to name property editors with the property name followed by 'Property', for example TFieldProperty for the property editor of the TField property.

  TPropertyEditor = class
public
function  AllEqual: Boolean; Virtual;
function  AutoFill: Boolean; Virtual;
procedure Edit; Virtual;     // Activated by double-clicking the property value
procedure ShowValue; Virtual; // Activated by control-clicking the property value
function  GetAttributes: TPropertyAttributes; Virtual;
function  GetEditLimit: Integer; Virtual;
function  GetName: ShortString; Virtual;
procedure GetProperties(Proc: TGetPropEditProc); Virtual;
function  GetHint(HintType: TPropEditHint; x, y: integer): String; Virtual;
function  GetDefaultValue: AnsiString; Virtual;
procedure GetValues(Proc: TGetStrProc); Virtual;
procedure SetValue(const NewValue: AnsiString); Virtual;
function  SubPropertiesNeedsUpdate: Boolean; Virtual;
function  IsDefaultValue: Boolean; Virtual;
function  IsNotDefaultValue: Boolean; Virtual;
// ... shortened
end;

A good example for a property editor is the TFont property editor.

One of the most common cases for property editors is properties which are classes. Because classes have many fields and can have a variety of formats, it's not possible for Lazarus to have the object inspector edit field able to edit it, like is done for strings and numeric types.

For classes, a convention is to have the value field show permanently the name of the class in parentheses, for example "(TFont)" and the "..." button shows a dialog to edit this class. This behaviour, except for the dialog, is implemented by a standard property editor for classes called TClassPropertyEditor, which can be inherited from when writing property editors for classes:

TClassPropertyEditor = class(TPropertyEditor)
public
constructor Create(Hook: TPropertyEditorHook; APropCount: Integer); Override;
function GetAttributes: TPropertyAttributes; Override;
procedure GetProperties(Proc: TGetPropEditProc); Override;
function GetValue: AnsiString; Override;
Write SetSubPropsTypeFilter
Default tkAny;
end;

Going back to the TFont example, inheriting from TClassPropertyEditor already offers part of the desired behavior and then the TFontPropertyEditor class only needs to implement showing the dialog in the Edit method and set the attributes for the editor:

  TFontPropertyEditor = class(TClassPropertyEditor)
public
procedure Edit; Override;
function  GetAttributes: TPropertyAttributes; Override;
end;

procedure TFontPropertyEditor.Edit;
var
FontDialog: TFontDialog;
begin
FontDialog := TFontDialog.Create(NIL);
try
FontDialog.Font    := TFont(GetObjectValue(TFont));
FontDialog.Options := FontDialog.Options + [fdShowHelp, fdForceFontExist];
if FontDialog.Execute then SetPtrValue(FontDialog.Font);
finally
FontDialog.Free;
end;
end;

function TFontPropertyEditor.GetAttributes: TPropertyAttributes;
begin
end;

### Component editors

Component editors control the behavior of right-clicking and double clicking components in the form designer.

Each component editor is a class, which should descend from TComponentEditor or one of its descendents and implement methods from this base class. They should be registered in the 'Register' procedure by using the function RegisterComponentEditor from the unit ComponentEditors. It is a standard to name component editors with the component name followed by 'Editor', for example TStringGridComponentEditor for the property editor of the TStringGrid component. Although user component editors should be based in TComponentEditor, most of its methods are actually from an ancestor, so it is necessary to also know TBaseComponentEditor:

  TBaseComponentEditor = class
protected
public
constructor Create(AComponent: TComponent;
procedure Edit; Virtual; Abstract;
procedure ExecuteVerb(Index: Integer); Virtual; Abstract;
function  GetVerb(Index: Integer): String; Virtual; Abstract;
function  GetVerbCount: Integer; Virtual; Abstract;
procedure PrepareItem(Index: Integer; const AnItem: TMenuItem); Virtual; Abstract;
procedure Copy; Virtual; Abstract;
function  IsInInlined: Boolean; Virtual; Abstract;
function  GetComponent: TComponent; Virtual; Abstract;
function  GetDesigner: TComponentEditorDesigner; Virtual; Abstract;
function  GetHook(out Hook: TPropertyEditorHook): Boolean; Virtual; Abstract;
procedure Modified; Virtual; Abstract;
end;

The most important method of a component editor is Edit, which is called when the component is double clicked. When the context menu for the component is invoked the GetVerbCount and GetVerb methods are called to build the menu. If one of the verbs (which means menu items in this case) are selected, ExecuteVerb is called. There is a default component editor (TDefaultEditor) which implements Edit to search the properties of the component for the most appropriate one to be edited. It usually chooses an event, which is edited by adding it's skeleton code in the code editor and setting the cursor to be in place to add code for it.

A simple and interesting example is the TCheckListBox component editor which creates a dialog to edit. More convenient then implementing all methods from TBaseComponentEditor is inheriting from TComponentEditor, and this is what TCheckListBoxEditor does. This base class adds empty implementations for most methods and some default ones for others. For Edit it calls ExecuteVerb(0), so if the first item will be identical to the double-click action, which is a convention for editor, there is no need to implement Edit. This basic action for the double-click and first menu item is often a dialog, and for TCheckListBox this is also done:

  TCheckListBoxComponentEditor = class(TComponentEditor)
protected
procedure DoShowEditor;
public
procedure ExecuteVerb(Index: Integer); override;
function  GetVerb(Index: Integer): String; override;
function  GetVerbCount: Integer; override;
end;

procedure TCheckGroupComponentEditor.DoShowEditor;
var
Dlg: TCheckGroupEditorDlg;
begin
Dlg := TCheckGroupEditorDlg.Create(NIL);
try
// .. shortened
Dlg.ShowModal;
// .. shortened
finally
Dlg.Free;
end;
end;

procedure TCheckGroupComponentEditor.ExecuteVerb(Index: Integer);
begin
case Index of
0: DoShowEditor;
end;
end;

function TCheckGroupComponentEditor.GetVerb(Index: Integer): String;
begin
Result := 'CheckBox Editor...';
end;

function TCheckGroupComponentEditor.GetVerbCount: Integer;
begin
Result := 1;
end;

## Design-time component debugging

To catch design-time-errors in a (newly created) dbgcomponent:

• open project C:\lazarus\ide\lazarus.lpi;
• run project;
• set breakpoint in dbgcomponent in main (1st) app;
• use dbgcomponent pascal code in second app;
• step through design-time component code in debug-session; do whatever is necessary;