TButton

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A TButton is a component that provides a basic push button control. It is available on the Standard tab of the Component Palette.

A TButton is one of the most basic controls on a Form. Clicking with the mouse on it (or change with the Tab ⇆ key on the button and pressed it with ↵ Enter), an action is triggered. This click is called an event. For this you need event handler that are called after the jump.

You can add a button to your form, by clicking the TButton (square button with an "OK" in the middle ) on the Standard component palette and place it with a click on your form.

The event handler for a mouse click can be quite easily reached in which a double-click on the pasted Button (or in the Object Inspector, select the event OnClick of your Button). The event handler for a Button1 on a form Form1 will look like this:

procedure TForm1.Button1Click(Sender: TObject);
begin

end;

Between the statements begin and end you could write code that is called when Button1 is clicked.

Almost all available beginner tutorials use TButtons as an easy entry into the Object Oriented Programming with Lazarus. Following tutorials are well suited for beginners to understand the use of buttons:

• The first GUI application for absolute beginners
• Your first Lazarus program tutorial for Lazarus
• Programming Example Object Oriented Programming with Free Pascal and Lazarus

A simple example

• Create a new application and drop a TButton on the form.
• Doubleclick this Button1 on the form (the default handler: OnClick is created for Button1, the source text editor opens).
• Add following code in the event handler:

procedure TForm1.Button1Click(Sender: TObject);
begin
  ShowMessage('Lazarus makes my day');  //A message will be displayed with the content...
end;

• Start your program (with Key F9).

Right-click

Each TButton has an (optional) PopupMenu property that will activate a connected TPopupMenu whenever the button is right-clicked.

Dynamically generated button

Sometimes, instead of creating buttons (or other components) with the Lazarus form designer, it is easier to create them dynamically at run time. This approach is useful especially if you have continually repeated buttons on a form.

This can be achieved as in the the following example (a quick calculator):

• Create a new blank GUI application with the form Form1 and add StdCtrls to the uses clause (here the TButton is).
• Change caption Form1 to QuickAdd.
• Create the OnCreate event handler of Form1 (go in the Object Inspector to the event OnCreate and click the button [...]).
• Add following code:

procedure TForm1.FormCreate(Sender: TObject);
var
  i:       Integer;
  aButton: TButton;
begin
  for i := 0 to 9 do begin                // create 10 Buttons 
    aButton := TButton.Create(Self);      // create Button, Owner is Form1, where the button is released later
    aButton.Parent  := Self;              // determine where it is to be displayed
    aButton.Width   := aButton.Height;    // Width should correspond to the height of the buttons
    aButton.Left    := i * aButton.Width; // Distance from left
    aButton.Caption := IntToStr(i);       // Captions of the buttons (0.9)
    aButton.OnClick := @aButtonClick;     // the event handler for the button -> will be created yet
  end;
  Self.Height := aButton.Height;          // Height of the form should correspond to the height of the buttons
  Self.Width  := aButton.Width * 10;      // Width of the form to match the width of all buttons
end;

• Now you must create the event handler for the button clicks.
• In the source editor, entering your class TForm1 in the section private.
• Add procedure aButtonClick(Sender: TObject); and then press the keys Ctrl+⇧ Shift+c (the code completion becomes active and creates the procedure TForm1.aButtonClick(Sender: TObject);.
• Paste following code:

procedure TForm1.aButtonClick(Sender: TObject);
const
  Cnt: Integer = 0;
var
  i: Integer;
begin
  if (Sender is TButton) and                  // called the event handler of a button out?
     TryStrToInt(TButton(Sender).Caption, i)  // then try to convert the label in a integer
  then begin
    Cnt := Cnt + i;                           // the adding counter is incremented by the number of entrechende
    Caption:='QuickAdd: '+IntToStr(Cnt);      // write the result to the caption of the form
  end;
end;

• Start your application.

  Note: You can assign every imaginable event handlers to your buttons, as long as this the form procedure <class>.<name of procedure>(Sender: TObject); has. Thus, you can use one from another class!

See also

• TButton doc
• TBitBtn
• TSpeedButton
• TColorButton
• 15-puzzle (Game)