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Revision as of 12:15, 20 July 2016 by Jma sp (talk | contribs) (Mediante código cuando creas el formulario)

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>> LCL Components >> TComboBox

Visión General

Esta página explica como utilizar el componente TComboBox. Cuando mencione hacer click en algo, a menos que diga explícitamente que haga click-derecho, siempre me referiré a realizar un click izquierdo en el elemento en cuestión.


Es una combinación de una caja de edición (edit box) y un listado (soltar-abajo(drop-down) permitiendo una o varias opciones a ser escogidas.

Comp Standard TComboBox.png

Modo de uso

Para utilizar un TComboBox en unaform, simplemente lo seleccionas de la paleta de componentes Standardy a continuación lo ubicas en el formulario mediante un click en el mismo.

En el componente ComboBox, las strings se almacenan en la propiedad Items, que es del tipo TStrings. Por tanto puedes asignar o eliminar cadenas en el ComboBox como en un TStringList o su padre TStrings.

Aquí hay unos cuantos ejemplo de uso para combobox ComboBox1 en un formulario Form1:

Llenar el ComboBox

Mediante el Inspector de Objetos

  • Selecciona el ComboBox en tu formulario mediante un click.
  • Ve al Inspector de Objetos en la solapa Propiedades de la propiedad Items.
  • Haz Click en el pulsador que tiene los tres puntos ... que habrirá el editor de cadenas.
  • Escribe las entradas de texto que necesites y confirma el contenido con OK.

Mediante código cuando creas el formulario

  • Create the OnCreate event handler for the form, by clicking on your form, use the Object Inspector, the tab events, select the OnCreate event and click the button [...] or double click the button in the form.
  • En el editor de código fuente ahora insertamos la sección de texto deseada, para nuestro ejemplo, escribiéndola como sigue:
procedure TForm1.FormCreate(Sender: TObject);  
  ComboBox1.Items.Clear;                  // Elimina todo el contenido previo.
  ComboBox1.Items.Add('Rojo');            // Añade la entrada del elemento 'Rojo'.
  ComboBox1.Items.Add('Verde');           // Añade la entrada del elemento 'Verde'.
  ComboBox1.Items.Add('Azul');            // Añade la entrada del elemento 'Azul'.
  ComboBox1.Items.Add('Color Aleatorio'); // Añade la entrada del elemento 'Color Aleatorio'.

ComboBox 01.PNG

Make that something happens after the selection

Like all components, even the TComboBox provides various events, that are called when the user use the combobox. To respond to a change of the selection in the ComboBox, you can use the OnChange event:

  • Doubleclick the ComboBox on the form or choose the OnChange event in the Object Inspector and click on the button [...].
  • The event handler is created, now you can insert your desired source, in our example we want to change the background color of the form:
procedure TForm1.ComboBox1Change(Sender: TObject);
  case ComboBox1.ItemIndex of  //what entry (which item) has currently been chosen
    0: Color:=clRed;
    1: Color:=clGreen;
    2: Color:=clBlue;
    3: Color:=Random($1000000);
  • Start your application, the selection changes the background color of the form.

Own drawed ComboBox

In general, it is advantageous to let the ComboBox show in the theme the user has chosen in his settings. In some cases (for example, to program a game with a colorful surface), you can deviate from this standard and draw it according to your own choice. This is how this works:


Note: Parameters of ComboBoxDrawItem:

If multiple controls (e.g. multiple ComboBoxes) access this event handle, you know which control caused the event. In our example, instead of ComboBox1.Canvas.FillRect(ARect) you could also write TComboBox(Control).Canvas.FillRect(ARect). However, you should still check in advance, whether it is a TComboBox:

  if Control is TComboBox then

Index: Specifies the item location, so you have access to the string <ComboBox>.Items[Index].
ARect: Describes the rectangle, which is necessary for drawing the background.
State: Status of the items, whether normal, focused, selected etc.

Draw a filled rectangle

  • You can modify the example Fill ComboBox by code when you create the form.
  • Change from ComboBox1 in the Object Inspector the property Style to csOwnerDrawFixed.
  • Create in the Object Inspector the event handler for the event OnDrawItem, by clicking on the button [...].
  • Add the following code to the handler:
procedure TForm1.ComboBox1DrawItem(Control: TWinControl; Index: Integer;
  ARect: TRect; State: TOwnerDrawState);
  ltRect: TRect;

  procedure FillColorfulRect(aCanvas: TCanvas; myRect: TRect);              //paint random color
  // Fills the rectangle with random colours
    y: Integer;
    for y:=myRect.Top to myRect.Bottom - 1 do begin
      aCanvas.Line(myRect.Left, y, myRect.Right, y);

  ComboBox1.Canvas.FillRect(ARect);                                         //first paint normal background
  ComboBox1.Canvas.TextRect(ARect, 22, ARect.Top, ComboBox1.Items[Index]);  //paint item text 

  ltRect.Left   := ARect.Left   + 2;                                        //rectangle for color
  ltRect.Right  := ARect.Left   + 20;
  ltRect.Top    := ARect.Top    + 1;
  ltRect.Bottom := ARect.Bottom - 1;

  ComboBox1.Canvas.Rectangle(ltRect);                                       //draw a border 

  if InflateRect(ltRect, -1, -1) then                                       //resize rectangle by one pixel
    if Index = 3 then
      FillColorfulRect(ComboBox1.Canvas, ltRect)                            //paint random color
    else begin
      case Index of
        0: ComboBox1.Canvas.Brush.Color := clRed;
        1: ComboBox1.Canvas.Brush.Color := clGreen;
        2: ComboBox1.Canvas.Brush.Color := clBlue;
      ComboBox1.Canvas.FillRect(ltRect);                                    //paint colors according to selection
  • Your example might look like:

ComboBoxBsp1.png -> ComboBoxBsp2.png

Preceded image

In this example, we load a few images in a TImageList and draw them in front of the items in the combobox. It is a simple example which only generally show what you can do. I don't run explicitly details, such as checking, whether the corresponding image exists etc. in this example. This should be done by you depending on the need.

  • Create an application analogous example Fill ComboBox by code when you create the form.
  • Change from ComboBox1 in the Object Inspector the property Style to csOwnerDrawFixed.
  • Add a TImageList from the component palette Common controls on your form.
  • The Height and Width of 16 pixels is preset in ImageList1. We allow this. To fit neatly the images into our combo box, we make the property ItemHeight from ComboBox1 to 18 in the Object Inspector.
  • Add four images in the ImageList:
    • Doubleclick ImageList1 or leftclick ImageList1 and select ImageList Editor....
    • Click on Add and select an image (see <Lazarus directory>/images/... there are various images or icons in 16x16px size).
    • Have you added four images, confirm your work with [OK].
  • Create in the Object Inspector the event handler for the event OnDrawItem, by clicking on the button [...].
  • Add the following code to the handler:
procedure TForm1.ComboBox1DrawItem(Control: TWinControl; Index: Integer;
  ARect: TRect; State: TOwnerDrawState);
  ComboBox1.Canvas.FillRect(ARect);                                         //first paint normal background
  ComboBox1.Canvas.TextRect(ARect, 20, ARect.Top, ComboBox1.Items[Index]);  //paint item text 
  ImageList1.Draw(ComboBox1.Canvas, ARect.Left + 1, ARect.Top + 1, Index);  //draw image according to index on canvas
  • Your example might look like:

ComboBoxBsp1.png -> ComboBoxBsp3.png

Return To: LCL Components  — Previous: TListBox Next: TScrollBar

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--Michl 22:14, 2 June 2014 (CEST)