TImageList

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TImageList is a component that provides a list of images that can be used within other components like TToolBar of TActionList.

Example with ComboBox

To use TImageList, drop an TImageList object onto the form. In this example we need six images.

All images must be the same size. If two sizes of image are to be used, then two TImageLists must be used.

Set the height and width of the images in the Object Inspector; In this case 50px wide by 18px high.

Set Style to csOwnerDrawFixed.

Double-click on the TImageList icon to open the TImageList editor. Make sure that the icons show correctly when selected. If they are smaller than expected, check the size in the Object Inspector <F11>. You may have to reload the images.

Place a ComboBox on the form and name it cbSymbols.

In the FormCreate event enter:

 cbSymbols.Items.Clear;
for I := 0 to 5 do
cbSymbols.ItemIndex := 0;

This will write 6 blank entries, and select the first.

In the OnDrawItem event for cbSymbols place the following code:

procedure TMyForm.cbSymbolsDrawItem(
aControl: TWinControl;
iIndex: Integer;
aRect: TRect;
aState: TOwnerDrawState );
var
cnv: TCanvas;
begin
if not (aControl is TComboBox) then exit;
cnv := TComboBox( aControl ).Canvas;
Imagelist1.Draw( cnv, aRect.Left+2, aRect.Top+2, iIndex );
end;

The adjustments (+2) are to center the image in the item, if necessary.

Example with SpeedButton

Add images to the ImageList1 like in the previous example and use:

   ImageList1.GetBitmap(iIndex,SpeedButton1.Glyph);

Multiple-resolution TImageList in Lazarus 1.9 and newer

In Lazarus 1.9 the TImageList was rewritten to support multiple resolutions of one image.

Every LCL control that supports ImageList has now a new [Images]Width property to decide what custom width at 96 PPI (100% scale) is to be used. Example: TToolBar.Images/ImageWidth, TListView.LargeImages/LargeImagesWidth.

Set the Scaled property to True and the image list will automatically pick up the scaled image in your High-DPI aware LCL application.

Demo applications

There are 2 demo applications in Lazarus sources:

• examples/imagelist_highdpi_designtime
• examples/imagelist_highdpi_runtime

Changes affecting backwards compatibility

• Old behavior: the image got sliced if too big or extended if too small.
• New behavior: the image is scaled to all resolutions in the image list.
• Reason: Image List now supports multiple resolutions.
• Remedy: use AddSliced (if the image consists of several icons to be added) or AddSlice (if one image from a custom rect has to be added - also rect outside the image is supported).

High-DPI alternative for Lazarus 1.8 and older: resize all images in TImageList

This is useful for example for High DPI scaling to get icons with higher resolution for TActionList, TMainMenu and TToolBar. Note: This may not work correctly with all widgetsets on all platforms. Note also: This code is not required for Laz 1.9+.

procedure ScaleImageList(ImgList: TImageList; NewWidth, NewHeight: Integer);
var
TempImgList: TImageList;
TempBmp1: TBitmap;
TempBmp2: TBitmap;
I: Integer;
begin
TempImgList := TImageList.Create(nil);
TempBmp1 := TBitmap.Create;
TempBmp1.PixelFormat := pf32bit;
TempBmp2 := TBitmap.Create;
TempBmp2.PixelFormat := pf32bit;
TempBmp2.SetSize(NewWidth, NewHeight);
try
TempImgList.Width := NewWidth;
TempImgList.Height := NewHeight;

for I := 0 to ImgList.Count - 1 do begin
// Load image for given index to temporary bitmap
ImgList.GetBitmap(I, TempBmp1);

// Clear transparent image background
TempBmp2.Canvas.Brush.Style := bsSolid;
TempBmp2.Canvas.Brush.Color := TempBmp2.TransparentColor;
TempBmp2.Canvas.FillRect(0, 0, TempBmp2.Width, TempBmp2.Height);

// Stretch image to new size
TempBmp2.Canvas.StretchDraw(Rect(0, 0, TempBmp2.Width, TempBmp2.Height), TempBmp1);
end;

ImgList.Assign(TempImgList);
finally
TempImgList.Free;
TempBmp1.Free;
TempBmp2.Free;
end;
end;