THTMListbox vs. Standard ListBox: When to Choose Which

Parsing libraries or regular expressions can help, but be careful with performance for long lists.

Drawing icons, checkboxes, or thumbnails

To show an icon or thumbnail before each item:

• Reserve space on the left (padding) when measuring.
• Draw the image in OnDrawItem at the desired coordinates.
• Use an image list (TImageList) for consistent size and performance.

Example:

// Draw icon at 2,2 offset inside item rectangle ImageList1.Draw(LB.Canvas, Rect.Left + 2, Rect.Top + 2, ItemImageIndex); 

When using variable-height items, ensure image height is considered in OnMeasureItem.

Selection and focus visuals

Custom selection rendering lets you highlight items in ways that match your app’s theme.

Choices:

• Use standard highlight colors (clHighlight/clHighlightText).
• Use gradient fills for selection (GradientFill API or manual drawing).
• Draw a custom focus rectangle when the control has focus.

Example custom gradient (simplified):

// Fill background with blend from Color1 to Color2 DrawGradient(LB.Canvas.Handle, Rect, Color1, Color2); 

Remember to still ensure good contrast for accessibility.

Handling high-DPI and scaling

• Use logical units when measuring (Canvas.TextHeight, DrawText with DT_EXTERNALLEADING).
• Query the current DPI (Screen.PixelsPerInch or per-monitor DPI APIs) and scale icon sizes and padding accordingly.
• If using TImageList, use a high-DPI-aware image list or supply multiple image sizes.

Performance considerations

• Avoid heavy parsing or creating fonts inside OnDrawItem — precompute styles and resources.
• Cache measured heights for items if content doesn’t change often.
• Use BeginPaint/EndPaint blocks appropriately; avoid unnecessary invalidates.
• For large lists, virtualize content (only render visible items) — many listbox controls already do this, but complex on-the-fly HTML parsing can still be costly.

Accessibility and keyboard support

• Ensure selected item text remains readable with high-contrast themes.
• Support keyboard focus rectangle and item activation (double-click/Enter) semantics.
• Provide tooltip or accessible name if items contain non-textual content.

Example: Combined features

A compact example showing icons, wrapped text, and HTML-like bold segments (conceptual):

procedure TForm1.ListboxDrawItem(Control: TWinControl; Index: Integer; Rect: TRect; State: TOwnerDrawState); var   LB: THTMListbox;   Txt: string;   TextRect: TRect;   IconWidth: Integer; begin   LB := Control as THTMListbox;   Txt := LB.Items[Index];   IconWidth := 24;   // Background   if odSelected in State then     LB.Canvas.Brush.Color := clHighlight   else     LB.Canvas.Brush.Color := LB.Color;   LB.Canvas.FillRect(Rect);   // Draw icon   ImageList1.Draw(LB.Canvas, Rect.Left + 4, Rect.Top + 4, ItemIconIndex[Index]);   // Text area after icon   TextRect := Rect;   Inc(TextRect.Left, IconWidth + 8);   InflateRect(TextRect, -4, 0);   // Simple "HTML" bold parsing example (very simplified)   if Pos('<b>', Txt) > 0 then   begin     // naive split: before <b>, bold, after     // set font style accordingly and draw segments...   end   else     DrawText(LB.Canvas.Handle, PChar(Txt), Length(Txt), TextRect, DT_WORDBREAK or DT_NOPREFIX); end; 

Testing and debugging tips

• Start with plain text drawing and progressively add complexity (icons, wrapping, HTML).
• Use a small dataset while developing measurement and drawing logic.
• Test with various DPI settings, long strings, non-Latin scripts, and RTL text if applicable.
• Profile with a large item count to find bottlenecks.

Summary

Customizing item rendering in THTMListbox lets you build rich, visually distinct lists by combining owner-draw events, HTML-like formatting, and images. Key takeaways:

• Use owner-draw modes and implement OnDrawItem and OnMeasureItem for full control.
• Precompute measurements and resources to keep drawing fast.
• Integrate HTML parsing carefully — prefer the component’s built-in HTML rendering if available.
• Consider DPI, accessibility, and performance while designing your visuals.

This should give you a solid foundation to tailor THTMListbox items to your app’s needs.