Building Dynamic Lists with THTMListbox: Examples & Best PracticesTHTMListbox is a flexible list-control component commonly used in Delphi and Lazarus environments to present, manage, and interact with collections of items. Unlike standard listbox controls, THTMListbox often provides richer rendering, HTML-like styling, custom item heights, and advanced event handling — making it well suited for modern desktop applications that require visually rich or interactive lists. This article explains core concepts, shows practical examples, and presents best practices for building dynamic, performant, and maintainable lists with THTMListbox.
When to choose THTMListbox
THTMListbox shines when you need:
- Rich item formatting (HTML-like markup, styled text, images).
- Heterogeneous item heights (items of different vertical size).
- Advanced interactivity (clickable regions inside items, embedded controls).
- Custom drawing and layout without rewriting a full owner-draw control.
If your needs are simple text-only lists with uniform appearance, the standard TListBox or TStringGrid might be sufficient and simpler.
Core concepts
THTMListbox typically exposes the following concepts and features (exact property/event names may vary by implementation):
- Items collection: the underlying list of item data (strings or objects).
- Item templates or HTML markup: a way to describe item appearance using tags or formatting codes.
- Owner-draw hooks / OnDrawItem: callbacks to customize painting.
- Virtual mode / OnDataRequest: populate item content on demand for large datasets.
- Selection model: single or multi-select, with keyboard support.
- Hit-testing and click regions: determining which part of an item was clicked (useful for inline buttons or links).
- Scrolling and lazy-loading: techniques to keep UI responsive when showing many items.
Example 1 — Basic usage and adding items
This example demonstrates initializing a THTMListbox, adding plain and formatted items, and handling a simple selection change event.
- Place a THTMListbox on a form (or create it at runtime).
- Add items in code:
// Delphi-style pseudocode procedure TForm1.FormCreate(Sender: TObject); begin HTMListbox1.Items.Clear; HTMListbox1.Items.Add('Plain item 1'); HTMListbox1.Items.Add('<b>Bold item</b>'); HTMListbox1.Items.Add('<img src="icon.png"/> Item with icon'); end; procedure TForm1.HTMListbox1SelectionChange(Sender: TObject); begin ShowMessage('Selected: ' + HTMListbox1.Items[HTMListbox1.ItemIndex]); end;
Notes:
- Use the component’s supported markup for styling items (bold, color, images).
- Use item objects (like associating TObject or a record) when attaching metadata to items.
Example 2 — Custom item objects and metadata
For interactive applications you’ll often attach metadata (IDs, flags, data objects) to items rather than embedding all info in strings. Use an item-object pattern or the Items.Objects[] array.
type PItemData = ^TItemData; TItemData = record ID: Integer; Timestamp: TDateTime; Meta: string; end; procedure TForm1.AddItemWithMetadata(const Text: string; ID: Integer); var DataPtr: PItemData; Index: Integer; begin New(DataPtr); DataPtr^.ID := ID; DataPtr^.Timestamp := Now; DataPtr^.Meta := 'source:api'; Index := HTMListbox1.Items.Add(Text); HTMListbox1.Items.Objects[Index] := TObject(DataPtr); end; procedure TForm1.FormDestroy(Sender: TObject); var i: Integer; Ptr: PItemData; begin for i := 0 to HTMListbox1.Items.Count - 1 do begin Ptr := PItemData(HTMListbox1.Items.Objects[i]); if Ptr <> nil then Dispose(Ptr); end; end;
Tips:
- Manage memory for allocated objects carefully (dispose on form close).
- Consider using reference-counted objects (TObject descendants) instead of raw pointers.
Example 3 — Virtual mode for large datasets
When dealing with thousands of items, avoid storing full rendered content for every item. Use a virtual mode where the list requests item data only when needed (OnDataRequest/OnGetItem).
Conceptual steps:
- Set VirtualMode := True (or equivalent).
- Implement an event to supply item text/markup for a given index.
- Maintain a lightweight data source (array, database cursor, in-memory collection).
Pseudo-event:
procedure TForm1.HTMListbox1GetItemData(Sender: TObject; Index: Integer; out Text: string); begin Text := MyDataSource.GetFormattedText(Index); // generate markup on demand end;
Benefits:
- Lower memory usage.
- Faster startup and smoother scrolling.
- You can implement lazy-loading from disk or network.
Example 4 — Inline actions and hit testing
THTMListbox often supports hit-testing so you can respond differently depending on where the user clicks inside an item (for example, click on an image to open, click on text to select).
Example approach:
- In OnMouseDown or a dedicated OnClickRegion event, translate mouse coordinates to item index and region.
- Regions could be image bounds, link ranges, or custom-defined rectangles inside the item layout.
procedure TForm1.HTMListbox1MouseDown(Sender: TObject; Button: TMouseButton; Shift: TShiftState; X, Y: Integer); var Index: Integer; Region: TClickRegion; begin Index := HTMListbox1.ItemAtPos(Point(X,Y), True); if Index < 0 then Exit; Region := HTMListbox1.HitTestRegion(Index, X, Y); case Region of crImage: OpenImageForItem(Index); crLink: OpenLinkForItem(Index); crText: HTMListbox1.ItemIndex := Index; end; end;
Best practices:
- Provide clear visual affordances (hover cursor change, small icons) for clickable regions.
- Make touch-friendly targets if your app may run on tablets.
Performance tips
- Use virtual mode for large lists (>1k items).
- Cache measured heights and rendered bitmaps if item layout is complex.
- Batch UI updates: disable repaint/update while inserting many items (BeginUpdate/EndUpdate).
- Avoid complex per-item allocations during painting; reuse objects/buffers.
- Throttle expensive operations (network, disk) and populate items asynchronously.
Accessibility and keyboard support
- Ensure keyboard navigation (Up/Down, PageUp/PageDown, Home/End) behaves predictably.
- Provide accessibility names/labels for items when possible (for screen readers).
- Preserve focus management when items are added/removed programmatically.
Styling and theming
- Prefer style templates instead of hard-coded colors/fonts so themes can be applied globally.
- Respect user DPI and font scaling: measure text with current canvas metrics.
- Use vector icons (or appropriately scaled bitmaps) to remain crisp at different DPIs.
Testing and debugging strategies
- Test with different data shapes: empty lists, single item, thousands of items, long texts, missing images.
- Simulate slow I/O to ensure UI remains responsive (e.g., load images asynchronously).
- Add logging around critical events (OnDrawItem, OnDataRequest) to find bottlenecks.
- Use tools to profile painting and memory usage if you see leaks or slowdowns.
Common pitfalls and how to avoid them
- Memory leaks from per-item allocated structures — dispose or use managed objects.
- Doing heavy work in paint handlers — move logic to background threads and cache results.
- Not handling changes to font or DPI — re-measure and refresh layout when such changes occur.
- Assuming fixed item heights — account for dynamic heights or enforce a consistent height if simpler.
Integration examples
- Messaging client: show avatars, sender name, message preview, and inline timestamps; use virtual mode for chat history.
- File explorer: per-item icons, file name with highlighted search matches, file size/date metadata.
- Notification center: group items by date with collapsible groups and inline action buttons (snooze, dismiss).
Summary
THTMListbox offers powerful capabilities for creating visually rich and interactive lists. Use its markup and hit-testing to deliver polished UI, adopt virtual mode and caching to scale to large datasets, and attach metadata to items for robust app logic. Pay attention to performance, memory management, accessibility, and theming to ensure a reliable, maintainable implementation.