positron-data-grid-pattern

name	positron-data-grid-pattern
description	Use when building a list, table, grid, or virtualized scrolling UI in Positron, or touching `DataGridInstance` / `PositronDataGrid`. Avoids the common mistake of mediating props into the instance via useEffect.

Positron Data Grid Pattern

Positron has a virtualized grid system built around an abstract DataGridInstance and a renderer component <PositronDataGrid />. The pattern is non-obvious from outside, and the wrong instinct is to build a React wrapper component that takes props and pushes them into the instance via useEffect. Don't. This skill exists so you don't make that mistake.

The core insight

A DataGridInstance subclass IS the data grid.

The instance owns:

• Items / data (or knows how to fetch it)
• The item renderer
• Layout (column widths, row heights, overrides, pinning)
• Selection, cursor, scroll position
• Outbound events (onDidUpdate, custom events from the subclass)

<PositronDataGrid instance={...} /> is a thin React renderer over any DataGridInstance subclass. It does not own state. It just observes the instance and paints.

The pattern

The shared shape, regardless of data strategy:

// 1. Pick or build a DataGridInstance subclass.
//    - Embed strategy: subclass PositronListInstance, or DataGridInstance directly.
//    - fetchData strategy: subclass DataGridInstance and override fetchData().

// 2. The caller creates exactly one instance for the component's lifetime.
const [instance] = useState(() => new MySubclass({ /* options */ }));

// 3. The caller subscribes to instance events as needed.
useEffect(() => {
    const d = instance.onDidSomething(payload => /* ... */);
    return () => d.dispose();
}, [instance]);

// 4. Dispose on unmount.
useEffect(() => () => instance.dispose(), [instance]);

// 5. Render the grid.
return <PositronDataGrid instance={instance} />;

How data gets into the instance depends on the strategy:

• Embed strategy (e.g. PositronListInstance): the caller pushes items in via a setter the subclass exposes. Example:

  useEffect(() => {
      instance.setItems(items);
  }, [instance, items]);
  

  This setter is specific to PositronListInstance; other embed subclasses can expose whatever shape they want.

• fetchData strategy (e.g. TableDataDataGridInstance): the caller does not push data in. The subclass owns its data source - typically wired to a comm/backend in its constructor or via a method that connects it. The base class then calls fetchData(...) whenever the viewport needs cells, and the subclass populates its cache and returns.

Either way: no React wrapper component sits between the caller and the instance. The instance is the API surface.

The anti-pattern (do NOT do this)

Don't build <MyList items={...} renderItem={...} onActivate={...} /> as a React component that internally:

• holds the instance in useMemo / useState
• pushes every prop into the instance via useEffect(() => instance.setX(props.x), [props.x])
• mirrors the instance's events back out as React props

Symptoms that you're heading the wrong way:

• A pile of small useEffects that all just call setters on the instance.
• A useEffect that re-creates the instance when one of its construction options changes (and a paired disposal effect to handle the swap).
• Comments explaining why a ref or a thunk indirection is needed to keep a closure stable inside the instance.

If you see those, delete the wrapper. Let the caller drive the instance directly.

Two data strategies

A DataGridInstance subclass can manage data in one of two ways. Pick based on dataset size.

Strategy A: embed the data in memory

For small-to-medium lists where the entire dataset fits comfortably in memory.

• The subclass holds the items in a private field.
• Caller pushes in via a setter (e.g. setItems).
• fetchData() is implemented as a no-op.

Example: PositronListInstance.

Strategy B: implement fetchData for lazy/virtualized data

For huge datasets where holding everything in memory is infeasible. The base class calls fetchData(rowStartIndex, rowCount, columnStartIndex, columnCount) (or similar) when it needs the cells for the visible viewport.

• The subclass connects to a backend (data explorer comms, DuckDB, etc.).
• fetchData populates the cache for the requested window.
• cell(col, row) returns from the cache.
• The subclass typically owns its own schema/columns, not just rows.

Examples: TableDataDataGridInstance, TableSummaryDataGridInstance, InlineTableDataGridInstance, ColumnSelectorDataGridInstance.

You generally know which you need before writing a line: is the dataset bounded and small? Embed. Is it backed by a query/file/comm and possibly enormous? Implement fetchData.

Customizing grid behavior

The subclass passes options to super(...) in its constructor - column/row headers, scrollbars (and overscroll), pinning, selection, resizing, automatic layout, etc. The same <PositronDataGrid /> renderer adapts to all of these via the instance's configuration.

If you need a behavior the base class doesn't expose, the path is usually:

1. Add the option to DataGridInstance's options type and constructor.
2. Plumb it down to whatever subsystem needs it (LayoutManager, scroll, etc.).
3. Update existing subclasses if their defaults change.

This is invasive - many subclasses share the base - so weigh the cost before going there.

Existing subclasses to read for reference

• PositronListInstance - single-column virtualized list (embed strategy).
• TableDataDataGridInstance - main data explorer table (fetchData strategy).
• TableSummaryDataGridInstance - column summary panel (fetchData strategy).
• InlineTableDataGridInstance - inline data preview in notebook outputs (fetchData strategy).
• ColumnSelectorDataGridInstance - column picker in modals (fetchData strategy).

Skim two before writing a new one.

Quick checklist when adding a new grid-style UI

• Is there an existing subclass that fits? (Don't reinvent.)
• If new: subclass DataGridInstance (or an existing subclass), implement what's required.
• Pick embed vs. fetchData based on dataset size.
• Caller creates the instance once via useState(() => new MySubclass(...)).
• Caller pushes in data via setters, subscribes to events, disposes on unmount.
• Render with <PositronDataGrid instance={instance} />.
• No React wrapper component mediating props into the instance.