virtualizer

A headless virtualization engine inspired by TanStack Virtual.

This crate provides the core algorithms needed to virtualize massive lists at interactive frame rates:

• Prefix sums over item sizes (Fenwick tree).
• Fast offset → index lookup.
• Overscanned visible ranges.
• Optional dynamic measurement via measure / resize_item.
• Scroll-to-index helpers (scroll_to_index_offset, Align).

It is UI-agnostic and can be used in TUIs/GUI frameworks.

Usage

use virtualizer::{Align, Virtualizer, VirtualizerOptions};

let mut v = Virtualizer::new(VirtualizerOptions::new(1_000_000, |_| 1));
v.set_viewport_size(20);
v.set_scroll_offset(100);

let mut items = Vec::new();
v.for_each_virtual_item(|it| items.push(it));
assert!(!items.is_empty());

let off = v.scroll_to_index_offset(1234, Align::Start);
v.set_scroll_offset(off);

If you want clamped scroll offsets (useful for scroll-to helpers), use set_scroll_offset_clamped or clamp_scroll_offset.

If your UI layer wants keys alongside items, use for_each_virtual_item_keyed.

If you want to scroll to a raw offset (TanStack scrollToOffset), set it via set_scroll_offset_clamped (or set_scroll_offset if you want to handle clamping yourself).

For an adapter-style walkthrough (rect/scroll events/dynamic measurement), see examples/adapter_sim.rs (cargo run -p virtualizer --example adapter_sim).

For more end-to-end adapter scenarios (tween scrolling, pinned headers, keyed reorders, dynamic measurement), see examples/README.md.

If you want adapter-level utilities (scroll anchoring for prepend, tween helpers), see the optional virtualizer-adapter crate in this workspace (../virtualizer-adapter/).

Reusing buffers (allocation-friendly)

If you call for_each_virtual_item every frame, consider reusing a Vec to avoid per-frame allocations:

use virtualizer::{Virtualizer, VirtualizerOptions};

let v = Virtualizer::new(VirtualizerOptions::new(10_000, |_| 1));
let mut items = Vec::new();
items.clear();
v.for_each_virtual_item(|it| items.push(it));

Similar APIs exist for keys and indexes: for_each_virtual_item_keyed and for_each_virtual_index.

Pinned/sticky rows (custom index selection)

If you need pinned/sticky rows (e.g. headers), set range_extractor to emit the exact set of indexes you want to render. The extractor is zero-allocation: it receives an emit callback.

Use IndexEmitter to make this ergonomic and correct (sorted + deduped):

use std::sync::Arc;
use virtualizer::{IndexEmitter, Range, VirtualizerOptions};

let mut opts = VirtualizerOptions::new(1000, |_| 1);
opts.range_extractor = Some(Arc::new(|r: Range, emit: &mut dyn FnMut(usize)| {
    let mut e = IndexEmitter::new(r, emit);
    e.emit_pinned(0);
    e.emit_overscanned();
}));

Custom keys

Use VirtualizerOptions::new_with_key if you want a non-u64 key type (e.g. for stable caching across reorders in your UI layer).

use virtualizer::{Virtualizer, VirtualizerOptions};

let opts = VirtualizerOptions::new_with_key(100, |_| 1, |i| format!("row-{i}"));
let v: Virtualizer<String> = Virtualizer::new(opts);
assert_eq!(v.key_for(7), "row-7");

Note: the key type must implement Hash + Eq with the default std feature, and Ord when built with --no-default-features (no_std + alloc).

If your data is reordered/changed while count stays the same, call sync_item_keys so measured sizes follow item keys. If you update get_item_key itself via set_get_item_key, the virtualizer automatically rebuilds per-index sizes from the key-based cache.

use virtualizer::{Virtualizer, VirtualizerOptions};

let mut v = Virtualizer::new(VirtualizerOptions::new(2, |_| 1));
v.measure(0, 10);
v.set_get_item_key(|i| if i == 0 { 1 } else { 0 }); // reorder
assert_eq!(v.item_size(1), Some(10));

Scroll margin

If your scroll offset is measured from a larger scroll container (e.g. window scrolling) and the list starts after some header/content, set scroll_margin so virtual item start values match the scroll container coordinate system:

use virtualizer::{Virtualizer, VirtualizerOptions};

let mut opts = VirtualizerOptions::new(10_000, |_| 35);
opts.scroll_margin = 50; // list begins 50px below scroll origin
let mut v = Virtualizer::new(opts);
v.set_viewport_and_scroll(500, 0);

Rects

TanStack Virtual exposes scrollRect and an initialRect option. This crate provides a platform- agnostic Rect { main, cross }:

• main: the virtualized axis size (e.g. height for vertical lists)
• cross: the cross axis size (e.g. width for vertical lists)

Use VirtualizerOptions::with_initial_rect(Some(rect)) for an initial value (SSR-like setups), and call set_scroll_rect(rect) from your adapter to keep it up-to-date.

Initial offset

TanStack Virtual allows initialOffset to be a number or a function. This crate uses InitialOffset:

• InitialOffset::Value(123)
• InitialOffset::Provider(|| 123)

Dynamic measurement

measure updates sizes but does not adjust scroll position. resize_item returns the applied scroll adjustment (delta) and also updates the virtualizer's internal scroll_offset to prevent jumps when items before the viewport change size.

To reset all measurements (TanStack measure()), call reset_measurements.

To persist and restore measurements (key → size), use export_measurement_cache / import_measurement_cache (see examples/measurement_cache.rs).

State & callbacks

If you want TanStack-style change notifications, set on_change (or call set_on_change) and drive is_scrolling via set_is_scrolling from your framework adapter.

For TanStack parity, you can also use notify_scroll_event(now_ms) + update_scrolling(now_ms) to implement isScrollingResetDelay behavior without requiring timers in the core.

If your adapter updates multiple pieces of state in a single frame (common in UI event handlers), use batch_update to coalesce them into a single on_change callback:

v.batch_update(|v| {
    v.set_scroll_rect(rect);
    v.set_scroll_offset_clamped(offset);
    v.notify_scroll_event(now_ms);
});

For even more ergonomic adapter code, you can use the higher-level "event" entry points which internally batch and mark scrolling:

• apply_scroll_offset_event / apply_scroll_offset_event_clamped
• apply_scroll_frame / apply_scroll_frame_clamped

no_std + alloc

Disable default features to build in no_std environments (requires alloc):

cargo build --no-default-features

License

Dual-licensed under MIT OR Apache-2.0. See LICENSE-MIT and LICENSE-APACHE.