Backends

Knopper renders to a TerminalBackend. Two ship today; the trait is the extension point (a future web/Sixel backend implements the same surface).

TerminalBackend

#![allow(unused)]
fn main() {
pub trait TerminalBackend {
    type Error;
    fn execute(&mut self, commands: &[BackendCommand]) -> Result<(), Self::Error>;
    fn sync_order(&mut self, order: &[NodeId]) -> Result<(), Self::Error>;
}
}

execute applies the translated patch commands; sync_order communicates the front-to-back stacking order (for backends with real z-ordering, like notcurses planes). The runtime calls both on every commit.

MockBackend

In-memory backend that records executed commands and exposes a queryable BackendState (entries by NodeId, cursor, surface count). This is the test backend — the entire CI suite runs against it, and host test harnesses should too.

Also available: MockRenderer for the simpler Renderer trait path.

NotcursesBackend (feature notcurses)

Real terminal rendering via notcurses planes — one plane per scene node, synced in render order. Requires the system notcurses library ≥ 3.0.11 (libnotcurses-dev on Ubuntu 26.04+; note Ubuntu 24.04's 3.0.7 is too old). The crate's vendored cargo feature is a docs-only path and does not link — install the system library.

knopper = { version = "0.1", features = ["notcurses"] }

Supporting types

• BackendCommand — the instruction set (create/update/remove surface, draw text/border, set cursor, …)
• BackendEntry / BackendState — recorded backend state (mock inspection)
• backend_commands(previous, patches) — the patch → command translation, public for custom backends