plushie/testing/backend

Test backend: record-of-functions defining backend capabilities.

The TestBackend type is a record of functions that implement each testing operation (find, click, type_text, etc.). This design lets tests be polymorphic over the backend – swap mock() for headless() or windowed() and the same test code runs at different fidelity levels.

Backend variants

• mock(): pure Gleam, no renderer process. Runs the Elm loop directly. Fast, deterministic, no external dependencies. Best for unit-testing logic, tree structure, and model state.
• headless(): starts the Rust binary with --headless for software rendering. Supports screenshots and pixel-level assertions. Requires the plushie binary on PATH or configured.
• windowed(): starts the Rust binary with GPU rendering and a visible window. For manual visual verification and interactive debugging.

Concurrency

Mock backends are synchronous – each operation completes immediately in the calling process. Headless and windowed backends communicate with a renderer actor over OTP messages, so operations involve a process.call with a timeout. All backends are single-session; concurrent access to the same session is not supported.

pub type Selector {
  ById(String)
  ByRole(String)
  ByLabel(String)
  Focused
}

pub type TestBackend(model) {
  TestBackend(
    start: fn(app.App(model, event.Event)) -> session.TestSession(
      model,
      event.Event,
    ),
    stop: fn(session.TestSession(model, event.Event)) -> Nil,
    find: fn(session.TestSession(model, event.Event), String) -> option.Option(
      element.Element,
    ),
    click: fn(session.TestSession(model, event.Event), String) -> session.TestSession(
      model,
      event.Event,
    ),
    type_text: fn(
      session.TestSession(model, event.Event),
      String,
      String,
    ) -> session.TestSession(model, event.Event),
    submit: fn(session.TestSession(model, event.Event), String) -> session.TestSession(
      model,
      event.Event,
    ),
    toggle: fn(session.TestSession(model, event.Event), String) -> session.TestSession(
      model,
      event.Event,
    ),
    select: fn(
      session.TestSession(model, event.Event),
      String,
      String,
    ) -> session.TestSession(model, event.Event),
    slide: fn(
      session.TestSession(model, event.Event),
      String,
      Float,
    ) -> session.TestSession(model, event.Event),
    model: fn(session.TestSession(model, event.Event)) -> model,
    tree: fn(session.TestSession(model, event.Event)) -> node.Node,
    reset: fn(session.TestSession(model, event.Event)) -> session.TestSession(
      model,
      event.Event,
    ),
    send_event: fn(
      session.TestSession(model, event.Event),
      event.Event,
    ) -> session.TestSession(model, event.Event),
  )
}

pub fn mock() -> TestBackend(model)