access_testing_window called without testing backend/adapter

Error message

access_testing_window called without testing backend/adapter

What it means

Error "access_testing_window called without testing backend/adapter" thrown in slint-ui/slint.

Source

Thrown at internal/backends/testing/internal_tests.rs:193

            position: i_slint_core::lengths::logical_point_from_api(
                i_slint_core::api::LogicalPosition::new(center_x, center_y),
            ),
            delta,
            phase,
        },
    ));
}

pub fn access_testing_window<R>(
    window: &i_slint_core::api::Window,
    callback: impl FnOnce(&TestingWindow) -> R,
) -> R {
    i_slint_core::window::WindowInner::from_pub(window)
        .window_adapter()
        .internal(i_slint_core::InternalToken)
        .and_then(|wa| (wa as &dyn core::any::Any).downcast_ref::<TestingWindow>())
        .map(callback)
        .expect("access_testing_window called without testing backend/adapter")
}

/// Runs a future to completion by polling the future and updating the mock time until the future is ready
pub fn block_on<R>(future: impl Future<Output = R>) -> R {
    let mut pinned = core::pin::pin!(future);
    let mut ctx = core::task::Context::from_waker(core::task::Waker::noop());
    loop {
        if let core::task::Poll::Ready(r) = pinned.as_mut().poll(&mut ctx) {
            return r;
        }
        let duration = i_slint_core::platform::duration_until_next_timer_update()
            .unwrap_or(core::time::Duration::from_secs(1));
        crate::testing_backend::mock_elapsed_time(duration.as_millis() as u64);
    }
}

View on GitHub (pinned to 3fd8f2ec03)

Solutions

Initialize the testing backend before accessing the testing window.
Verify the window was created by the testing adapter.

When it happens

Trigger: Thrown at internal/backends/testing/internal_tests.rs:193 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of slint-ui/slint@3fd8f2ec03 (2026-08-19). Data as JSON: /api/errors/a919c818a1f64a90. Report an issue: GitHub.