Crate i_slint_backend_testing

NOTE: This library is an internal crate of the Slint project. This crate should not be used directly by applications using Slint. You should use the slint crate instead.

WARNING: This crate does not follow the semver convention for versioning and can only be used with version = "=x.y.z" in Cargo.toml.

Preliminary Slint Testing API

This crate provides the preliminary API that we’re developing to enable different user interface (UI) testing scenarios for Slint applications.

To use this functionality, you need to be cautious when importing dependencies since this crate does not adhere to semver and may introduce breaking changes in any patch release. Additionally, the version of this crate must match the version of Slint. To indicate that you specifically want this version, include the = symbol in the version string.

[dependencies]
slint = { version = "x.y.z", ... }
i-slint-backend-testing = "=x.y.z"

Testing Backend

By default, Slint applications will select a backend and renderer suitable for application display on the screen, by means of utilizing a windowing system - if present - or directly rendering to the framebuffer.

For automated testing in CI environments without a windowing system / display, it might still be desirable to run tests. The Slint Testing Backend simulates a windowing system without requiring one: No pixels are rendered and text is measured by a fixed font size.

Use init_integration_test_with_system_time() for integration tests where your test code requires Slint to provide an event loop, for example when spawning threads and calling slint::invoke_from_event_loop(). If you want to not only simulate the windowing system but also the system time, use init_integration_test_with_mock_time() to initialize the backend and then call mock_elapsed_time() to advance animations and move timers closer to their next timeout.

Use init_no_event_loop() for unit tests when your test code does not require an event loop. Note that system time is also mocked in this scenario, so use mock_elapsed_time() to advance animations and timers.

Preliminary User Interface Testing API

We’re developing APIs to faciliate the creation of automated tests for Slint based UIs. A building block is the ability to introspect and modify the state from within what would be a regular application process.

A regular application might have a main() entry-point like so:

fn main() -> Result<(), slint::PlatformError>
{
    let app = App::new()?;
    // ... set up state, callbacks, models, ...

    app.run()
}

In addition, it may be desirable to create an integration test that verifies how the application behaves when simulating user input. The objective of the user interface testing API provided in this crate is to faciliate locating, mutation, and verifying state of elements in your UI. ElementHandle provides a view for these elements.

The example below assumes that somehwere in the UI you have declared a Button with the text “Submit” and you may want to verify how the application behaves when simulation the activation. This is done by locating and triggering it via its accessibility interface, that every Button implements.

import { Button } from "std-widgets.slint";
component Form {
    callback submit();
    VerticalLayout {
        // ...
        Button {
            text: "Submit";
            clicked => { root.submit(); }
        }
    }
}

export component App {
    callback submit <=> form.submit;
    // ...
    form := Form {
        // ...
    }
}

#[test]
fn test_basic_user_interface()
{
    i_slint_backend_testing::init_no_event_loop();
    let app = App::new().unwrap();
    // ... set up state, callbacks, models, ...
    let submitted = std::rc::Rc::new(std::cell::RefCell::new(false));

    app.on_submit({
        let submitted = submitted.clone();
        move || { *submitted.borrow_mut() = true; }
    });

    let buttons: Vec<_> = i_slint_backend_testing::ElementHandle::find_by_accessible_label(&app, "Submit").collect();
    assert_eq!(buttons.len(), 1);
    let button = &buttons[0];

    button.invoke_accessible_default_action();

    assert!(*submitted.borrow());
}

Simulating events / Asynchronous testing

When testing user interfaces it may be desirable to not only invoke accessible actions on elements, but it may also be useful to simulate touch or mouse input. For example a mouse click on a button is a sequence:

1. An initial mouse move event to a location over the button
2. A mouse press event.
3. In real life, a certain amount of time would elapse now.
4. Finally, the user lifts the finger again from the mouse and a mouse release event is triggered.

To simulate this behaviour, ElementHandle provides functions such as ElementHandle::single_click() and ElementHandle::double_click(). Since these functions simulate a sequence of events with a period of idle time between the events, these functions are async and return a std::future::Future, which resolves when the last event in the sequence was sent.

Calling these functions requires running the test function itself as a future and running an event loop in the background. This can be accomplished using slint::spawn_local(), slint::run_event_loop(), and slint::quit_event_loop(). The following example wraps the core functions for testing in an async closure:

use slint::platform::PointerEventButton;

#[test]
fn test_click() {
    i_slint_backend_testing::init_integration_test_with_system_time();

    slint::spawn_local(async move {
        slint::slint! {
            export component App inherits Window {
                out property <int> click-count: 0;
                ta := TouchArea {
                    clicked => { root.click-count += 1; }
                }
            }
        }

        let app = App::new().unwrap();

        let mut it = ElementHandle::find_by_element_id(&app, "App::ta");
        let elem = it.next().unwrap();
        assert!(it.next().is_none());

        assert_eq!(app.get_click_count(), 0);
        elem.single_click(PointerEventButton::Left).await;
        assert_eq!(app.get_click_count(), 1);

        slint::quit_event_loop().unwrap();
    })
    .unwrap();
    slint::run_event_loop().unwrap();
}

After initializing the testing backend with support for using the system time, an async closure is spawned, which does the actual testing. In the subsequent run_event_loop() call, the event loop is started, and that will start polling the async closure passed to spawn_local().

In this closure we can now call .await on the future ElementHandle::single_click() returns, which will keep running the event loop until the click is complete, and then continue with the test function.

Structs

• ElementHandle
  ElementHandle wraps an existing element in a Slint UI. An ElementHandle does not keep the corresponding element in the UI alive. Use Self::is_valid() to verify that it is still alive.
• ElementQuery
  Use ElementQuery to form a query into the tree of UI elements and then locate one or multiple matching elements.

Enums

• AccessibleRole
  This enum represents the different values for the accessible-role property, used to describe the role of an element in the context of assistive technology such as screen readers.

Traits

• ElementRoot
  Trait for type that can be searched for element. This is implemented for everything that implements ComponentHandle

Functions

• init_integration_test_with_mock_time
  Initialize the testing backend with support for simple event loop. This function can only be called once per process, so make sure to use integration tests with only one #[test] function. (Or in a doc test) Must be called before any call that would otherwise initialize the rendering backend. Calling it when the rendering backend is already initialized will panic.
• init_integration_test_with_system_time
  Initialize the testing backend with support for simple event loop. This function can only be called once per process, so make sure to use integration tests with only one #[test] function. (Or in a doc test) Must be called before any call that would otherwise initialize the rendering backend. Calling it when the rendering backend is already initialized will panic.
• init_no_event_loop
  Initialize the testing backend without support for event loop. This means that each test thread can use its own backend, but global functions that needs an event loop such as slint::invoke_from_event_loop or Timers won’t work. Must be called before any call that would otherwise initialize the rendering backend. Calling it when the rendering backend is already initialized will panic.
• mock_elapsed_time
  Advance the simulated mock time by the specified duration. Use in combination with init_integration_test_with_mock_time() or init_no_event_loop().