xa11y_core/

provider.rs

use crate::element::ElementData;
use crate::error::{Error, Result};
use crate::event_provider::Subscription;
use crate::selector::{matches_simple, Combinator, Selector, SelectorGroup, SelectorSegment};
use crate::shell::ShellSurfaceKind;

/// Platform backend trait for accessibility tree access.
///
/// Providers implement lazy, on-demand tree navigation. Elements are identified
/// by their [`ElementData`] (which contains a provider-specific `handle` for
/// looking up the underlying platform object).
///
/// # Action model
///
/// Common actions are first-class methods with proper typed signatures.
/// Platform-specific or custom actions use [`perform_action`](Self::perform_action)
/// as an escape hatch — it takes a `snake_case` action name string.
///
/// Providers should check platform permissions in their constructor (`new()`)
/// and return [`Error::PermissionDenied`](crate::Error::PermissionDenied) if
/// required permissions are not granted.
pub trait Provider: Send + Sync {
    // ── Tree navigation ─────────────────────────────────────────────

    /// Get direct children of an element.
    ///
    /// If `element` is `None`, returns top-level application elements.
    fn get_children(&self, element: Option<&ElementData>) -> Result<Vec<ElementData>>;

    /// List an application's windows. Backends whose native window list is
    /// wider than their accessibility tree roots can override this.
    fn app_windows(&self, app: &ElementData) -> Result<Vec<ElementData>> {
        self.get_children(Some(app))
    }

    /// Get the parent of an element.
    ///
    /// Returns `None` for top-level (application) elements.
    fn get_parent(&self, element: &ElementData) -> Result<Option<ElementData>>;

    /// Enumerate top-level applications visible to this provider.
    ///
    /// Backends return one `ElementData` per application, always with
    /// `role=Application` as the **enumeration** contract. The direct-attach
    /// path ([`app_by_pid`](Self::app_by_pid)) is the one exception: macOS
    /// deliberately preserves `Role::Unknown` for a process that exposes no
    /// AX application (Window Server), so that `windows()` fails honestly
    /// instead of pretending the process has no windows. Windows synthesizes
    /// one Application node per
    /// process (UIA exposes applications as their top-level windows; the
    /// provider groups them by pid — and calls the process's first window the
    /// node's representative — see `xa11y-windows`). AT-SPI can return several
    /// native registrations sharing a pid; the core collapses those into one
    /// public `App` and uses [`app_roots`](Self::app_roots) for process-wide
    /// traversal. The application's top-level
    /// windows are its `app_windows` results, filtered to `Window|Dialog`.
    /// This is the dedicated discovery primitive: it replaces the previous
    /// `find_elements(None, application_selector, .., depth=0)` idiom and
    /// lets each backend batch the platform-specific enumeration (CGWindowList
    /// on macOS, the AT-SPI registry on Linux, UIA's desktop root on Windows)
    /// in one place.
    ///
    /// Required: every `Provider` implements this explicitly. There's no
    /// default impl — discovery and subtree search have different cost
    /// profiles, and silently routing app discovery through `get_children`
    /// would hide the difference from implementors.
    fn list_apps(&self) -> Result<Vec<ElementData>>;

    /// Whether [`list_apps`](Self::list_apps) can expose more than one
    /// `Application` entry for a single process id.
    ///
    /// Only the AT-SPI backend can (a per-process entry plus per-event-loop
    /// pieces such as a tray or a tooltip context); macOS and Windows
    /// guarantee one entry per pid. When `false`, [`App::windows_with`]
    /// skips the same-pid merge — and the full `list_apps` re-enumeration it
    /// requires — because the calling entry's descendants already cover the
    /// process, so no cross-entry merge can add anything.
    ///
    /// Defaults to `false`. A provider that overrides this with `true` must
    /// answer `get_children` per same-pid entry and give each entry (and its
    /// windows) a stable identity durable across `list_apps` calls, since
    /// the merge deduplicates by that identity.
    fn splits_app_across_entries(&self) -> bool {
        false
    }

    /// Return every native application root that belongs to the same process
    /// as `app`.
    ///
    /// Most accessibility APIs expose exactly one root per process, so the
    /// default is the supplied root. AT-SPI providers override this because a
    /// process may register several independent roots.
    fn app_roots(&self, app: &ElementData) -> Result<Vec<ElementData>> {
        Ok(vec![app.clone()])
    }

    /// Find the application owning process `pid` — one attempt, no polling.
    ///
    /// This is the single-shot discovery primitive behind `App::by_pid`; the
    /// core's lookup loop owns the retry/timeout policy. Returns
    /// [`Error::SelectorNotMatched`] when no accessible application for `pid`
    /// exists *yet* — the signal the poll loop retries on — and any other
    /// error when the lookup itself failed (permissions, platform API
    /// failure), which short-circuits the poll.
    ///
    /// The default implementation filters [`list_apps`](Self::list_apps) by
    /// `pid`, and its not-matched error carries enumeration diagnostics (how
    /// many apps were visible and how many had no resolvable pid) so a
    /// timeout on an opaque CI runner says *why* the app was invisible.
    /// Backends override this where the platform supports reaching a process
    /// directly (`AXUIElementCreateApplication` on macOS, a UIA `ProcessId`
    /// property search on Windows): direct attach is immune to enumeration
    /// blind spots such as top-level windows that are still unnamed while the
    /// app starts up.
    fn app_by_pid(&self, pid: u32) -> Result<ElementData> {
        let apps = self.list_apps()?;
        let total = apps.len();
        let unresolved = apps.iter().filter(|a| a.pid.is_none()).count();
        if let Some(data) = apps.into_iter().find(|a| a.pid == Some(pid)) {
            return Ok(data);
        }
        // The enumeration counts are structured diagnosis fields (tenet 6),
        // not prose baked into the selector — bindings expose them as
        // attributes, and `App::by_pid_with` merges in the full candidate
        // list at its terminal failure site. Cheap to build (two counters),
        // so attaching it per poll tick is fine.
        Err(
            Error::selector_not_matched(format!("application[pid={pid}]")).diagnose(
                crate::error::Diagnosis {
                    last_observed: Some(format!(
                        "enumerated {total} running apps, {unresolved} without a resolvable pid"
                    )),
                    ..Default::default()
                },
            ),
        )
    }

    /// Identify the application that currently holds the system foreground /
    /// input focus — one attempt, no polling.
    ///
    /// Each backend uses its platform's canonical foreground mechanism: the
    /// system-wide `AXUIElement`'s focused-application attribute (macOS),
    /// `GetForegroundWindow` + `ElementFromHandle` (Windows), and the focused
    /// element's `Application` ancestor in the AT-SPI registry (Linux). The
    /// returned `ElementData` has the same shape [`list_apps`](Self::list_apps)
    /// produces for that app — in particular its `pid` lines up — so the core
    /// can tag the foreground entry within an enumeration by pid.
    ///
    /// Returns [`Error::SelectorNotMatched`] when no application currently
    /// holds focus (e.g. focus rests on the desktop / shell, or the screen is
    /// locked). That is a legitimate state, not a failure: the core treats it
    /// as "nothing is foreground" — every listed app stays untagged — rather
    /// than surfacing an error. Any other error (permission, platform API
    /// failure) is a real failure and propagates.
    ///
    /// Required: every `Provider` implements this explicitly. Like
    /// [`list_apps`](Self::list_apps) there is no default impl — there is no
    /// portable way to derive the foreground app, so a silent no-op default
    /// would hide an unimplemented backend (tenet 1).
    fn focused_app(&self) -> Result<ElementData>;

    /// Enumerate OS shell surfaces — taskbars, panels, docks, menu bars,
    /// status items, the desktop, and any transient shell flyout currently
    /// open. One entry per surface, each a real platform element usable as a
    /// search root.
    ///
    /// The listing is live: transient surfaces
    /// ([`ShellSurfaceKind::Flyout`]) appear only while they are on screen,
    /// and enumerating NEVER opens, closes, or presses anything. Required —
    /// no default impl, for the same reason [`list_apps`](Self::list_apps)
    /// has none: enumeration is platform-specific, and a silent empty default
    /// would hide an unimplemented backend (tenet 1).
    ///
    /// The kind is part of the primitive's contract, so it travels in the
    /// signature where the compiler sees it rather than being smuggled
    /// through `raw`. Backends return the platform root untouched;
    /// `ShellSurface::list_with` additionally stamps `raw["shell_kind"]` on
    /// the surface root, in one place, so a consumer holding only an
    /// [`Element`](crate::Element) can read the kind back. That stamp is a
    /// readback, not a selector: it is on the root alone, a rooted locator
    /// only emits descendants of its root, and `TreeNode` carries no `raw`.
    fn list_shell_surfaces(&self) -> Result<Vec<(ShellSurfaceKind, ElementData)>>;

    /// Search for elements matching a selector.
    ///
    /// The selector is already parsed by the core — providers match against it
    /// during traversal and can prune subtrees that can't match.
    ///
    /// `root` is the subtree root to search under (always present; use
    /// [`list_apps`](Self::list_apps) to discover application roots first).
    /// If `limit` is `Some(n)`, stops after finding `n` matches.
    /// If `max_depth` is `Some(d)`, does not descend deeper than `d` levels.
    ///
    /// This is a thin convenience wrapper that re-uses
    /// [`find_elements_group`](Self::find_elements_group) with a single-clause
    /// group. Backends do **not** override this method — they override
    /// `find_elements_group`, and the single-clause case runs through the
    /// same native code path as multi-clause queries.
    fn find_elements(
        &self,
        root: &ElementData,
        selector: &Selector,
        limit: Option<usize>,
        max_depth: Option<u32>,
    ) -> Result<Vec<ElementData>> {
        let group = SelectorGroup {
            clauses: vec![selector.clone()],
        };
        self.find_elements_group(root, &group, limit, max_depth)
    }

    /// Search for elements matching any clause of a comma-separated selector
    /// group, scoped to `root`'s subtree.
    ///
    /// This is the **primary search primitive** each backend should override.
    /// A native implementation performs ONE platform-level subtree query/walk
    /// (e.g. `FindAllBuildCache(TreeScope_Subtree)` on Windows, one DFS over
    /// AT-SPI children on Linux, one AX walk on macOS) and evaluates every
    /// clause inline against each visited element. This avoids the
    /// per-clause-walk perf cliff and also keeps the cross-clause dedup
    /// correct: because everything happens inside one call, each platform
    /// node is visited once and platform identity is stable.
    ///
    /// `root` is non-optional: app discovery now goes through
    /// [`list_apps`](Self::list_apps) and any rootless multi-app search is
    /// performed by the caller (e.g. `Locator`) by enumerating apps and
    /// invoking this method per app.
    ///
    /// The default implementation traverses via [`get_children`](Self::get_children)
    /// and uses tree-path identity for cross-clause merging. It's the
    /// fallback for backends that haven't shipped a native override yet —
    /// the same path the test/mock provider exercises.
    fn find_elements_group(
        &self,
        root: &ElementData,
        group: &SelectorGroup,
        limit: Option<usize>,
        max_depth: Option<u32>,
    ) -> Result<Vec<ElementData>> {
        crate::selector::find_elements_in_tree_group(
            |el| self.get_children(el),
            Some(root),
            group,
            limit,
            max_depth,
        )
    }

    /// Narrow candidates through remaining selector segments (Child/Descendant
    /// combinators), deduplicate, apply final :nth and limit.
    fn narrow_multi_segment(
        &self,
        mut candidates: Vec<ElementData>,
        segments: &[SelectorSegment],
        max_depth: u32,
        limit: Option<usize>,
    ) -> Result<Vec<ElementData>> {
        for segment in segments {
            let mut next_candidates = Vec::new();
            for candidate in &candidates {
                match segment.combinator {
                    Combinator::Child => {
                        let children = self.get_children(Some(candidate))?;
                        for child in children {
                            if matches_simple(&child, &segment.simple) {
                                next_candidates.push(child);
                            }
                        }
                    }
                    Combinator::Descendant => {
                        let sub_selector = Selector {
                            segments: vec![SelectorSegment {
                                combinator: Combinator::Root,
                                simple: segment.simple.clone(),
                            }],
                        };
                        let mut sub_results =
                            self.find_elements(candidate, &sub_selector, None, Some(max_depth))?;
                        next_candidates.append(&mut sub_results);
                    }
                    Combinator::Root => unreachable!(),
                }
            }
            let mut seen = std::collections::HashSet::new();
            next_candidates.retain(|e| seen.insert(e.handle));
            candidates = next_candidates;
        }

        // Apply :nth on last segment
        if let Some(nth) = segments.last().and_then(|s| s.simple.nth) {
            if nth <= candidates.len() {
                candidates = vec![candidates.remove(nth - 1)];
            } else {
                candidates.clear();
            }
        }

        if let Some(limit) = limit {
            candidates.truncate(limit);
        }

        Ok(candidates)
    }

    // ── Common actions ──────────────────────────────────────────────

    /// Click / tap / invoke the element.
    fn press(&self, element: &ElementData) -> Result<()>;

    /// Set keyboard focus to the element.
    fn focus(&self, element: &ElementData) -> Result<()>;

    /// Remove keyboard focus from the element.
    fn blur(&self, element: &ElementData) -> Result<()>;

    /// Toggle a checkbox or switch.
    fn toggle(&self, element: &ElementData) -> Result<()>;

    /// Select an item in a list, tab group, or menu.
    fn select(&self, element: &ElementData) -> Result<()>;

    /// Expand a collapsible element (combo box, tree item, disclosure).
    fn expand(&self, element: &ElementData) -> Result<()>;

    /// Collapse an expanded element.
    fn collapse(&self, element: &ElementData) -> Result<()>;

    /// Show the element's context menu or dropdown.
    fn show_menu(&self, element: &ElementData) -> Result<()>;

    /// Increment a slider or spinner by one step.
    fn increment(&self, element: &ElementData) -> Result<()>;

    /// Decrement a slider or spinner by one step.
    fn decrement(&self, element: &ElementData) -> Result<()>;

    /// Scroll the element into the visible area.
    fn scroll_into_view(&self, element: &ElementData) -> Result<()>;

    // ── Typed operations ────────────────────────────────────────────

    /// Set the text value of the element.
    fn set_value(&self, element: &ElementData, value: &str) -> Result<()>;

    /// Set the numeric value of the element (slider, spinner).
    fn set_numeric_value(&self, element: &ElementData, value: f64) -> Result<()>;

    /// Insert text at the current cursor position.
    fn type_text(&self, element: &ElementData, text: &str) -> Result<()>;

    /// Select a text range (0-based character offsets).
    fn set_text_selection(&self, element: &ElementData, start: u32, end: u32) -> Result<()>;

    // ── Generic action escape hatch ─────────────────────────────────

    /// Perform an action by `snake_case` name.
    ///
    /// This is the escape hatch for platform-specific actions not covered by
    /// the first-class methods above. The provider converts the name to the
    /// platform's convention (e.g. `"custom_thing"` → `"AXCustomThing"` on
    /// macOS) and invokes it.
    ///
    /// Well-known action names (`"press"`, `"focus"`, etc.) should also work
    /// here — providers should delegate to the corresponding method.
    fn perform_action(&self, element: &ElementData, action: &str) -> Result<()>;

    // ── Window management ──────────────────────────────────────────

    /// `App::windows` filters `app_windows` to `Role::Window | Role::Dialog`.
    /// By default `app_windows` returns the Application node's children.
    /// Windows overrides it because native dialogs can be UIA descendants
    /// of another window; listing them does not make them separate roots
    /// for an app-wide tree walk.
    ///
    /// Linux refines the contract: an app may register several AT-SPI
    /// Application entries (per-process instance plus per-event-loop pieces
    /// such as a tray or the GTK IM context), so `App::windows_with` starts
    /// from this entry's filtered children, then merges the filtered children
    /// of every same-pid Application entry and deduplicates by stable id (see
    /// `same_window_identity`). The result is process-complete rather than a
    /// strict direct-children filter, and merged windows are not this node's
    /// children, so no single z-order spans them. Providers that split an app
    /// across entries must answer `app_windows` per entry; the merge is
    /// core's job.
    ///
    /// The filter lives in `App::windows_with`; backends only override
    /// `app_windows` when native window listing differs from tree navigation.
    ///
    /// Activate the window: bring it to the foreground and give it focus.
    ///
    /// Window verbs may use the platform accessibility API or a native
    /// window-manager protocol. A native provider must establish a durable
    /// association from this accessibility element to exactly one native
    /// window before it mutates state; process lifetime, destroyed/reused
    /// identifiers, and same-process ambiguity are errors rather than guesses.
    /// Titles are never sufficient identity by themselves.
    ///
    /// Unless a backend documents a stronger postcondition, success means the
    /// semantic platform request was accepted for delivery. Window managers
    /// commonly apply it asynchronously; consumers can re-read the element's
    /// tri-state window flags or bounds to observe completion. Unsupported
    /// capabilities and refused requests must remain distinct errors, and a
    /// backend must never substitute simulated input.
    ///
    /// Required: every `Provider` implements this explicitly. There is no
    /// portable default — each platform's activation mechanism is unique
    /// (AXRaise on macOS, `SetForegroundWindow` on Windows, GrabFocus on
    /// Linux), and a silent no-op default would hide an unimplemented
    /// backend (tenet 1).
    fn activate(&self, element: &ElementData) -> Result<()>;

    /// Minimize the window.
    fn minimize(&self, element: &ElementData) -> Result<()>;

    /// Maximize the window.
    ///
    /// Distinct from [`Self::enter_fullscreen`]: maximizing leaves the window
    /// in its platform's maximized state (the macOS zoom equivalent, Windows
    /// `WindowVisualState_Maximized`) while fullscreen is the native
    /// fullscreen state. A platform without the operation fails surfaceably
    /// rather than substituting the other.
    fn maximize(&self, element: &ElementData) -> Result<()>;

    /// Put the window in native fullscreen.
    ///
    /// Distinct from [`Self::maximize`]: fullscreen is the platform's native
    /// fullscreen state (macOS `AXFullScreen`), not a maximized window.
    /// [`Self::restore`] clears it. A platform without an accessibility API
    /// for fullscreen fails surfaceably.
    fn enter_fullscreen(&self, element: &ElementData) -> Result<()>;

    /// Restore the window to its normal state (from minimized/maximized/fullscreen).
    fn restore(&self, element: &ElementData) -> Result<()>;

    /// Close the window.
    fn close(&self, element: &ElementData) -> Result<()>;

    /// Move the window in the coordinates of [`ElementData::bounds`]
    /// (physical pixels on Windows, logical points elsewhere).
    fn move_to(&self, element: &ElementData, x: i32, y: i32) -> Result<()>;

    /// Resize the window in the units of [`ElementData::bounds`].
    fn resize_to(&self, element: &ElementData, width: u32, height: u32) -> Result<()>;

    // ── Events ──────────────────────────────────────────────────────

    /// Subscribe to all accessibility events for an application.
    ///
    /// The element should be an application-level element (role=Application).
    /// The provider extracts the PID from `element.pid`.
    ///
    /// Returns a [`Subscription`] that receives events until dropped.
    fn subscribe(&self, element: &ElementData) -> Result<Subscription>;
}

// Blanket impl so shared references to a provider act as providers themselves.
// Used by the umbrella crate's singleton (a `&'static dyn Provider` wrapped in
// `Arc<_>`) and by any caller that wants to share a provider via `&T`. The
// orphan rules keep this collision-free for downstream crates because `xa11y-core`
// owns the `Provider` trait.
impl<T: Provider + ?Sized> Provider for &T {
    fn get_children(&self, element: Option<&ElementData>) -> Result<Vec<ElementData>> {
        (**self).get_children(element)
    }
    fn app_windows(&self, app: &ElementData) -> Result<Vec<ElementData>> {
        (**self).app_windows(app)
    }
    fn get_parent(&self, element: &ElementData) -> Result<Option<ElementData>> {
        (**self).get_parent(element)
    }
    fn list_apps(&self) -> Result<Vec<ElementData>> {
        (**self).list_apps()
    }
    // Delegated explicitly (despite having a default impl) so a concrete
    // provider's override isn't bypassed when it's used through a shared
    // reference — the default body on `&T` would re-route and silently
    // answer `false` for a Linux provider passed by reference.
    fn splits_app_across_entries(&self) -> bool {
        (**self).splits_app_across_entries()
    }
    fn app_roots(&self, app: &ElementData) -> Result<Vec<ElementData>> {
        (**self).app_roots(app)
    }
    // Delegated explicitly (despite having a default impl) so a concrete
    // provider's PID-direct override isn't bypassed when it's used through a
    // shared reference — the default body on `&T` would re-route through
    // `list_apps` and silently lose the override.
    fn app_by_pid(&self, pid: u32) -> Result<ElementData> {
        (**self).app_by_pid(pid)
    }
    fn focused_app(&self) -> Result<ElementData> {
        (**self).focused_app()
    }
    fn list_shell_surfaces(&self) -> Result<Vec<(ShellSurfaceKind, ElementData)>> {
        (**self).list_shell_surfaces()
    }
    fn find_elements(
        &self,
        root: &ElementData,
        selector: &Selector,
        limit: Option<usize>,
        max_depth: Option<u32>,
    ) -> Result<Vec<ElementData>> {
        (**self).find_elements(root, selector, limit, max_depth)
    }
    fn find_elements_group(
        &self,
        root: &ElementData,
        group: &SelectorGroup,
        limit: Option<usize>,
        max_depth: Option<u32>,
    ) -> Result<Vec<ElementData>> {
        (**self).find_elements_group(root, group, limit, max_depth)
    }
    fn narrow_multi_segment(
        &self,
        candidates: Vec<ElementData>,
        segments: &[SelectorSegment],
        max_depth: u32,
        limit: Option<usize>,
    ) -> Result<Vec<ElementData>> {
        (**self).narrow_multi_segment(candidates, segments, max_depth, limit)
    }
    fn press(&self, element: &ElementData) -> Result<()> {
        (**self).press(element)
    }
    fn focus(&self, element: &ElementData) -> Result<()> {
        (**self).focus(element)
    }
    fn blur(&self, element: &ElementData) -> Result<()> {
        (**self).blur(element)
    }
    fn toggle(&self, element: &ElementData) -> Result<()> {
        (**self).toggle(element)
    }
    fn select(&self, element: &ElementData) -> Result<()> {
        (**self).select(element)
    }
    fn expand(&self, element: &ElementData) -> Result<()> {
        (**self).expand(element)
    }
    fn collapse(&self, element: &ElementData) -> Result<()> {
        (**self).collapse(element)
    }
    fn show_menu(&self, element: &ElementData) -> Result<()> {
        (**self).show_menu(element)
    }
    fn increment(&self, element: &ElementData) -> Result<()> {
        (**self).increment(element)
    }
    fn decrement(&self, element: &ElementData) -> Result<()> {
        (**self).decrement(element)
    }
    fn scroll_into_view(&self, element: &ElementData) -> Result<()> {
        (**self).scroll_into_view(element)
    }
    fn set_value(&self, element: &ElementData, value: &str) -> Result<()> {
        (**self).set_value(element, value)
    }
    fn set_numeric_value(&self, element: &ElementData, value: f64) -> Result<()> {
        (**self).set_numeric_value(element, value)
    }
    fn type_text(&self, element: &ElementData, text: &str) -> Result<()> {
        (**self).type_text(element, text)
    }
    fn set_text_selection(&self, element: &ElementData, start: u32, end: u32) -> Result<()> {
        (**self).set_text_selection(element, start, end)
    }
    fn perform_action(&self, element: &ElementData, action: &str) -> Result<()> {
        (**self).perform_action(element, action)
    }
    fn activate(&self, element: &ElementData) -> Result<()> {
        (**self).activate(element)
    }
    fn minimize(&self, element: &ElementData) -> Result<()> {
        (**self).minimize(element)
    }
    fn maximize(&self, element: &ElementData) -> Result<()> {
        (**self).maximize(element)
    }
    fn enter_fullscreen(&self, element: &ElementData) -> Result<()> {
        (**self).enter_fullscreen(element)
    }
    fn restore(&self, element: &ElementData) -> Result<()> {
        (**self).restore(element)
    }
    fn close(&self, element: &ElementData) -> Result<()> {
        (**self).close(element)
    }
    fn move_to(&self, element: &ElementData, x: i32, y: i32) -> Result<()> {
        (**self).move_to(element, x, y)
    }
    fn resize_to(&self, element: &ElementData, width: u32, height: u32) -> Result<()> {
        (**self).resize_to(element, width, height)
    }
    fn subscribe(&self, element: &ElementData) -> Result<Subscription> {
        (**self).subscribe(element)
    }
}