swiftui_native/views/

closures.rs

//! Closure arguments.

use super::*;
use crate::abi;
use crate::resolve;
use core::ffi::c_void;
use crate::mangle::swift_type;
// ── Closure arguments ──

//
// A Swift closure is two words — an entry point and a context — so it drops
// straight into the integer argument list like any other pair of registers.
// What varies is the *shape* of the callback, and that is what the thunks in
// `abi` are for: which registers the argument arrives in, and whether the
// closure has to write a result through x8.
//
// Every context here is leaked. SwiftUI stores an escaping closure for as long
// as the view lives, and a non-escaping one is called before the modifier
// returns, so leaking is either necessary or harmless; freeing is neither.

/// The two words of a Swift `() -> ()` closure that runs `action`.
pub(crate) fn action_words(action: impl Fn() + 'static) -> [u64; 2] {
    let ctx = Box::leak(Box::new(abi::ClosureCtx::new(invoke_action, boxed_ctx(Box::new(action) as Box<dyn Fn()>))))
        as *mut abi::ClosureCtx;
    [abi::swift_closure_thunk as *const () as u64, ctx as u64]
}

/// The two words of a `() -> ()` closure that runs an already-boxed action.
///
/// [`action_words`] takes the closure by value and boxes it; this takes the box
/// straight, which is what an `Option<Box<dyn Fn()>>` argument already holds.
pub(crate) fn boxed_action_words(action: Box<dyn Fn()>) -> [u64; 2] {
    let ctx = Box::leak(Box::new(abi::ClosureCtx::new(
        super::controls::run_boxed_action,
        super::boxed_ctx(action),
    ))) as *mut abi::ClosureCtx;
    [abi::swift_closure_thunk as *const () as u64, ctx as u64]
}

/// The two words of a `@ViewBuilder () -> V` closure returning `view`.
///
/// The view is built here and handed back whenever SwiftUI asks, which is what
/// a `ViewBuilder` closure over a fixed subtree amounts to. `V` is the view's
/// own type — the caller has to name it in the `Content.Type` it passes, and
/// `view.meta`/`view.wt` are what to name it with.
pub(crate) fn view_words(view: &ViewHandle) -> [u64; 2] {
    let ctx = Box::leak(Box::new(abi::ClosureCtx::new(
        emit_tuple,
        Child::leak(view, Content::Concrete),
    ))) as *mut abi::ClosureCtx;
    [abi::swift_closure_thunk as *const () as u64, ctx as u64]
}

/// The two words of a one-argument, no-result closure.
pub(crate) fn wide_words(
    body: unsafe extern "C" fn(u64, u64, u64, *mut c_void),
    payload: *mut c_void,
) -> [u64; 2] {
    let ctx = Box::leak(Box::new(abi::WideClosureCtx::new(body, payload)))
        as *mut abi::WideClosureCtx;
    [abi::wide_closure_thunk as *const () as u64, ctx as u64]
}

/// The two words of a two-argument, no-result closure.
pub(crate) fn pair_words(
    body: unsafe extern "C" fn(*const u8, *const u8, *mut c_void),
    payload: *mut c_void,
) -> [u64; 2] {
    let ctx = Box::leak(Box::new(abi::PairClosureCtx::new(body, payload)))
        as *mut abi::PairClosureCtx;
    [abi::pair_closure_thunk as *const () as u64, ctx as u64]
}

/// A callback that receives an enum's case number.
///
/// Whether the case arrives as a value or as an address is not the closure's
/// choice: a `@frozen` enum is loadable and travels in x0, a resilient one of
/// the very same single byte travels by pointer. `indirect` records which, and
/// `getEnumTag` turns either into a case number.
pub(crate) struct TagCallback {
    pub(crate) f: Box<dyn Fn(i64)>,
    pub(crate) meta: *const c_void,
    pub(crate) indirect: bool,
}

pub(crate) unsafe extern "C" fn invoke_tag(a0: u64, _a1: u64, _a2: u64, payload: *mut c_void) {
    unsafe {
        let cb = &*(payload as *const TagCallback);
        let tag = if cb.indirect {
            abi::read_enum_tag(a0 as *const u8, cb.meta)
        } else {
            abi::read_enum_tag(a0.to_le_bytes().as_ptr(), cb.meta)
        };
        (cb.f)(i64::from(tag));
    }
}

/// The two words of an `(E) -> ()` closure delivering `E`'s case number.
pub(crate) unsafe fn tag_words(f: impl Fn(i64) + 'static, meta: *const c_void, indirect: bool) -> [u64; 2] {
    let cb = Box::leak(Box::new(TagCallback { f: Box::new(f), meta, indirect }));
    wide_words(invoke_tag, cb as *mut TagCallback as *mut c_void)
}

pub(crate) unsafe extern "C" fn invoke_bool(a0: u64, _a1: u64, _a2: u64, payload: *mut c_void) {
    let f = unsafe { boxed_ref::<dyn Fn(bool)>(payload) };
    f(a0 & 1 != 0);
}

/// The two words of a `(Bool) -> ()` closure.
pub(crate) fn bool_words(f: impl Fn(bool) + 'static) -> [u64; 2] {
    wide_words(invoke_bool, boxed_ctx(Box::new(f) as Box<dyn Fn(bool)>))
}

pub(crate) unsafe extern "C" fn invoke_url(a0: u64, _a1: u64, _a2: u64, payload: *mut c_void) {
    unsafe {
        let f = boxed_ref::<dyn Fn(&str)>(payload);
        // `URL` is resilient, so it arrives by address; `absoluteString` is the
        // one accessor that turns it back into something Rust can read.
        let (s0, s1): (u64, u64);
        core::arch::asm!("blr {f}", f = in(reg) resolve::url_absolute_string(),
            in("x20") a0,
            lateout("x0") s0, lateout("x1") s1,
            lateout("x2") _, lateout("x3") _, lateout("x4") _, lateout("x5") _,
            lateout("x6") _, lateout("x7") _, lateout("x8") _, lateout("x9") _,
            lateout("x10") _, lateout("x11") _, lateout("x12") _, lateout("x13") _,
            lateout("x14") _, lateout("x15") _, lateout("x16") _, lateout("x17") _,
            lateout("lr") _, lateout("x21") _, clobber_abi("C"),
        );
        let mut raw = [0u8; 16];
        raw[..8].copy_from_slice(&s0.to_le_bytes());
        raw[8..].copy_from_slice(&s1.to_le_bytes());
        f(&abi::read_swift_string(raw));
    }
}

/// The two words of a `(URL) -> ()` closure, delivering the absolute string.
pub(crate) fn url_words(f: impl Fn(&str) + 'static) -> [u64; 2] {
    wide_words(invoke_url, boxed_ctx(Box::new(f) as Box<dyn Fn(&str)>))
}

/// A callback receiving two enum case numbers.
pub(crate) struct TagPairCallback {
    pub(crate) f: Box<dyn Fn(i64, i64)>,
    pub(crate) meta: *const c_void,
}

pub(crate) unsafe extern "C" fn invoke_tag_pair(a: *const u8, b: *const u8, payload: *mut c_void) {
    unsafe {
        let cb = &*(payload as *const TagPairCallback);
        // Both are frozen single-byte enums, so the registers hold the values.
        let ta = abi::read_enum_tag((a as u64).to_le_bytes().as_ptr(), cb.meta);
        let tb = abi::read_enum_tag((b as u64).to_le_bytes().as_ptr(), cb.meta);
        (cb.f)(i64::from(ta), i64::from(tb));
    }
}

pub(crate) unsafe extern "C" fn invoke_word_pair(a: *const u8, b: *const u8, payload: *mut c_void) {
    let f = unsafe { boxed_ref::<dyn Fn(u64, u64)>(payload) };
    f(a as u64, b as u64);
}

/// Apply a modifier with one generic parameter, given the whole argument list.
///
/// [`generic1_modifier`] leads with a pointer to the value, which is right when
/// the generic parameter *is* the argument. A `@ViewBuilder` modifier is the
/// other case: the generic parameter is what the closure returns, so the
/// argument list is the closure's two words and the type arguments still have
/// to follow.
pub(crate) unsafe fn generic1_modifier_ints(
    func: *const c_void,
    oret: *const c_void,
    view: &ViewHandle,
    arg_meta: *const c_void,
    arg_wt: *const c_void,
    leading: &[u64],
    floats: [f64; 4],
) -> ViewHandle {
    unsafe {
        let subst = [view.meta, arg_meta, view.wt, arg_wt];
        let (meta, wt, size) = opaque_result_type(oret, &subst);
        let mut ints = Vec::with_capacity(leading.len() + 4);
        ints.extend_from_slice(leading);
        ints.push(view.meta as u64);
        ints.push(arg_meta as u64);
        ints.push(view.wt as u64);
        ints.push(arg_wt as u64);
        let r = abi::call_modifier_generic(func, view.value_ptr(), &ints, floats, size);
        wrap_owned(r, meta, wt)
    }
}

/// Apply a `@ViewBuilder content:` modifier whose content is a fixed subtree.
pub(crate) unsafe fn content_modifier(
    func: *const c_void,
    oret: *const c_void,
    view: &ViewHandle,
    leading: &[u64],
    content: &ViewHandle,
) -> ViewHandle {
    unsafe {
        let mut ints = leading.to_vec();
        ints.extend_from_slice(&view_words(content));
        generic1_modifier_ints(
            func,
            oret,
            view,
            content.meta,
            content.wt,
            &ints,
            [0.0; 4],
        )
    }
}

/// Read a `@frozen` static property that fits in one register.
///
/// The counterpart to [`resilient_static`]: a frozen type of eight bytes or
/// fewer is loadable, so its getter returns the value in x0 rather than
/// writing it through x8.
pub(crate) unsafe fn frozen_static(meta: *const c_void, getter: &'static core::ffi::CStr) -> u64 {
    unsafe {
        let f = resolve::sym(getter);
        assert!(!f.is_null(), "missing static: {}", crate::diag::symbol_name(getter));
        abi::static_regs(f, meta)[0]
    }
}

/// A member of `SwiftUI.AccessibilityActionKind`.
///
/// Three of these are platform-specific, and asking for one where it does not
/// exist is not a missing symbol — the symbol is there on every platform and
/// traps with "Unavailable code reached" when called. Availability is in the
/// SDK's declarations, not in the export trie, so it is written out here.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum ActionKind {
    /// Every platform.
    Default,
    /// Every platform.
    Escape,
    /// iOS, tvOS and watchOS only.
    /// Unavailable on macOS: SwiftUI ships a trap stub there, so
    /// reaching this aborts with "Unavailable code reached".
    #[cfg(not(target_os = "macos"))]
    MagicTap,
    /// macOS only.
    ShowMenu,
    /// macOS only.
    Delete,
}

impl ActionKind {
    pub(crate) fn getter(self) -> &'static core::ffi::CStr {
        match self {
            Self::Default => c"$s7SwiftUI23AccessibilityActionKindV7defaultACvgZ",
            Self::Escape => c"$s7SwiftUI23AccessibilityActionKindV6escapeACvgZ",
            #[cfg(not(target_os = "macos"))]
            Self::MagicTap => c"$s7SwiftUI23AccessibilityActionKindV8magicTapACvgZ",
            Self::ShowMenu => c"$s7SwiftUI23AccessibilityActionKindV8showMenuACvgZ",
            Self::Delete => c"$s7SwiftUI23AccessibilityActionKindV6deleteACvgZ",
        }
    }
}

swift_statics! {
    /// A member of `SwiftUI.SubmitTriggers`.
    pub enum SubmitTrigger {
        Text = c"$s7SwiftUI14SubmitTriggersV4textACvgZ",
        Search = c"$s7SwiftUI14SubmitTriggersV6searchACvgZ",
        Collection = c"$s7SwiftUI14SubmitTriggersV10collectionACvgZ",
    }
}

swift_statics! {
    /// A member of `SwiftUI.EventModifiers`.
    pub enum EventModifier {
        Shift = c"$s7SwiftUI14EventModifiersV5shiftACvgZ",
        Control = c"$s7SwiftUI14EventModifiersV7controlACvgZ",
        Option = c"$s7SwiftUI14EventModifiersV6optionACvgZ",
        Command = c"$s7SwiftUI14EventModifiersV7commandACvgZ",
        CapsLock = c"$s7SwiftUI14EventModifiersV8capsLockACvgZ",
        Function = c"$s7SwiftUI14EventModifiersV8functionACvgZ",
        NumericPad = c"$s7SwiftUI14EventModifiersV10numericPadACvgZ",
        All = c"$s7SwiftUI14EventModifiersV3allACvgZ",
    }
}

/// Apply `.accessibilityAction(_:_:)`.
pub fn accessibility_action(view: &ViewHandle, kind: ActionKind, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::a11y_action_kind_ma());
        let k = resilient_static(meta, kind.getter());
        let c = action_words(action);
        a11y_modifier(resolve::a11y_action_fn(), view, &[k.as_ptr() as u64, c[0], c[1]], [0.0; 4])
    }
}

/// Apply `.accessibilityAdjustableAction(_:)`.
///
/// The direction reaches the callback as a case number: 0 is `.increment`,
/// 1 is `.decrement`.
pub fn accessibility_adjustable_action(view: &ViewHandle, action: impl Fn(i64) + 'static) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::a11y_adjustment_direction_ma());
        // Resilient, so the case arrives by address even though it is one byte.
        let c = tag_words(action, meta, true);
        a11y_modifier(resolve::a11y_adjustable_action_fn(), view, &c, [0.0; 4])
    }
}

/// Apply `.accessibilityScrollAction(_:)`.
///
/// The edge reaches the callback as a case number, in `Edge`'s declared order:
/// top, leading, bottom, trailing.
pub fn accessibility_scroll_action(view: &ViewHandle, action: impl Fn(i64) + 'static) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::edge_enum_ma());
        // `Edge` is frozen, so the case travels in the register itself.
        let c = tag_words(action, meta, false);
        a11y_modifier(resolve::a11y_scroll_action_fn(), view, &c, [0.0; 4])
    }
}

/// Apply `.renameAction(_:)`.
pub fn rename_action(view: &ViewHandle, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        opaque_modifier(
            resolve::rename_action_fn(),
            resolve::rename_action_oret(),
            view,
            &action_words(action),
            [0.0; 4],
        )
    }
}

/// Apply `.onSubmit(of:_:)`.
pub fn on_submit(view: &ViewHandle, trigger: SubmitTrigger, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::submit_triggers_ma());
        let t = resilient_static(meta, trigger.getter());
        let c = action_words(action);
        opaque_modifier(
            resolve::on_submit_fn(),
            resolve::on_submit_oret(),
            view,
            &[t.as_ptr() as u64, c[0], c[1]],
            [0.0; 4],
        )
    }
}

/// Apply `.onDeleteCommand(perform:)`.
pub fn on_delete_command(view: &ViewHandle, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        opaque_modifier(
            resolve::on_delete_command_fn(),
            resolve::on_delete_command_oret(),
            view,
            &action_words(action),
            [0.0; 4],
        )
    }
}

/// Apply `.onExitCommand(perform:)`.
pub fn on_exit_command(view: &ViewHandle, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        opaque_modifier(
            resolve::on_exit_command_fn(),
            resolve::on_exit_command_oret(),
            view,
            &action_words(action),
            [0.0; 4],
        )
    }
}

/// Apply `.onMoveCommand(perform:)`.
///
/// The direction arrives as a case number: up, down, left, right.
pub fn on_move_command(view: &ViewHandle, action: impl Fn(i64) + 'static) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::move_command_direction_ma());
        let c = tag_words(action, meta, true);
        opaque_modifier(
            resolve::on_move_command_fn(),
            resolve::on_move_command_oret(),
            view,
            &c,
            [0.0; 4],
        )
    }
}

/// Apply `.onInteractiveResizeChange(_:)`.
pub fn on_interactive_resize_change(view: &ViewHandle, action: impl Fn(bool) + 'static) -> ViewHandle {
    unsafe {
        opaque_modifier(
            resolve::on_interactive_resize_fn(),
            resolve::on_interactive_resize_oret(),
            view,
            &bool_words(action),
            [0.0; 4],
        )
    }
}

/// Apply `.onScrollVisibilityChange(threshold:_:)`.
pub fn on_scroll_visibility_change(
    view: &ViewHandle,
    threshold: f64,
    action: impl Fn(bool) + 'static,
) -> ViewHandle {
    unsafe {
        opaque_modifier(
            resolve::on_scroll_visibility_fn(),
            resolve::on_scroll_visibility_oret(),
            view,
            &bool_words(action),
            [threshold, 0.0, 0.0, 0.0],
        )
    }
}

/// Apply `.onOpenURL(perform:)`.
pub fn on_open_url(view: &ViewHandle, action: impl Fn(&str) + 'static) -> ViewHandle {
    unsafe {
        opaque_modifier(
            resolve::on_open_url_fn(),
            resolve::on_open_url_oret(),
            view,
            &url_words(action),
            [0.0; 4],
        )
    }
}

/// Apply `.onScrollPhaseChange(_:)`.
///
/// Both phases arrive as case numbers, in `ScrollPhase`'s declared order:
/// idle, tracking, interacting, decelerating, animating.
pub fn on_scroll_phase_change(view: &ViewHandle, action: impl Fn(i64, i64) + 'static) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::scroll_phase_ma());
        let cb = Box::leak(Box::new(TagPairCallback { f: Box::new(action), meta }));
        let c = pair_words(invoke_tag_pair, cb as *mut TagPairCallback as *mut c_void);
        opaque_modifier(
            resolve::on_scroll_phase_change_fn(),
            resolve::on_scroll_phase_change_oret(),
            view,
            &c,
            [0.0; 4],
        )
    }
}

/// A `SwiftUI.Transaction` being handed to a `.transaction` callback.
///
/// Only for the duration of the call: the pointer is the one SwiftUI passed
/// `inout`, and writing through it after the callback returns writes to a dead
/// stack slot.
#[derive(Clone, Copy, Debug)]
pub struct TransactionRef(*mut c_void);

impl TransactionRef {
    /// Set `disablesAnimations`.
    pub fn set_disables_animations(self, value: bool) {
        unsafe {
            abi::call_out(
                resolve::transaction_disables_animations_set(),
                abi::ints(&[u64::from(value)]),
                [0.0; 4],
                self.0 as u64,
                core::ptr::null_mut(),
            );
        }
    }
}

pub(crate) unsafe extern "C" fn invoke_transaction(a0: u64, _a1: u64, _a2: u64, payload: *mut c_void) {
    let f = unsafe { boxed_ref::<dyn Fn(TransactionRef)>(payload) };
    f(TransactionRef(a0 as *mut c_void));
}

/// Apply `.transaction(_:)`.
///
/// The transaction is `inout`, so it arrives by address whether or not it would
/// otherwise be loadable, and the callback can write to it in place.
pub fn transaction(view: &ViewHandle, action: impl Fn(TransactionRef) + 'static) -> ViewHandle {
    unsafe {
        let c = wide_words(invoke_transaction, boxed_ctx(Box::new(action) as Box<dyn Fn(TransactionRef)>));
        opaque_modifier(resolve::transaction_fn(), resolve::transaction_oret(), view, &c, [0.0; 4])
    }
}

/// Apply `.accessibilityChildren(children:)`.
pub fn accessibility_children(view: &ViewHandle, children: &ViewHandle) -> ViewHandle {
    unsafe {
        content_modifier(
            resolve::a11y_children_fn(),
            resolve::a11y_children_oret(),
            view,
            &[],
            children,
        )
    }
}

/// Apply `.accessibilityRepresentation(representation:)`.
pub fn accessibility_representation(view: &ViewHandle, representation: &ViewHandle) -> ViewHandle {
    unsafe {
        content_modifier(
            resolve::a11y_representation_fn(),
            resolve::a11y_representation_oret(),
            view,
            &[],
            representation,
        )
    }
}

swift_statics! {
    /// A member of `SwiftUI.AccessibilityActionCategory`.
    pub enum ActionCategory {
        Default = c"$s7SwiftUI27AccessibilityActionCategoryV7defaultACvgZ",
        Edit = c"$s7SwiftUI27AccessibilityActionCategoryV4editACvgZ",
    }
}

/// Apply `.accessibilityActions(category:_:)`.
pub fn accessibility_actions(
    view: &ViewHandle,
    category: ActionCategory,
    content: &ViewHandle,
) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::a11y_action_category_ma());
        let c = resilient_static(meta, category.getter());
        content_modifier(
            resolve::a11y_actions_fn(),
            resolve::a11y_actions_oret(),
            view,
            &[c.as_ptr() as u64],
            content,
        )
    }
}

/// Apply `.toolbarTitleMenu(content:)`.
pub fn toolbar_title_menu(view: &ViewHandle, content: &ViewHandle) -> ViewHandle {
    unsafe {
        content_modifier(
            resolve::toolbar_title_menu_fn(),
            resolve::toolbar_title_menu_oret(),
            view,
            &[],
            content,
        )
    }
}

/// Apply `.sectionActions(content:)`.
pub fn section_actions(view: &ViewHandle, content: &ViewHandle) -> ViewHandle {
    unsafe {
        content_modifier(
            resolve::section_actions_fn(),
            resolve::section_actions_oret(),
            view,
            &[],
            content,
        )
    }
}

/// Apply `.tabItem(_:)`.
pub fn tab_item(view: &ViewHandle, label: &ViewHandle) -> ViewHandle {
    unsafe {
        content_modifier(resolve::tab_item_fn(), resolve::tab_item_oret(), view, &[], label)
    }
}

#[cfg(test)]
mod closure_tests {
    use super::*;
    use std::sync::atomic::{AtomicI64, Ordering};

    /// Call a closure the way Swift does: argument in x0, context in x20.
    ///
    /// Constructing a modifier proves only that the closure's two words were
    /// accepted. Whether the callback runs, and whether it reads the argument
    /// out of the right register, is not visible in the view's type or in a
    /// dump of its value — so it is checked here directly.
    unsafe fn call_closure(words: [u64; 2], arg: u64) {
        unsafe {
            core::arch::asm!("blr {f}", f = in(reg) words[0],
                in("x0") arg, in("x20") words[1],
                lateout("x1") _, lateout("x2") _, lateout("x3") _, lateout("x4") _,
                lateout("x5") _, lateout("x6") _, lateout("x7") _, lateout("x8") _,
                lateout("x9") _, lateout("x10") _, lateout("x11") _, lateout("x12") _,
                lateout("x13") _, lateout("x14") _, lateout("x15") _, lateout("x16") _,
                lateout("x17") _, lateout("lr") _, lateout("x21") _, clobber_abi("C"),
            );
        }
    }

    static SEEN: AtomicI64 = AtomicI64::new(-1);

    fn record(v: i64) {
        SEEN.store(v, Ordering::SeqCst);
    }

    #[test]
    fn frozen_enum_case_arrives_in_the_register() {
        unsafe {
            // `Edge` is frozen and one byte, so the case number *is* x0.
            let meta = abi::metadata0(resolve::edge_enum_ma());
            let words = tag_words(record, meta, false);
            for case in 0..4u64 {
                SEEN.store(-1, Ordering::SeqCst);
                call_closure(words, case);
                assert_eq!(SEEN.load(Ordering::SeqCst), case as i64, "Edge case {case}");
            }
        }
    }

    #[test]
    fn resilient_enum_case_arrives_by_address() {
        unsafe {
            // `MoveCommandDirection` is the same one byte but not frozen, so
            // x0 holds a pointer to it rather than the case itself.
            let meta = abi::metadata0(resolve::move_command_direction_ma());
            let words = tag_words(record, meta, true);
            for case in 0..4u32 {
                let mut buf = vec![0u8; abi::type_bytes(meta)];
                abi::inject_enum_tag(&mut buf, meta, case);
                SEEN.store(-1, Ordering::SeqCst);
                call_closure(words, buf.as_ptr() as u64);
                assert_eq!(SEEN.load(Ordering::SeqCst), i64::from(case), "direction {case}");
            }
        }
    }

    fn record_bool(v: bool) {
        SEEN.store(i64::from(v), Ordering::SeqCst);
    }

    #[test]
    fn bool_closure_reads_the_low_bit() {
        let words = bool_words(record_bool);
        unsafe {
            SEEN.store(-1, Ordering::SeqCst);
            call_closure(words, 1);
            assert_eq!(SEEN.load(Ordering::SeqCst), 1);
            call_closure(words, 0);
            assert_eq!(SEEN.load(Ordering::SeqCst), 0);
        }
    }

    /// A round trip through the enum witnesses: inject a case, read it back.
    #[test]
    fn enum_tag_round_trips() {
        unsafe {
            let meta = abi::metadata0(resolve::scroll_phase_ma());
            let mut buf = vec![0u8; abi::type_bytes(meta)];
            for case in 0..5u32 {
                abi::inject_enum_tag(&mut buf, meta, case);
                assert_eq!(abi::read_enum_tag(buf.as_ptr(), meta), case);
            }
        }
    }

    /// The context must survive the call that installed it, so the words a
    /// closure is built from have to point at something leaked rather than at
    /// a temporary.
    #[test]
    fn closure_context_outlives_construction() {
        let words = {
            let view = text("gone by now");
            view_words(&view)
        };
        assert_ne!(words[0], 0);
        assert_ne!(words[1], 0);
        let mut out = 0u64;
        unsafe {
            core::arch::asm!("blr {f}", f = in(reg) words[0],
                in("x8") &mut out as *mut u64, in("x20") words[1],
                lateout("x0") _, lateout("x1") _, lateout("x2") _, lateout("x3") _,
                lateout("x4") _, lateout("x5") _, lateout("x6") _, lateout("x7") _,
                lateout("x9") _, lateout("x10") _, lateout("x11") _, lateout("x12") _,
                lateout("x13") _, lateout("x14") _, lateout("x15") _, lateout("x16") _,
                lateout("x17") _, lateout("lr") _, lateout("x21") _, clobber_abi("C"),
            );
        }
        assert_ne!(out, 0, "the ViewBuilder closure handed back nothing");
    }
}

/// The two words of a `(V) -> ()` closure that ignores its argument.
///
/// SwiftUI hands these callbacks a `DragSession`, a `PencilSqueezeGesturePhase`
/// and the like — values this crate has no reader for yet. The callback still
/// fires; only the argument is dropped, and the wide thunk forwards x0 through
/// x2 so it does not matter whether the value came in registers or by address.
pub(crate) fn ignoring_value_words(action: impl Fn() + 'static) -> [u64; 2] {
    wide_words(invoke_ignoring_value, boxed_ctx(Box::new(action) as Box<dyn Fn()>))
}

pub(crate) unsafe extern "C" fn invoke_ignoring_value(
    _a0: u64,
    _a1: u64,
    _a2: u64,
    payload: *mut c_void,
) {
    let f = unsafe { boxed_ref::<dyn Fn()>(payload) };
    f();
}

/// Apply `.accessibilityZoomAction(_:)`.
pub fn accessibility_zoom_action(view: &ViewHandle, action: impl Fn() + 'static) -> ViewHandle {
    unsafe { a11y_modifier(resolve::a11y_zoom_action_fn(), view, &ignoring_value_words(action), [0.0; 4]) }
}

/// Apply `.onContinueUserActivity(_:perform:)`.
pub fn on_continue_user_activity(view: &ViewHandle, kind: &str, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        let s = abi::swift_string(kind);
        let c = ignoring_value_words(action);
        opaque_modifier(
            resolve::on_continue_user_activity_fn(),
            resolve::on_continue_user_activity_oret(),
            view,
            &[
                u64::from_le_bytes(s[..8].try_into().expect("lo")),
                u64::from_le_bytes(s[8..].try_into().expect("hi")),
                c[0],
                c[1],
            ],
            [0.0; 4],
        )
    }
}

/// Apply `.presentationBackground(alignment:content:)`.
pub fn presentation_background(
    view: &ViewHandle,
    alignment: Alignment,
    content: &ViewHandle,
) -> ViewHandle {
    unsafe {
        let a = alignment.words();
        content_modifier(
            resolve::presentation_background_fn(),
            resolve::presentation_background_oret(),
            view,
            &[a[0], a[1]],
            content,
        )
    }
}

swift_statics! {
    /// A content type, as `UTType`'s own static properties name them.
    pub enum ContentType {
        PlainText = c"$s22UniformTypeIdentifiers6UTTypeV9plainTextACvgZ",
        Text = c"$s22UniformTypeIdentifiers6UTTypeV4textACvgZ",
        Url = c"$s22UniformTypeIdentifiers6UTTypeV3urlACvgZ",
    }
}

/// The two words of a closure returning a single word.
pub(crate) fn word_closure(
    body: unsafe extern "C" fn(*mut c_void, u64) -> u64,
    payload: *mut c_void,
) -> [u64; 2] {
    let ctx = Box::leak(Box::new(abi::WordClosureCtx::new(body, payload)))
        as *mut abi::WordClosureCtx;
    [abi::word_closure_thunk as *const () as u64, ctx as u64]
}

unsafe impl Send for ItemProvider {}
unsafe impl Sync for ItemProvider {}

/// A Swift `[NSItemProvider]` built from Rust.
pub(crate) unsafe fn provider_array(items: &[&str]) -> u64 {
    unsafe {
        let meta = crate::type_by_name!(swift_type!(objc NSItemProvider).as_bytes());
        let stride = abi::type_bytes(meta);
        let mut bytes = vec![0u8; stride * items.len()];
        for (i, t) in items.iter().enumerate() {
            let p = ItemProvider::plain_text(t);
            bytes[i * stride..i * stride + 8].copy_from_slice(&(p.0 as u64).to_le_bytes());
        }
        abi::swift_array(meta, items.len(), &bytes)
    }
}

/// Whether a drop should be accepted — `onDrop`'s `isTargeted`-style predicate.
pub(crate) type DropDecide = Box<dyn Fn(&[ItemProvider]) -> bool>;

/// What a drop handler is told, with no answer expected.
pub(crate) type DropObserve = Box<dyn Fn(&[ItemProvider])>;

/// What an item-provider closure carries.
pub(crate) struct ProviderPayload {
    array: u64,
    decide: Option<DropDecide>,
    observe: Option<DropObserve>,
}

pub(crate) unsafe extern "C" fn emit_providers(payload: *mut c_void, _arg: u64) -> u64 {
    unsafe {
        let p = &*(payload as *const ProviderPayload);
        swift_runtime_sys::RuntimeRaw::swift_retain(p.array as *mut c_void);
        p.array
    }
}

pub(crate) unsafe extern "C" fn emit_first_provider(payload: *mut c_void, _arg: u64) -> u64 {
    unsafe {
        let p = &*(payload as *const ProviderPayload);
        let items = read_provider_array(p.array);
        items.first().map_or(0, |i| i.0 as u64)
    }
}

pub(crate) unsafe extern "C" fn decide_on_providers(payload: *mut c_void, arg: u64) -> u64 {
    unsafe {
        let p = &*(payload as *const ProviderPayload);
        let items = read_provider_array(arg);
        u64::from(p.decide.as_ref().map(|f| f(&items)).unwrap_or(false))
    }
}

pub(crate) unsafe extern "C" fn observe_providers(
    a0: u64,
    _a1: u64,
    _a2: u64,
    payload: *mut c_void,
) {
    unsafe {
        let p = &*(payload as *const ProviderPayload);
        if let Some(f) = p.observe.as_ref() {
            f(&read_provider_array(a0));
        }
    }
}

pub(crate) fn provider_payload(
    array: u64,
    decide: Option<DropDecide>,
    observe: Option<DropObserve>,
) -> *mut c_void {
    Box::leak(Box::new(ProviderPayload { array, decide, observe })) as *mut ProviderPayload
        as *mut c_void
}

swift_statics! {
    /// Where a container background sits.
    /// Where a `.containerBackground` is drawn.
    ///
    /// **`Navigation` and `NavigationSplitView` are unavailable on macOS** —
    /// SwiftUI ships trap stubs for them there, so reaching one aborts with
    /// "Unavailable code reached" rather than failing to resolve. `Window` and
    /// `Presentation` work on every platform. Measured, not assumed: each case
    /// was called on macOS and the two named here are the ones that trap.
    pub enum ContainerBackgroundPlacement {
        Window = c"$s7SwiftUI28ContainerBackgroundPlacementV6windowACvgZ",
        /// Unavailable on macOS: SwiftUI ships a trap stub there, so
        /// reaching this aborts with "Unavailable code reached".
        #[cfg(not(target_os = "macos"))]
        Navigation = c"$s7SwiftUI28ContainerBackgroundPlacementV10navigationACvgZ",
        Presentation = c"$s7SwiftUI28ContainerBackgroundPlacementV12presentationACvgZ",
        /// Unavailable on macOS: SwiftUI ships a trap stub there, so
        /// reaching this aborts with "Unavailable code reached".
        #[cfg(not(target_os = "macos"))]
        NavigationSplitView = c"$s7SwiftUI28ContainerBackgroundPlacementV19navigationSplitViewACvgZ",
    }
}

swift_cases! {
    /// `SwiftUI.VerticalEdge`.
    pub enum VerticalEdge {
        Top = 0,
        Bottom = 1,
    }
}

/// Apply `.containerBackground(for:alignment:content:)`.
pub fn container_background(
    view: &ViewHandle,
    placement: ContainerBackgroundPlacement,
    alignment: Alignment,
    content: &ViewHandle,
) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::container_background_placement_ma());
        let p = resilient_static(meta, placement.getter());
        let a = alignment.words();
        content_modifier(
            resolve::container_background_fn(),
            resolve::container_background_oret(),
            view,
            &[p.as_ptr() as u64, a[0], a[1]],
            content,
        )
    }
}

/// The edge, alignment and spacing the two safe-area modifiers share.
unsafe fn safe_area_args(
    edge: VerticalEdge,
    alignment: HorizontalAlignment,
    spacing: Option<f64>,
) -> Vec<u64> {
    unsafe {
        let e = abi::enum_argument(abi::metadata0(resolve::vertical_edge_ma()), edge.tag());
        let a = frozen_static(abi::metadata0(resolve::horizontal_alignment_ma()), alignment.getter());
        let mut ints = vec![e, a];
        ints.extend(opt_f64_words(spacing));
        ints
    }
}

/// Apply `.safeAreaInset(edge:alignment:spacing:content:)`.
pub fn safe_area_inset(
    view: &ViewHandle,
    edge: VerticalEdge,
    alignment: HorizontalAlignment,
    spacing: Option<f64>,
    content: &ViewHandle,
) -> ViewHandle {
    unsafe {
        let args = safe_area_args(edge, alignment, spacing);
        content_modifier(
            resolve::safe_area_inset_fn(),
            resolve::safe_area_inset_oret(),
            view,
            &args,
            content,
        )
    }
}

/// Apply `.safeAreaBar(edge:alignment:spacing:content:)`.
pub fn safe_area_bar(
    view: &ViewHandle,
    edge: VerticalEdge,
    alignment: HorizontalAlignment,
    spacing: Option<f64>,
    content: &ViewHandle,
) -> ViewHandle {
    unsafe {
        let args = safe_area_args(edge, alignment, spacing);
        content_modifier(
            resolve::safe_area_bar_fn(),
            resolve::safe_area_bar_oret(),
            view,
            &args,
            content,
        )
    }
}

/// Apply `.userActivity(_:isActive:_:)`.
pub fn user_activity(view: &ViewHandle, kind: &str, active: bool, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        let s = abi::swift_string(kind);
        let c = ignoring_value_words(action);
        opaque_modifier(
            resolve::user_activity_fn(),
            resolve::user_activity_oret(),
            view,
            &[
                u64::from_le_bytes(s[..8].try_into().expect("lo")),
                u64::from_le_bytes(s[8..].try_into().expect("hi")),
                u64::from(active),
                c[0],
                c[1],
            ],
            [0.0; 4],
        )
    }
}

swift_statics! {
    /// A haptic or audible cue.
    pub enum SensoryFeedback {
        Success = c"$s7SwiftUI15SensoryFeedbackV7successACvgZ",
        Warning = c"$s7SwiftUI15SensoryFeedbackV7warningACvgZ",
        Error = c"$s7SwiftUI15SensoryFeedbackV5errorACvgZ",
        Selection = c"$s7SwiftUI15SensoryFeedbackV9selectionACvgZ",
        Impact = c"$s7SwiftUI15SensoryFeedbackV6impactACvgZ",
        Alignment = c"$s7SwiftUI15SensoryFeedbackV9alignmentACvgZ",
        Decrease = c"$s7SwiftUI15SensoryFeedbackV8decreaseACvgZ",
        Increase = c"$s7SwiftUI15SensoryFeedbackV8increaseACvgZ",
        LevelChange = c"$s7SwiftUI15SensoryFeedbackV11levelChangeACvgZ",
        PathComplete = c"$s7SwiftUI15SensoryFeedbackV12pathCompleteACvgZ",
        Start = c"$s7SwiftUI15SensoryFeedbackV5startACvgZ",
        Stop = c"$s7SwiftUI15SensoryFeedbackV4stopACvgZ",
    }
}

/// A byte buffer as the argument registers it occupies.
pub(crate) fn words(bytes: &[u8]) -> Vec<u64> {
    bytes
        .chunks(8)
        .map(|c| {
            let mut w = [0u8; 8];
            w[..c.len()].copy_from_slice(c);
            u64::from_le_bytes(w)
        })
        .collect()
}

swift_statics! {
    /// Where a search field is placed in the surrounding chrome.
    ///
    /// A struct rather than an enum, so each case is a static getter returning a
    /// value small enough to travel in a register.
    pub enum SearchFieldPlacement {
        Automatic = c"$s7SwiftUI20SearchFieldPlacementV9automaticACvgZ",
        ToolbarPrincipal = c"$s7SwiftUI20SearchFieldPlacementV16toolbarPrincipalACvgZ",
        NavigationColumn = c"$s7SwiftUI20SearchFieldPlacementV16navigationColumnACvgZ",
        /// Unavailable on macOS: SwiftUI ships a trap stub there, so
        /// reaching this aborts with "Unavailable code reached".
        #[cfg(not(target_os = "macos"))]
        NavigationBarDrawer = c"$s7SwiftUI20SearchFieldPlacementV19navigationBarDrawerACvgZ",
        Sidebar = c"$s7SwiftUI20SearchFieldPlacementV7sidebarACvgZ",
        Toolbar = c"$s7SwiftUI20SearchFieldPlacementV7toolbarACvgZ",
    }
}

/// Apply a modifier with *two* generic `View` parameters beyond `Self`.
///
/// Distinct from `generics::generic2_modifier_ints`, which is one generic
/// parameter carrying two conformances.
///
/// The tail is every metadata argument in generic-parameter order and then every
/// witness table in requirement order — not the two interleaved, which is what
/// repeating [`generic1_modifier_ints`] would give.
pub(crate) unsafe fn two_view_modifier(
    func: *const c_void,
    oret: *const c_void,
    view: &ViewHandle,
    leading: &[u64],
    a: &ViewHandle,
    b: &ViewHandle,
) -> ViewHandle {
    unsafe {
        // `Self`, then the two content views — which are passed as `AnyView`
        // words, so only the first of each triple is the receiver's.
        // Three distinct types: the receiver, and each of the two contents,
        // which `view_words` now hands back at its own type rather than boxed.
        let subst = [view.meta, a.meta, b.meta, view.wt, a.wt, b.wt];
        let (meta, wt, size) = opaque_result_type(oret, &subst);
        let mut ints = leading.to_vec();
        ints.extend_from_slice(&view_words(a));
        ints.extend_from_slice(&view_words(b));
        ints.extend_from_slice(&[view.meta as u64, a.meta as u64, b.meta as u64]);
        ints.extend_from_slice(&[view.wt as u64, a.wt as u64, b.wt as u64]);
        let r = abi::call_modifier_generic(func, view.value_ptr(), &ints, [0.0; 4], size);
        wrap_owned(r, meta, wt)
    }
}

swift_statics! {
    /// `SwiftUI.SearchSuggestionsPlacement.Set`, an option set.
    pub enum SearchSuggestionsPlacementSet {
        Menu = c"$s7SwiftUI26SearchSuggestionsPlacementV3SetV4menuAEvgZ",
        Content = c"$s7SwiftUI26SearchSuggestionsPlacementV3SetV7contentAEvgZ",
    }
}

swift_statics! {
    /// `SwiftUI.TextInputFormattingControlPlacement.Set`, an option set.
    pub enum TextInputFormattingControlPlacementSet {
        All = c"$s7SwiftUI35TextInputFormattingControlPlacementV3SetV3allAEvgZ",
        Default = c"$s7SwiftUI35TextInputFormattingControlPlacementV3SetV7defaultAEvgZ",
        ContextMenu = c"$s7SwiftUI35TextInputFormattingControlPlacementV3SetV11contextMenuAEvgZ",
        AccessoryBar = c"$s7SwiftUI35TextInputFormattingControlPlacementV3SetV12accessoryBarAEvgZ",
        /// Unavailable on macOS: SwiftUI ships a trap stub there, so
        /// reaching this aborts with "Unavailable code reached".
        #[cfg(not(target_os = "macos"))]
        InputAssistant = c"$s7SwiftUI35TextInputFormattingControlPlacementV3SetV14inputAssistantAEvgZ",
        FontPanel = c"$s7SwiftUI35TextInputFormattingControlPlacementV3SetV9fontPanelAEvgZ",
    }
}

swift_statics! {
    /// A Foundation `CharacterSet`, by the static that names it.
    ///
    /// `Whitespaces` and `Decimals` are the ones the parity suite compares.
    /// `.alphanumerics` is bound and correct, but it cannot be compared *across
    /// processes*: which concrete class backs it — a bitmap `NSCharacterSet` or
    /// a CF-bridged `__NSCFCharacterSet` — depends on what else in the process
    /// has touched Foundation, and `dump` prints the two differently. That is a
    /// property of Foundation's caching, not of the call.
    pub enum CharacterSetKind {
        Alphanumerics = c"$s10Foundation12CharacterSetV13alphanumericsACvgZ",
        Letters = c"$s10Foundation12CharacterSetV7lettersACvgZ",
        Decimals = c"$s10Foundation12CharacterSetV13decimalDigitsACvgZ",
        Whitespaces = c"$s10Foundation12CharacterSetV11whitespacesACvgZ",
    }
}

swift_statics! {
    /// Which phases of a key press to observe.
    pub enum KeyPressPhases {
        Down = c"$s7SwiftUI8KeyPressV6PhasesV4downAEvgZ",
        Up = c"$s7SwiftUI8KeyPressV6PhasesV2upAEvgZ",
        Repeat = c"$s7SwiftUI8KeyPressV6PhasesV6repeatAEvgZ",
        All = c"$s7SwiftUI8KeyPressV6PhasesV3allAEvgZ",
    }
}

/// Apply `.navigationTransition(_:)` with the automatic transition.
///
/// The static is declared on the *protocol*, in an extension constrained to
/// `Self == AutomaticNavigationTransition`, so reading it means naming that
/// concrete type — and the conformance comes from a runtime lookup rather than
/// an exported witness-table symbol.
pub fn navigation_transition(view: &ViewHandle) -> ViewHandle {
    unsafe {
        let meta = abi::metadata0(resolve::automatic_navigation_transition_ma());
        let wt = abi::get_view_wt_for_proto(meta, resolve::navigation_transition_proto());
        assert!(!wt.is_null(), "AutomaticNavigationTransition: NavigationTransition");
        let value = abi::static_out(
            resolve::navigation_transition_automatic(), meta, abi::type_bytes(meta));
        generic1_modifier_ints(
            resolve::navigation_transition_fn(),
            resolve::navigation_transition_oret(),
            view,
            meta,
            wt,
            &[value.as_ptr() as u64],
            [0.0; 4],
        )
    }
}

swift_statics! {
    /// A line height, as `AttributedString.LineHeight`'s statics name it.
    ///
    /// The initialiser taking an `NSParagraphStyle` is SwiftUI-internal, so the
    /// statics are the public way in — and `exact(points:)` and
    /// `multiple(factor:)` are the only two that take an argument.
    pub enum LineHeightKind {
        Variable = c"$s10Foundation16AttributedStringV8CoreTextE10LineHeightV8variableAFvgZ",
        Normal = c"$s10Foundation16AttributedStringV8CoreTextE10LineHeightV6normalAFvgZ",
        Tight = c"$s10Foundation16AttributedStringV8CoreTextE10LineHeightV5tightAFvgZ",
        Loose = c"$s10Foundation16AttributedStringV8CoreTextE10LineHeightV5looseAFvgZ",
    }
}

/// Apply `.lineHeight(_:)`.
///
/// `AttributedString.LineHeight` is nine bytes and resilient, so its static
/// getters write through x8 and the optional wrapping one goes to the modifier
/// by address. Passing `None` writes the optional's `.none` tag and leaves the
/// payload alone.
pub fn line_height(view: &ViewHandle, height: Option<LineHeightKind>) -> ViewHandle {
    unsafe {
        let payload = abi::metadata0(resolve::line_height_ma());
        let opt = abi::resolve_type_by_mangled_name(
            b"10Foundation16AttributedStringV8CoreTextE10LineHeightVSg");
        assert!(!opt.is_null(), "Optional<AttributedString.LineHeight>");
        let mut buf = vec![0u8; abi::type_bytes(opt)];
        if let Some(kind) = height {
            let v = resilient_static(payload, kind.getter());
            buf[..v.len()].copy_from_slice(&v);
        }
        abi::store_optional_tag(&mut buf, payload, height.is_some());
        opaque_modifier(
            resolve::line_height_fn(),
            resolve::line_height_oret(),
            view,
            &[buf.as_ptr() as u64],
            [0.0; 4],
        )
    }
}

/// Apply `.refreshable(action:)`.
///
/// The action is a Swift `async` closure built by [`crate::asyncfn`]: it runs
/// `action` and resumes immediately, never suspending.
pub fn refreshable(view: &ViewHandle, action: impl Fn() + Send + Sync + 'static) -> ViewHandle {
    unsafe {
        static mut RECORD: crate::asyncfn::FunctionPointer =
            crate::asyncfn::FunctionPointer::new();
        let closure = crate::asyncfn::prepare(
            crate::static_mut(&raw mut RECORD),
            0,
            action,
        );
        opaque_modifier(
            resolve::refreshable_fn(),
            resolve::refreshable_oret(),
            view,
            &closure,
            [0.0; 4],
        )
    }
}

/// [`generic1_modifier_ints`] for out-of-crate diagnostics.
#[doc(hidden)]
pub unsafe fn generic1_modifier_ints_for_test(
    func: *const c_void,
    oret: *const c_void,
    view: &ViewHandle,
    arg_meta: *const c_void,
    arg_wt: *const c_void,
    leading: &[u64],
) -> ViewHandle {
    unsafe { generic1_modifier_ints(func, oret, view, arg_meta, arg_wt, leading, [0.0; 4]) }
}

swift_statics! {
    /// How urgently a `task` should run.
    /// `.medium` is deliberately absent. The stdlib marks it
    /// `@_alwaysEmitIntoClient`, so unlike its neighbours it has no symbol to
    /// call — `dlsym` returns null for `$sScP6mediumScPvgZ` even with
    /// `libswift_Concurrency` loaded, and naming it could only panic.
    pub enum TaskPriority {
        High = c"$sScP4highScPvgZ",
        UserInitiated = c"$sScP13userInitiatedScPvgZ",
        Utility = c"$sScP7utilityScPvgZ",
        Low = c"$sScP3lowScPvgZ",
        Background = c"$sScP10backgroundScPvgZ",
    }
}

/// Apply `.task(id:priority:_:)`.
pub fn task(
    view: &ViewHandle,
    id: i64,
    priority: TaskPriority,
    file: &str,
    line: i64,
    action: impl Fn() + Send + Sync + 'static,
) -> ViewHandle {
    unsafe {
        static mut RECORD: crate::asyncfn::FunctionPointer =
            crate::asyncfn::FunctionPointer::new();
        let closure = crate::asyncfn::prepare(
            crate::static_mut(&raw mut RECORD),
            1,
            action,
        );
        let ctx = Box::leak(Box::new(abi::IsolatedContext::nonisolated()))
            as *mut abi::IsolatedContext;
        let pm = abi::metadata0(resolve::task_priority_ma());
        let p = resilient_static(pm, priority.getter());
        let name = abi::swift_string(&format!("View.task @ {file}:{line}"));
        let n0 = u64::from_le_bytes(name[..8].try_into().expect("name lo"));
        let n1 = u64::from_le_bytes(name[8..].try_into().expect("name hi"));
        let meta = resolve::int_meta();
        let wt = resolve::int_equatable_wt();
        let modifier = abi::generic_metadata2(resolve::task_modifier_ma(), meta, wt);
        assert!(!modifier.is_null(), "_TaskValueModifier2<Int> metadata");
        let mut value = vec![0u8; abi::type_bytes(modifier)];
        let mut boxed = id;
        core::arch::asm!(
            "sub sp, sp, #16",
            "stp {s0}, {s1}, [sp]",
            "blr {f}",
            "add sp, sp, #16",
            f = in(reg) resolve::task_modifier_init(),
            s0 = in(reg) meta, s1 = in(reg) wt,
            in("x0") &mut boxed as *mut i64,
            in("x1") n0, in("x2") n1,
            in("x3") 0u64, in("x4") 0u64,
            in("x5") p.as_ptr(),
            in("x6") closure[0], in("x7") ctx as u64,
            in("x8") value.as_mut_ptr(),
            lateout("x9") _, lateout("x10") _, lateout("x11") _, lateout("x12") _,
            lateout("x13") _, lateout("x14") _, lateout("x15") _, lateout("x16") _,
            lateout("x17") _, lateout("lr") _, lateout("x20") _, lateout("x21") _,
            clobber_abi("C"),
        );
        apply_modifier(view, modifier, &value)
    }
}

/// Apply `.task { … }` — the spelling with no `id:`.
///
/// A different modifier from [`task`], not a default-argument version of it:
/// that one builds `_TaskValueModifier2<ID>` and re-runs when the id changes,
/// this one builds `_TaskModifier2` and runs once. `_TaskModifier2` is not
/// generic, so there are no generic words to spill.
///
/// The `name` carries the call site — SwiftUI records `View.task @ file:line` —
/// so a value comparison against a reference only agrees if the caller passes
/// the reference's own file and line, which is why they are parameters.
pub fn task_once(
    view: &ViewHandle,
    priority: TaskPriority,
    file: &str,
    line: i64,
    action: impl Fn() + Send + Sync + 'static,
) -> ViewHandle {
    unsafe {
        static mut RECORD: crate::asyncfn::FunctionPointer =
            crate::asyncfn::FunctionPointer::new();
        let closure = crate::asyncfn::prepare(
            crate::static_mut(&raw mut RECORD),
            2,
            action,
        );
        let ctx = Box::leak(Box::new(abi::IsolatedContext::nonisolated()))
            as *mut abi::IsolatedContext;
        let pm = abi::metadata0(resolve::task_priority_ma());
        let p = resilient_static(pm, priority.getter());
        let name = abi::swift_string(&format!("View.task @ {file}:{line}"));
        let n0 = u64::from_le_bytes(name[..8].try_into().expect("name lo"));
        let n1 = u64::from_le_bytes(name[8..].try_into().expect("name hi"));

        let modifier = abi::metadata0(resolve::task_modifier2_ma());
        assert!(!modifier.is_null(), "_TaskModifier2 metadata");
        let mut value = vec![0u8; abi::type_bytes(modifier)];
        // init(name:executorPreference:priority:action:)
        //   x0/x1 = name, x2/x3 = TaskExecutor? existential (nil),
        //   x4 = &priority, x5 = action fn, x6 = action ctx, x8 = @out
        core::arch::asm!("blr {f}", f = in(reg) resolve::task_modifier2_init(),
            in("x0") n0, in("x1") n1,
            in("x2") 0u64, in("x3") 0u64,
            in("x4") p.as_ptr(),
            in("x5") closure[0], in("x6") ctx as u64,
            in("x8") value.as_mut_ptr(),
            lateout("x7") _, lateout("x9") _, lateout("x10") _, lateout("x11") _,
            lateout("x12") _, lateout("x13") _, lateout("x14") _, lateout("x15") _,
            lateout("x16") _, lateout("x17") _, lateout("lr") _, lateout("x20") _,
            lateout("x21") _, clobber_abi("C"),
        );
        apply_modifier(view, modifier, &value)
    }
}