swiftui_native/views/

compositing.rs

//! Compositing, 3D, and the rest of the gesture family.

use super::*;
use crate::abi;
use crate::resolve;
use core::ffi::c_void;
use crate::mangle::{self, swift_type};
mangle::modifier_types! {
    /// `.onChange(of:initial:_:)` over an `Int` adds two modifiers; these
    /// compose as `ModifiedContent<ModifiedContent<Receiver, inner>, outer>`.
    pub(crate) ON_CHANGE_INNER_T = ty("_ValueActionModifier2").of(vec![Type::Verbatim("Si".into())]);
    /// Watching a `String`.
    pub(crate) ON_CHANGE_STR_T = ty("_ValueActionModifier2").of(vec![Type::Verbatim("SS".into())]);
    /// Watching a `Bool`.
    pub(crate) ON_CHANGE_BOOL_T = ty("_ValueActionModifier2").of(vec![Type::Verbatim("Sb".into())]);
    /// Watching a `Double`.
    pub(crate) ON_CHANGE_F64_T = ty("_ValueActionModifier2").of(vec![Type::Verbatim("Sd".into())]);
    /// The outer half of `.onChange(of:initial:_:)`.
    pub(crate) ON_CHANGE_OUTER_T = ty("_AppearanceActionModifier");
}
mangle::modifier_types! {
    /// `ModifiedContent<AnyView, _ClipEffect<Rectangle>>`
    pub(crate) CLIPPED_T = ty("_ClipEffect").of(vec![ty("Rectangle")]);
    /// `ModifiedContent<AnyView, _CompositingGroupEffect>`
    pub(crate) COMPOSITING_T = ty("_CompositingGroupEffect");
    /// `ModifiedContent<AnyView, _DrawingGroupEffect>`
    pub(crate) DRAWING_GROUP_T = ty("_DrawingGroupEffect");
    /// `ModifiedContent<AnyView, _Rotation3DEffect>`
    pub(crate) ROTATION3D_T = ty("_Rotation3DEffect");
    /// `ModifiedContent<AnyView, AddGestureModifier<DragGesture, DefaultGestureCombiner>>`
    pub(crate) DRAG_T = ty("AddGestureModifier").of(vec![ty("DragGesture"), ty("DefaultGestureCombiner")]);
    /// `ModifiedContent<AnyView, AddGestureModifier<MagnifyGesture, DefaultGestureCombiner>>`
    pub(crate) MAGNIFY_T = ty("AddGestureModifier").of(vec![ty("MagnifyGesture"), ty("DefaultGestureCombiner")]);
    /// `ModifiedContent<AnyView, AddGestureModifier<RotateGesture, DefaultGestureCombiner>>`
    pub(crate) ROTATE_T = ty("AddGestureModifier").of(vec![ty("RotateGesture"), ty("DefaultGestureCombiner")]);
    /// `.popover(isPresented:content:)`
    /// `ModifiedContent<AnyView, PopoverPresentationModifier<PopoverConditionalStateProvider<AnyView>>>`
    pub(crate) POPOVER_T = ty("PopoverPresentationModifier").of(vec![ty("PopoverConditionalStateProvider").of(vec![ty("AnyView")])]);
}







/// Apply `.clipped()`.
pub fn clipped(view: &ViewHandle) -> ViewHandle {
    unsafe {
        let (_, _, size) = CLIPPED_T.applied_to(view.meta);
        let r = abi::call_modifier(resolve::clipped_fn(), view.value_ptr(), view.meta, view.wt, &[0], [0.0; 4], size);
        wrap_modified(r, &CLIPPED_T, view)
    }
}

/// Apply `.compositingGroup()`.
pub fn compositing_group(view: &ViewHandle) -> ViewHandle {
    unsafe {
        let (_, _, size) = COMPOSITING_T.applied_to(view.meta);
        let r = abi::call_modifier(resolve::compositing_group_fn(), view.value_ptr(), view.meta, view.wt, &[], [0.0; 4], size);
        wrap_modified(r, &COMPOSITING_T, view)
    }
}

/// Apply `.drawingGroup()` — render the subtree offscreen through Metal.
pub fn drawing_group(view: &ViewHandle) -> ViewHandle {
    unsafe {
        let (_, _, size) = DRAWING_GROUP_T.applied_to(view.meta);
        // `ColorRenderingMode` is a resilient enum, so it is address-only and
        // passed by pointer rather than by value.
        let mut mode = [0u8]; // .nonLinear
        let r = abi::call_modifier(resolve::drawing_group_fn(), view.value_ptr(), view.meta, view.wt,
            &[0, mode.as_mut_ptr() as u64],
            [0.0; 4],
            size,
        );
        wrap_modified(r, &DRAWING_GROUP_T, view)
    }
}

/// Apply `.rotation3DEffect(_:axis:)`.
pub fn rotation_3d(view: &ViewHandle, degrees: f64, ax: f64, ay: f64, az: f64) -> ViewHandle {
    unsafe {
        let (_, _, size) = ROTATION3D_T.applied_to(view.meta);
        let mut result = vec![0u8; size];
        let this = view.value_ptr();
        let radians = degrees * std::f64::consts::PI / 180.0;
        let (anchor_x, anchor_y) = (0.5f64, 0.5f64);
        // rotation3DEffect(_:axis:anchor:anchorZ:perspective:)
        //   every argument is a CGFloat or an Angle, so they all travel in
        //   d0-d7: angle, axis x/y/z, anchor x/y, anchorZ, perspective
        core::arch::asm!("blr {f}", f = in(reg) resolve::rotation3d_fn(),
            in("d0") radians,
            in("d1") ax, in("d2") ay, in("d3") az,
            in("d4") anchor_x, in("d5") anchor_y,
            in("d6") 0.0f64, in("d7") 1.0f64,
            in("x0") view.meta,
            in("x1") view.wt,
            in("x8") result.as_mut_ptr(),
            in("x20") this,
            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"),
        );
        wrap_modified(result, &ROTATION3D_T, view)
    }
}

/// Attach a gesture value that is already built.
pub(crate) unsafe fn attach_gesture(
    view: &ViewHandle,
    gesture: &mut [u8],
    g_meta: *const c_void,
    modifier: &ModifierType,
) -> ViewHandle {
    unsafe {
        let g_wt = abi::get_view_wt_for_proto(g_meta, resolve::gesture_proto());
        assert!(!g_wt.is_null(), "gesture type: Gesture");
        let (_, _, size) = modifier.applied_to(view.meta);
        let mut result = vec![0u8; size];
        let this = view.value_ptr();
        core::arch::asm!("blr {f}", f = in(reg) resolve::gesture_fn(),
            in("x0") gesture.as_mut_ptr(),
            in("x1") 3u64, // GestureMask.all
            in("x2") view.meta,
            in("x3") g_meta,
            in("x4") view.wt,
            in("x5") g_wt,
            in("x8") result.as_mut_ptr(),
            in("x20") this,
            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"),
        );
        wrap_modified(result, modifier, view)
    }
}

/// Attach a `DragGesture`.
pub fn drag_gesture(view: &ViewHandle, minimum_distance: f64) -> ViewHandle {
    unsafe {
        let g_meta = abi::metadata0(resolve::drag_gesture_ma());
        let space_meta = abi::metadata0(resolve::local_space_ma());
        let mut gesture = vec![0u8; abi::type_bytes(g_meta)];
        // DragGesture.init<C: CoordinateSpaceProtocol>(minimumDistance:coordinateSpace:)
        //   d0 = distance, x0 = @in C (LocalCoordinateSpace is zero-size),
        //   x1 = C.Type, x2 = C: CoordinateSpaceProtocol WT, x8 = @out
        let mut space = [0u8; 8];
        core::arch::asm!("blr {f}", f = in(reg) resolve::drag_gesture_init(),
            in("d0") minimum_distance,
            in("x0") space.as_mut_ptr(),
            in("x1") space_meta,
            in("x2") resolve::local_space_wt(),
            in("x8") gesture.as_mut_ptr(),
            in("x20") g_meta,
            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"),
        );
        attach_gesture(view, &mut gesture, g_meta, &DRAG_T)
    }
}

/// Attach a `DragGesture` with an `onEnded` handler.
///
/// [`drag_gesture`] attaches a bare `DragGesture` with nothing listening, which
/// builds the same value Swift would but can never be seen to fire. This wires
/// the action SwiftUI calls when the drag ends, so the gesture is observable.
///
/// The result type is taken from `.gesture`'s opaque-result-type descriptor
/// rather than a hand-written mangling: the type is
/// `ModifiedContent<AnyView, AddGestureModifier<_EndedGesture<DragGesture>, _>>`,
/// and spelling that out by hand is how earlier bindings resolved to null.
pub fn on_drag_gesture(view: &ViewHandle, minimum_distance: f64, action: impl Fn() + 'static) -> ViewHandle {
    unsafe {
        drag_ended(
            view,
            minimum_distance,
            invoke_value_action,
            boxed_ctx(Box::new(action) as Box<dyn Fn()>),
        )
    }
}

/// The same, with the value SwiftUI delivered.
///
/// [`on_drag_gesture`]'s handler is told only *that* a drag ended. This one is
/// told where it started, where it finished and how fast it was going — see
/// [`DragValue`].
pub fn on_drag_ended_value(
    view: &ViewHandle,
    minimum_distance: f64,
    action: impl Fn(DragValue) + 'static,
) -> ViewHandle {
    unsafe {
        drag_ended(
            view,
            minimum_distance,
            invoke_drag_value,
            boxed_ctx(Box::new(action) as Box<dyn Fn(DragValue)>),
        )
    }
}

/// Build the gesture and attach it — the part both handlers share.
///
/// # Safety
/// `payload` must be what `body` expects.
unsafe fn drag_ended(
    view: &ViewHandle,
    minimum_distance: f64,
    body: unsafe extern "C" fn(*const u8, *mut c_void),
    payload: *mut c_void,
) -> ViewHandle {
    unsafe {
        let g_meta = abi::metadata0(resolve::drag_gesture_ma());
        let space_meta = abi::metadata0(resolve::local_space_ma());
        let mut gesture = vec![0u8; abi::type_bytes(g_meta)];
        let mut space = [0u8; 8];
        core::arch::asm!("blr {f}", f = in(reg) resolve::drag_gesture_init(),
            in("d0") minimum_distance,
            in("x0") space.as_mut_ptr(),
            in("x1") space_meta,
            in("x2") resolve::local_space_wt(),
            in("x8") gesture.as_mut_ptr(),
            in("x20") g_meta,
            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"),
        );
        gesture_ended(view, &mut gesture, g_meta, body, payload)
    }
}

/// Wrap any built gesture in `.onEnded(_:)` and attach it.
///
/// Nothing below is drag-specific: `Gesture.onEnded` is an extension on the
/// protocol, `_EndedGesture` takes whatever gesture it is given, and the result
/// type comes from `.gesture`'s own opaque-result-type descriptor. So magnify
/// and rotate get handlers by handing their gesture to this — they had none at
/// all, which made them the only gestures the DSL could attach but never hear
/// from.
///
/// # Safety
/// `gesture` must be a live value of type `g_meta`, and `payload` what `body`
/// expects.
pub(crate) unsafe fn gesture_ended(
    view: &ViewHandle,
    gesture: &mut [u8],
    g_meta: *const c_void,
    body: unsafe extern "C" fn(*const u8, *mut c_void),
    payload: *mut c_void,
) -> ViewHandle {
    unsafe {
        let g_wt = abi::get_view_wt_for_proto(g_meta, resolve::gesture_proto());
        assert!(!g_wt.is_null(), "gesture type: Gesture");

        // `_EndedGesture<Content: Gesture>`: one generic parameter and one
        // conformance requirement, so the accessor takes two key arguments.
        let ended_meta =
            abi::generic_metadata2(resolve::ended_gesture_ma(), g_meta, g_wt);
        let mut ended = vec![0u8; abi::type_bytes(ended_meta)];

        // The handler takes `DragGesture.Value` by pointer and returns nothing,
        // which is exactly the shape `value_closure_thunk` was written for.
        let ctx = Box::leak(Box::new(abi::ValueClosureCtx::new(body, payload)))
            as *mut abi::ValueClosureCtx;

        // Gesture.onEnded(_:) — x0/x1 = closure, x2 = Self.Type,
        // x3 = Self: Gesture WT, x8 = @out, x20 = @in_guaranteed self.
        core::arch::asm!("blr {f}", f = in(reg) resolve::gesture_on_ended(),
            in("x0") abi::value_closure_thunk as *const () as u64,
            in("x1") ctx as u64,
            in("x2") g_meta,
            in("x3") g_wt,
            in("x8") ended.as_mut_ptr(),
            in("x20") gesture.as_mut_ptr(),
            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"),
        );

        let ended_wt = abi::get_view_wt_for_proto(ended_meta, resolve::gesture_proto());
        assert!(!ended_wt.is_null(), "_EndedGesture<G>: Gesture");

        let subst = [
            view.meta,
            ended_meta,
            view.wt,
            ended_wt,
        ];
        let (meta, wt, size) = opaque_result_type(resolve::gesture_oret(), &subst);
        let mut result = vec![0u8; size];
        let this = view.value_ptr();
        core::arch::asm!("blr {f}", f = in(reg) resolve::gesture_fn(),
            in("x0") ended.as_mut_ptr(),
            in("x1") 3u64, // GestureMask.all
            in("x2") view.meta,
            in("x3") ended_meta,
            in("x4") view.wt,
            in("x5") ended_wt,
            in("x8") result.as_mut_ptr(),
            in("x20") this,
            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"),
        );
        wrap_owned(result, meta, wt)
    }
}

/// Body for a `(Value) -> ()` handler that ignores the value.
pub(crate) unsafe extern "C" fn invoke_value_action(_value: *const u8, payload: *mut c_void) {
    let action = unsafe { boxed_ref::<dyn Fn()>(payload) };
    action();
}

/// Attach a `MagnifyGesture`.
pub fn magnify_gesture(view: &ViewHandle, minimum_scale_delta: f64) -> ViewHandle {
    unsafe {
        let g_meta = abi::metadata0(resolve::magnify_gesture_ma());
        let mut gesture = vec![0u8; abi::type_bytes(g_meta)];
        core::arch::asm!("blr {f}", f = in(reg) resolve::magnify_gesture_init(),
            in("d0") minimum_scale_delta,
            in("x8") gesture.as_mut_ptr(),
            in("x20") g_meta,
            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"),
        );
        attach_gesture(view, &mut gesture, g_meta, &MAGNIFY_T)
    }
}

/// Attach a `RotateGesture`. The minimum delta is an `Angle`, i.e. radians.
pub fn rotate_gesture(view: &ViewHandle, minimum_degrees: f64) -> ViewHandle {
    unsafe {
        let g_meta = abi::metadata0(resolve::rotate_gesture_ma());
        let mut gesture = vec![0u8; abi::type_bytes(g_meta)];
        let radians = minimum_degrees * std::f64::consts::PI / 180.0;
        core::arch::asm!("blr {f}", f = in(reg) resolve::rotate_gesture_init(),
            in("d0") radians,
            in("x8") gesture.as_mut_ptr(),
            in("x20") g_meta,
            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"),
        );
        attach_gesture(view, &mut gesture, g_meta, &ROTATE_T)
    }
}



/// The Rust side of an `onChange` handler.
pub(crate) type ChangeHandler = Box<dyn Fn(i64, i64)>;

pub(crate) unsafe extern "C" fn invoke_change(old: *const u8, new: *const u8, payload: *mut c_void) {
    let handler = unsafe { &*(payload as *const ChangeHandler) };
    unsafe { handler(*(old as *const i64), *(new as *const i64)) };
}

/// Apply `.onChange(of:initial:_:)`, watching an `Int`.
///
/// The handler is called with the old and new values. It is leaked, since
/// SwiftUI holds it for the lifetime of the view.
pub fn on_change(
    view: &ViewHandle,
    value: i64,
    handler: impl Fn(i64, i64) + 'static,
) -> ViewHandle {
    unsafe {
        let mut v = value;
        on_change_of(
            view,
            (&raw mut v).cast(),
            resolve::int_meta(),
            &ON_CHANGE_INNER_T,
            invoke_change,
            boxed_ctx(Box::new(handler) as Box<dyn Fn(i64, i64)>),
        )
    }
}

/// Apply `.onChange(of:initial:_:)`, watching a `String`.
///
/// `onChange` is one of the modifiers an application reaches for most, and it
/// could only ever watch an `Int` — a search field, a selected identifier, a
/// mode name all had to be turned into a number first. `V` is constrained to
/// `Equatable` and nothing else, so anything with a witness can be watched;
/// these are the three the standard library exports one for besides `Int`.
pub fn on_change_str(
    view: &ViewHandle,
    value: &str,
    handler: impl Fn(String, String) + 'static,
) -> ViewHandle {
    unsafe {
        // `of:` is `@in V`, which is *consumed* — the callee takes ownership,
        // so the string is handed over and not dropped here.
        let mut v = abi::swift_string(value);
        on_change_of(
            view,
            v.as_mut_ptr(),
            resolve::string_meta(),
            &ON_CHANGE_STR_T,
            invoke_change_str,
            boxed_ctx(Box::new(handler) as Box<dyn Fn(String, String)>),
        )
    }
}

/// Apply `.onChange(of:initial:_:)`, watching a `Bool`.
pub fn on_change_bool(
    view: &ViewHandle,
    value: bool,
    handler: impl Fn(bool, bool) + 'static,
) -> ViewHandle {
    unsafe {
        let mut v = u8::from(value);
        on_change_of(
            view,
            &raw mut v,
            resolve::bool_meta(),
            &ON_CHANGE_BOOL_T,
            invoke_change_bool,
            boxed_ctx(Box::new(handler) as Box<dyn Fn(bool, bool)>),
        )
    }
}

/// Apply `.onChange(of:initial:_:)`, watching a `Double`.
pub fn on_change_f64(
    view: &ViewHandle,
    value: f64,
    handler: impl Fn(f64, f64) + 'static,
) -> ViewHandle {
    unsafe {
        let mut v = value;
        on_change_of(
            view,
            (&raw mut v).cast(),
            resolve::double_meta(),
            &ON_CHANGE_F64_T,
            invoke_change_f64,
            boxed_ctx(Box::new(handler) as Box<dyn Fn(f64, f64)>),
        )
    }
}

pub(crate) unsafe extern "C" fn invoke_change_str(
    old: *const u8,
    new: *const u8,
    payload: *mut c_void,
) {
    unsafe {
        let f = boxed_ref::<dyn Fn(String, String)>(payload);
        let read = |p: *const u8| {
            let mut b = [0u8; 16];
            b.copy_from_slice(core::slice::from_raw_parts(p, 16));
            // A borrowed read: both strings belong to SwiftUI for the duration
            // of the call, so this copies out rather than taking ownership.
            abi::read_swift_string(b)
        };
        f(read(old), read(new));
    }
}

pub(crate) unsafe extern "C" fn invoke_change_bool(
    old: *const u8,
    new: *const u8,
    payload: *mut c_void,
) {
    unsafe {
        let f = boxed_ref::<dyn Fn(bool, bool)>(payload);
        f(*old & 1 != 0, *new & 1 != 0);
    }
}

pub(crate) unsafe extern "C" fn invoke_change_f64(
    old: *const u8,
    new: *const u8,
    payload: *mut c_void,
) {
    unsafe {
        let f = boxed_ref::<dyn Fn(f64, f64)>(payload);
        f(*(old as *const f64), *(new as *const f64));
    }
}

/// The half of `onChange` that does not depend on what is being watched.
///
/// `onChange<V: Equatable>` is generic, so everything type-specific is a
/// register: the value's address, its metadata, and the mangled name of the
/// `_ValueActionModifier2<V>` the result nests in.
///
/// # Safety
/// `value` must point at a live value of type `meta`, which the call consumes;
/// `inner` must be `_ValueActionModifier2` over that same type; and `payload`
/// must be what `body` expects.
unsafe fn on_change_of(
    view: &ViewHandle,
    value: *mut u8,
    meta: *const c_void,
    inner: &ModifierType,
    body: unsafe extern "C" fn(*const u8, *const u8, *mut c_void),
    payload: *mut c_void,
) -> ViewHandle {
    unsafe {
        let ctx = Box::leak(Box::new(abi::PairClosureCtx::new(body, payload)))
            as *mut abi::PairClosureCtx;

        let eq_wt = abi::get_view_wt_for_proto(meta, resolve::equatable_proto());
        assert!(!eq_wt.is_null(), "{}: Equatable", crate::diag::type_name(meta));

        let (inner_m, _, _) = inner.applied_to(view.meta);
        let (_, _, size) = ON_CHANGE_OUTER_T.applied_to(inner_m);
        let mut result = vec![0u8; size];
        let this = view.value_ptr();
        // onChange<V: Equatable>(of: V, initial: Bool, _: (V, V) -> ())
        //   x0 = @in V, x1 = initial, x2 = closure fn, x3 = closure ctx,
        //   x4 = Self.Type, x5 = V.Type, x6 = Self: View WT,
        //   x7 = V: Equatable WT, x8 = @out, x20 = self
        core::arch::asm!("blr {f}", f = in(reg) resolve::on_change_fn(),
            in("x0") value,
            in("x1") 0u64,
            in("x2") abi::pair_closure_thunk as *const c_void,
            in("x3") ctx,
            in("x4") view.meta,
            in("x5") meta,
            in("x6") view.wt,
            in("x7") eq_wt,
            in("x8") result.as_mut_ptr(),
            in("x20") this,
            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"),
        );
        // `onChange` adds two modifiers, so the result nests twice: the
        // value-action modifier over the receiver, then the appearance-action
        // modifier over that.
        let (nested, _, _) = inner.applied_to(view.meta);
        let (meta, wt, _) = ON_CHANGE_OUTER_T.applied_to(nested);
        wrap_owned(result, meta, wt)
    }
}



/// Apply `.popover(isPresented:content:)`.
///
/// The public `popover` is fully inlined — it has a symbol but nothing ever
/// calls it — so this goes through `popoverCore`, which is what Swift itself
/// ends up calling.
/// Shared tail of `popover` and `popover_bound`.
///
/// # Safety
/// `binding` must be a `Binding<Bool>`.
pub(crate) unsafe fn popover_with_binding(
    view: &ViewHandle,
    content: &ViewHandle,
    binding: &[u8],
) -> ViewHandle {
    unsafe {
        let b0 = u64::from_le_bytes(binding[0..8].try_into().unwrap());
        let b1 = u64::from_le_bytes(binding[8..16].try_into().unwrap());
        let b2 = binding[16] as u64;

        // PopoverAttachmentAnchor.rect(.bounds): the Source is one word, the
        // enum pads to sixteen and carries its tag in the seventeenth byte,
        // where `.rect` is case 0.
        let anchor_meta = abi::metadata0(resolve::popover_anchor_ma());
        let mut anchor = vec![0u8; abi::type_bytes(anchor_meta)];
        let source: u64;
        core::arch::asm!("blr {f}", f = in(reg) resolve::anchor_source_bounds(),
            lateout("x0") source,
            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("x20") _, lateout("x21") _,
            clobber_abi("C"),
        );
        anchor[..8].copy_from_slice(&source.to_le_bytes());

        let ctx = Box::leak(Box::new(abi::ClosureCtx::new(
            emit_tuple,
            // `POPOVER_T` names
            // `PopoverPresentationModifier<PopoverConditionalStateProvider<AnyView>>`:
            // the content's type is part of the modifier's own name, so the
            // child stays erased until that name is composed per content.
            Child::leak(content, Content::Erased),
        ))) as *mut abi::ClosureCtx;

        let (_, _, size) = POPOVER_T.applied_to(view.meta);
        let mut result = vec![0u8; size];
        let this = view.value_ptr();
        // popoverCore(isPresented:attachmentAnchor:arrowEdges:isDetachable:content:)
        //   x0-x2 = Binding<Bool>, x3 = @in anchor,
        //   x4 = Edge.Set? (nil is 0x0100 — value then tag),
        //   x5 = isDetachable, x6 = content fn, x7 = content ctx,
        //   [sp] = Self.Type, [sp+8] = C.Type,
        //   [sp+16] = Self: View WT, [sp+24] = C: View WT
        core::arch::asm!(
            "sub sp, sp, #32",
            "stp {sm}, {cm}, [sp]",
            "stp {sw}, {cw}, [sp, #16]",
            "blr {f}",
            "add sp, sp, #32",
            f = in(reg) resolve::popover_core_fn(),
            sm = in(reg) view.meta,
            cm = in(reg) resolve::anyview_meta(),
            sw = in(reg) view.wt,
            cw = in(reg) resolve::anyview_wt(),
            in("x0") b0, in("x1") b1, in("x2") b2,
            in("x3") anchor.as_mut_ptr(),
            in("x4") 0x0100u64,
            in("x5") 0u64,
            in("x6") abi::swift_closure_thunk as *const c_void,
            in("x7") ctx,
            in("x8") result.as_mut_ptr(),
            in("x20") this,
            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"),
        );
        wrap_modified(result, &POPOVER_T, view)
    }
}

/// `popover` with a fixed flag. It cannot be dismissed: SwiftUI treats a
/// `Binding.constant` as read-only, so the presentation has no way to write
/// `false` back. Use [`popover_bound`] for one the user can close.
pub fn popover(view: &ViewHandle, content: &ViewHandle, presented: bool) -> ViewHandle {
    unsafe {
        let binding = constant_binding(
            &[u8::from(presented)],
            resolve::bool_meta(),
            mangle::bound(&swift_type!(SwiftUI.Binding), &[mangle::BOOL]).as_bytes(),
        );
        popover_with_binding(view, content, &binding)
    }
}

/// `popover` bound to Rust state, so dismissing it writes `false` back.
pub fn popover_bound(view: &ViewHandle, content: &ViewHandle, state: &BoundState<bool>) -> ViewHandle {
    unsafe {
        let binding = bool_binding_of(state);
        popover_with_binding(view, content, &binding)
    }
}


// ── What a drag delivered ───────────────────────────────────────────────────

/// A `DragGesture.Value`, read out of the struct SwiftUI hands the handler.
///
/// `on_drag_gesture` used to throw this away — the handler was `() -> ()` and
/// the value went into `invoke_value_action`, which ignores its argument. A
/// drag handler that cannot see where the drag went is not much of a drag
/// handler: it can tell you that something was dragged and nothing else.
///
/// The fields come from the struct's own layout rather than from accessors,
/// because SwiftUI exports exactly one — `translation` — and none of the
/// others:
///
/// ```text
///   +0   time           Date            (seconds from the 2001 epoch)
///   +8   location       CGPoint
///   +24  startLocation  CGPoint
///   +40  _velocity      _Velocity<CGSize>
///   +56  platform       (empty)
/// ```
///
/// That layout is read from the metadata at run time rather than trusted: a
/// field added in front of `location` in a later SwiftUI would otherwise turn
/// every drag into silent nonsense. If the shape is not the five fields above,
/// the reader hands back zeroes rather than reading somewhere it should not.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct DragValue {
    /// Where the drag is now, in the gesture's coordinate space.
    pub location: (f64, f64),
    /// Where it began.
    pub start_location: (f64, f64),
    /// How fast it was moving, in points per second.
    ///
    /// `_Velocity<CGSize>` wraps a single `CGSize` and nothing else — checked
    /// by following the field record's symbolic reference to the descriptor,
    /// which reports one stored field — so the two doubles at the field's own
    /// offset are the velocity, with no wrapper to step over.
    pub velocity: (f64, f64),
    /// Seconds since Apple's reference date (1 January 2001), which is what a
    /// `Date` stores. Useful for comparing two values, not as a wall clock.
    pub time: f64,
}

impl DragValue {
    /// `location - startLocation`, which is what Swift's `translation`
    /// computes. Cross-checked against SwiftUI's own getter in the tests.
    pub fn translation(&self) -> (f64, f64) {
        (self.location.0 - self.start_location.0, self.location.1 - self.start_location.1)
    }
}

/// The offsets of `DragGesture.Value`'s stored fields, from its metadata.
///
/// Read rather than hardcoded: this is a resilient struct in a system
/// framework, and a field added in front of `location` would otherwise turn
/// every drag into silent nonsense. Five fields at these offsets is what this
/// build of SwiftUI has; a different shape returns `None` and the reader hands
/// back zeroes rather than reading somewhere it should not.
fn drag_value_offsets() -> Option<&'static [usize; 4]> {
    static CACHE: std::sync::OnceLock<Option<[usize; 4]>> = std::sync::OnceLock::new();
    CACHE
        .get_or_init(|| unsafe {
            let meta = abi::metadata0(resolve::drag_value_ma());
            let offs = abi::struct_field_offsets(meta)?;
            // time, location, startLocation, _velocity, platform.
            if offs.len() != 5 {
                return None;
            }
            Some([offs[0], offs[1], offs[2], offs[3]])
        })
        .as_ref()
}

/// Read the value at `p`, which SwiftUI passed by address.
///
/// # Safety
/// `p` must point at a live `DragGesture.Value`.
pub(crate) unsafe fn read_drag_value(p: *const u8) -> DragValue {
    let Some(&[time, location, start, velocity]) = drag_value_offsets() else {
        return DragValue {
            location: (0.0, 0.0),
            start_location: (0.0, 0.0),
            velocity: (0.0, 0.0),
            time: 0.0,
        };
    };
    unsafe {
        let point = |at: usize| {
            let f = p.add(at) as *const f64;
            (*f, *f.add(1))
        };
        let v = DragValue {
            time: *(p.add(time) as *const f64),
            location: point(location),
            start_location: point(start),
            velocity: point(velocity),
        };
        debug_assert_eq!(
            v.translation(),
            swift_translation(p),
            "computed translation disagrees with SwiftUI's own getter — the \
             field offsets have moved"
        );
        v
    }
}

/// What SwiftUI's own `translation` getter says, for checking against
/// [`DragValue::translation`].
///
/// Only the one accessor is exported: `velocity`, `predictedEndLocation` and
/// `predictedEndTranslation` resolve to nothing, so they are computed here or
/// not at all. This is the one crosscheck available, and it covers `location`
/// and `startLocation` between them — which are the fields a wrong offset
/// would corrupt.
///
/// # Safety
/// `p` must point at a live `DragGesture.Value`.
unsafe fn swift_translation(p: *const u8) -> (f64, f64) {
    unsafe {
        let (w, h): (f64, f64);
        // A getter on a 56-byte struct: self is `@in_guaranteed` in x20, and
        // `CGSize` comes back in d0/d1.
        core::arch::asm!("blr {f}", f = in(reg) resolve::drag_value_translation(),
            in("x20") p,
            lateout("d0") w, lateout("d1") h,
            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"),
        );
        (w, h)
    }
}

/// Body for a `(DragGesture.Value) -> ()` handler that reads the value.
pub(crate) unsafe extern "C" fn invoke_drag_value(value: *const u8, payload: *mut c_void) {
    let f = unsafe { boxed_ref::<dyn Fn(DragValue)>(payload) };
    f(unsafe { read_drag_value(value) });
}

// ── What a magnify or a rotate delivered ────────────────────────────────────

/// A `MagnifyGesture.Value`.
///
/// `magnify_gesture` attaches the gesture and listens to nothing — it was the
/// only kind of gesture the DSL could add but never hear from. The layout is
/// the same shape as the drag's and read the same way:
///
/// ```text
///   +0   time           Date
///   +8   magnification  CGFloat
///   +16  velocity       CGFloat
///   +24  startAnchor    UnitPoint
///   +40  startLocation  CGPoint
///   +56  platform       (empty)
/// ```
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct MagnifyValue {
    /// How far the pinch has scaled, as a multiplier — 1.0 is no change.
    pub magnification: f64,
    /// How fast it is scaling, in multiples per second.
    pub velocity: f64,
    /// Where the pinch is centred, in unit space — (0.5, 0.5) is the middle.
    pub start_anchor: (f64, f64),
    /// Where it began, in the gesture's coordinate space.
    pub start_location: (f64, f64),
    /// Seconds since Apple's reference date.
    pub time: f64,
}

/// A `RotateGesture.Value`.
///
/// The same layout as [`MagnifyValue`] with `rotation`/`velocity` as `Angle`,
/// which stores radians and nothing else.
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct RotateValue {
    /// How far it has turned, in radians. `Angle` stores radians; the degrees
    /// are computed, so the conversion is here rather than through an accessor.
    pub radians: f64,
    /// How fast it is turning, in radians per second.
    pub velocity: f64,
    /// Where the rotation is centred, in unit space.
    pub start_anchor: (f64, f64),
    /// Where it began, in the gesture's coordinate space.
    pub start_location: (f64, f64),
    /// Seconds since Apple's reference date.
    pub time: f64,
}

impl RotateValue {
    /// The rotation in degrees, which is what `Angle.degrees` reports.
    pub fn degrees(&self) -> f64 {
        self.radians.to_degrees()
    }
}

/// The offsets shared by `MagnifyGesture.Value` and `RotateGesture.Value`.
///
/// Both are six fields at 0, 8, 16, 24, 40, 56 in this build of SwiftUI, read
/// from the metadata for the same reason the drag's are: a field inserted in
/// front of `magnification` would otherwise be read as the magnification.
fn pinch_value_offsets(accessor: *const c_void) -> Option<[usize; 5]> {
    unsafe {
        let offs = abi::struct_field_offsets(abi::metadata0(accessor))?;
        // time, amount, velocity, startAnchor, startLocation, platform.
        if offs.len() != 6 {
            return None;
        }
        Some([offs[0], offs[1], offs[2], offs[3], offs[4]])
    }
}

/// Read the five readable fields, which both gestures share.
///
/// # Safety
/// `p` must point at a live value of the type `accessor` names.
unsafe fn read_pinch_value(p: *const u8, accessor: *const c_void) -> [f64; 7] {
    let Some([time, amount, velocity, anchor, start]) = pinch_value_offsets(accessor) else {
        return [0.0; 7];
    };
    unsafe {
        let at = |o: usize| *(p.add(o) as *const f64);
        let pair = |o: usize| (at(o), at(o + 8));
        let (ax, ay) = pair(anchor);
        let (sx, sy) = pair(start);
        [at(amount), at(velocity), ax, ay, sx, sy, at(time)]
    }
}

/// Body for a `(MagnifyGesture.Value) -> ()` handler.
unsafe extern "C" fn invoke_magnify_value(value: *const u8, payload: *mut c_void) {
    let f = unsafe { boxed_ref::<dyn Fn(MagnifyValue)>(payload) };
    let v = unsafe { read_pinch_value(value, resolve::magnify_value_ma()) };
    f(MagnifyValue {
        magnification: v[0],
        velocity: v[1],
        start_anchor: (v[2], v[3]),
        start_location: (v[4], v[5]),
        time: v[6],
    });
}

/// Body for a `(RotateGesture.Value) -> ()` handler.
unsafe extern "C" fn invoke_rotate_value(value: *const u8, payload: *mut c_void) {
    let f = unsafe { boxed_ref::<dyn Fn(RotateValue)>(payload) };
    let v = unsafe { read_pinch_value(value, resolve::rotate_value_ma()) };
    f(RotateValue {
        radians: v[0],
        velocity: v[1],
        start_anchor: (v[2], v[3]),
        start_location: (v[4], v[5]),
        time: v[6],
    });
}

/// Attach a `MagnifyGesture` with an `onEnded` handler.
pub fn on_magnify_ended(
    view: &ViewHandle,
    minimum_scale_delta: f64,
    action: impl Fn(MagnifyValue) + 'static,
) -> ViewHandle {
    unsafe {
        let g_meta = abi::metadata0(resolve::magnify_gesture_ma());
        let mut gesture = vec![0u8; abi::type_bytes(g_meta)];
        core::arch::asm!("blr {f}", f = in(reg) resolve::magnify_gesture_init(),
            in("d0") minimum_scale_delta,
            in("x8") gesture.as_mut_ptr(),
            in("x20") g_meta,
            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"),
        );
        gesture_ended(
            view,
            &mut gesture,
            g_meta,
            invoke_magnify_value,
            boxed_ctx(Box::new(action) as Box<dyn Fn(MagnifyValue)>),
        )
    }
}

/// Attach a `RotateGesture` with an `onEnded` handler.
pub fn on_rotate_ended(
    view: &ViewHandle,
    minimum_degrees: f64,
    action: impl Fn(RotateValue) + 'static,
) -> ViewHandle {
    unsafe {
        let g_meta = abi::metadata0(resolve::rotate_gesture_ma());
        let mut gesture = vec![0u8; abi::type_bytes(g_meta)];
        // `minimumAngleDelta` is an `Angle`, which stores radians — the same
        // conversion `rotate_gesture` does, and the reason this is spelled out
        // rather than left to look like a plain float argument.
        let radians = minimum_degrees * std::f64::consts::PI / 180.0;
        core::arch::asm!("blr {f}", f = in(reg) resolve::rotate_gesture_init(),
            in("d0") radians,
            in("x8") gesture.as_mut_ptr(),
            in("x20") g_meta,
            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"),
        );
        gesture_ended(
            view,
            &mut gesture,
            g_meta,
            invoke_rotate_value,
            boxed_ctx(Box::new(action) as Box<dyn Fn(RotateValue)>),
        )
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    /// The pinch readers pick the right field out of a real layout.
    ///
    /// A synthetic magnify or rotate event is not something AppKit will build —
    /// there is no public constructor for one — so unlike the drag, these
    /// readers cannot be driven end to end. What *can* be checked is the part
    /// that would be wrong if the offsets moved: a buffer is filled with a
    /// distinct value at each of the offsets the metadata reports, and the
    /// reader has to come back with them in the right order.
    ///
    /// This is a weaker check than the drag's behavioural one and worth saying
    /// so: it proves the reader agrees with the metadata, not that SwiftUI puts
    /// the magnification where its metadata says. The drag case covers that for
    /// the shared mechanism.
    #[test]
    fn pinch_reader_matches_the_live_layout() {
        for accessor in [resolve::magnify_value_ma(), resolve::rotate_value_ma()] {
            let offsets = pinch_value_offsets(accessor).expect("six stored fields");
            let [time, amount, velocity, anchor, start] = offsets;

            // A distinct value per field, so a swapped pair cannot pass.
            let size = unsafe { abi::type_bytes(abi::metadata0(accessor)) };
            let mut buf = vec![0u8; size];
            let put = |b: &mut [u8], at: usize, v: f64| {
                b[at..at + 8].copy_from_slice(&v.to_le_bytes());
            };
            put(&mut buf, time, 11.0);
            put(&mut buf, amount, 22.0);
            put(&mut buf, velocity, 33.0);
            put(&mut buf, anchor, 44.0);
            put(&mut buf, anchor + 8, 55.0);
            put(&mut buf, start, 66.0);
            put(&mut buf, start + 8, 77.0);

            let got = unsafe { read_pinch_value(buf.as_ptr(), accessor) };
            assert_eq!(got, [22.0, 33.0, 44.0, 55.0, 66.0, 77.0, 11.0]);
        }
    }

    /// `Angle` stores radians, so the degrees are computed.
    #[test]
    fn rotate_value_converts_to_degrees() {
        let v = RotateValue {
            radians: std::f64::consts::FRAC_PI_2,
            velocity: 0.0,
            start_anchor: (0.5, 0.5),
            start_location: (0.0, 0.0),
            time: 0.0,
        };
        assert!((v.degrees() - 90.0).abs() < 1e-9);
    }

    /// A drag's translation is the difference of its two points.
    #[test]
    fn drag_value_translation_is_the_difference() {
        let v = DragValue {
            location: (142.0, 40.0),
            start_location: (70.0, 40.0),
            velocity: (164.0, 0.0),
            time: 0.0,
        };
        assert_eq!(v.translation(), (72.0, 0.0));
    }
}