swiftui_native/

objc.rs

//! Minimal Objective-C runtime bridge.
//!
//! AppKit and UIKit classes are reached through `objc_msgSend` rather than the Swift
//! calling convention — that is their native ABI, so nothing is lost.
//!
//! Every send goes through a correctly-typed, *non-variadic* view of
//! `objc_msgSend`. On arm64 variadic arguments are passed on the stack, so
//! calling a variadic declaration with real arguments delivers them where the
//! receiver never looks and the callee reads uninitialised registers instead.

use core::ffi::{c_char, c_void};

unsafe extern "C" {
    fn dlsym(handle: *mut c_void, symbol: *const c_char) -> *mut c_void;
    fn objc_msgSend(receiver: *mut c_void, sel: *const c_void, ...) -> *mut c_void;
    fn sel_registerName(name: *const c_char) -> *const c_void;

    fn objc_allocateClassPair(
        superclass: *mut c_void,
        name: *const c_char,
        extra_bytes: usize,
    ) -> *mut c_void;
    fn objc_registerClassPair(class: *mut c_void);
    fn objc_getClass(name: *const c_char) -> *mut c_void;
    fn objc_getProtocol(name: *const c_char) -> *mut c_void;
    fn class_addMethod(
        class: *mut c_void,
        name: *const c_void,
        imp: *const c_void,
        types: *const c_char,
    ) -> bool;
    fn class_addProtocol(class: *mut c_void, protocol: *mut c_void) -> bool;
}

const RTLD_DEFAULT: *mut c_void = -2isize as *mut c_void;

#[repr(C)]
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct NSPoint {
    pub x: f64,
    pub y: f64,
}

#[repr(C)]
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct NSSize {
    pub width: f64,
    pub height: f64,
}

#[repr(C)]
#[derive(Clone, Copy, Debug, PartialEq)]
pub struct NSRect {
    pub origin: NSPoint,
    pub size: NSSize,
}

impl NSRect {
    pub fn new(x: f64, y: f64, width: f64, height: f64) -> Self {
        Self { origin: NSPoint { x, y }, size: NSSize { width, height } }
    }
}

/// An autorelease pool, drained when it goes out of scope.
///
/// Objective-C's convention is that a method returning an object it does not
/// own hands back an *autoreleased* one — alive until the enclosing pool
/// drains. Cocoa's own event loop provides that pool; a Rust program has none,
/// so without this every temporary AppKit or UIKit object accumulates for the
/// life of the process. It is not a slow leak: capturing a few hundred frames
/// costs tens of megabytes.
///
/// Every entry point here that talks to AppKit or UIKit holds one for the
/// duration of the call. Code that loops over such calls wants its own around
/// the loop as well, so peak memory tracks one iteration rather than all of
/// them.
///
/// ```ignore
/// for frame in 0..600 {
///     let _pool = objc::AutoreleasePool::new();
///     capture(frame);
/// }
/// ```
pub struct AutoreleasePool {
    context: *mut c_void,
}

impl AutoreleasePool {
    pub fn new() -> Self {
        unsafe extern "C" {
            fn objc_autoreleasePoolPush() -> *mut c_void;
        }
        Self { context: unsafe { objc_autoreleasePoolPush() } }
    }
}

impl Default for AutoreleasePool {
    fn default() -> Self {
        Self::new()
    }
}

impl Drop for AutoreleasePool {
    fn drop(&mut self) {
        unsafe extern "C" {
            fn objc_autoreleasePoolPop(context: *mut c_void);
        }
        unsafe { objc_autoreleasePoolPop(self.context) };
    }
}

/// The address of `objc_msgSend`, for building a typed view of it.
///
/// Every call site needs its own signature — see the module note on why a
/// variadic declaration will not do — so they transmute this rather than
/// calling `objc_msgSend` directly.
pub fn msg_send_addr() -> *const c_void {
    objc_msgSend as *const c_void
}

/// Register (or look up) a selector.
pub fn sel(name: &core::ffi::CStr) -> *const c_void {
    unsafe { sel_registerName(name.as_ptr().cast()) }
}

/// Look up an Objective-C class by its `OBJC_CLASS_$_Name` symbol.
pub fn class(symbol: &core::ffi::CStr) -> *mut c_void {
    let c = unsafe { dlsym(RTLD_DEFAULT, symbol.as_ptr()) };
    assert!(!c.is_null(), "Objective-C class not found: {symbol:?}");
    c
}

macro_rules! send {
    ($name:ident, ($($arg:ident: $ty:ty),*) -> $ret:ty) => {
        /// Send a message with a specific, non-variadic signature.
                /// # Safety
        /// `recv` must be a live Objective-C object that responds to `s`, and the
        /// declared argument and return types must match that selector's. Nothing
        /// here checks either.
pub unsafe fn $name(recv: *mut c_void, s: *const c_void $(, $arg: $ty)*) -> $ret {
            type F = unsafe extern "C" fn(*mut c_void, *const c_void $(, $ty)*) -> $ret;
            let f: F = unsafe { core::mem::transmute(objc_msgSend as *const c_void) };
            unsafe { f(recv, s $(, $arg)*) }
        }
    };
}

send!(send, () -> *mut c_void);
send!(send_id, (a: *mut c_void) -> *mut c_void);
send!(send_bool, (a: bool) -> *mut c_void);
send!(send_i64, (a: i64) -> *mut c_void);
send!(send_u64, (a: u64) -> *mut c_void);
send!(send_f64, (a: f64) -> *mut c_void);
send!(send_rect, (a: NSRect) -> *mut c_void);
send!(send_rect_id, (a: NSRect, b: *mut c_void) -> *mut c_void);
send!(send_u64_id, (a: u64, b: *mut c_void) -> *mut c_void);
send!(send_id_bool, (a: *mut c_void, b: bool) -> bool);
send!(send_ret_size, () -> NSSize);
send!(send_ret_rect, () -> NSRect);
send!(send_ret_u64, () -> u64);
send!(send_ret_f64, () -> f64);
send!(send_ret_ptr, () -> *const u8);
send!(send_bool_id, (a: bool, b: *mut c_void) -> *mut c_void);
send!(send_i64_id, (a: i64, b: *mut c_void) -> *mut c_void);
send!(send_f64_id, (a: f64, b: *mut c_void) -> *mut c_void);
send!(send_id_id, (a: *mut c_void, b: *mut c_void) -> *mut c_void);
send!(send_id_ret_bool, (a: *mut c_void) -> bool);
send!(send_id_ret_i64, (a: *mut c_void) -> i64);
send!(send_id_ret_f64, (a: *mut c_void) -> f64);
send!(send_ret_bool, () -> bool);

/// Does `receiver` implement `selector`?
///
/// Used to pick between an API and its newer replacement at runtime rather than
/// at build time.
///
/// # Safety
/// `receiver` must be a live object or class.
pub unsafe fn responds(receiver: *mut c_void, selector: *const c_void) -> bool {
    unsafe { send_id(receiver, sel(c"respondsToSelector:"), selector as *mut c_void) as usize & 1 == 1 }
}

/// Create an `NSString` from a Rust `&str`.
/// # Safety
/// Foundation must be loaded. The returned object is `+1`.
pub unsafe fn nsstring(s: &str) -> *mut c_void {
    let alloc = unsafe { send(class(c"OBJC_CLASS_$_NSString"), sel(c"alloc")) };
    type InitBytes =
        unsafe extern "C" fn(*mut c_void, *const c_void, *const u8, u64, u64) -> *mut c_void;
    let f: InitBytes = unsafe { core::mem::transmute(objc_msgSend as *const c_void) };
    // 4 = NSUTF8StringEncoding
    let out = unsafe {
        f(alloc, sel(c"initWithBytes:length:encoding:"), s.as_ptr(), s.len() as u64, 4)
    };
    assert!(!out.is_null(), "NSString creation failed for {s:?}");
    out
}

/// `[NSApplication sharedApplication]`, pinned to the light appearance so that
/// rendering does not depend on the host's system setting.
#[cfg(target_os = "macos")]
/// # Safety
/// AppKit must be loaded and this must run on the main thread, as every
/// `NSApplication` call must.
pub unsafe fn shared_app() -> *mut c_void {
    let app = unsafe { send(class(c"OBJC_CLASS_$_NSApplication"), sel(c"sharedApplication")) };
    unsafe {
        let name = nsstring("NSAppearanceNameAqua");
        let appearance = send_id(
            class(c"OBJC_CLASS_$_NSAppearance"),
            sel(c"appearanceNamed:"),
            name,
        );
        if !appearance.is_null() {
            send_id(app, sel(c"setAppearance:"), appearance);
        }
    }
    app
}

/// `[UIApplication sharedApplication]`.
///
/// Null unless the process actually went through `UIApplicationMain` — which a
/// plain binary run under `simctl spawn` has not. Rendering does not need it;
/// showing a window does.
///
/// # Safety
/// Calling into UIKit off the main thread is not supported.
#[cfg(not(target_os = "macos"))]
pub unsafe fn shared_app() -> *mut c_void {
    unsafe { send(class(c"OBJC_CLASS_$_UIApplication"), sel(c"sharedApplication")) }
}

// ═══════════════════════════════════════════════════════════════════════════
// Defining classes at runtime
// ═══════════════════════════════════════════════════════════════════════════
//
// The Objective-C counterpart of what [`crate::state`] does on the Swift side:
// build the type, give it its methods, register it. UIKit will not start an
// application without a delegate *class* to name, and writing one in Objective-C
// would mean shipping a source file — so it is synthesised here instead.

/// A class being built. Add methods, then [`register`](ClassBuilder::register).
pub struct ClassBuilder {
    class: *mut c_void,
}

impl ClassBuilder {
    /// Begin a new class, or return `None` if the name is already taken.
    ///
    /// # Safety
    /// `superclass` must name a registered class.
    pub unsafe fn new(superclass: &core::ffi::CStr, name: &core::ffi::CStr) -> Option<Self> {
        unsafe {
            let sup = objc_getClass(superclass.as_ptr().cast());
            assert!(!sup.is_null(), "superclass not found: {superclass:?}");
            let class = objc_allocateClassPair(sup, name.as_ptr(), 0);
            if class.is_null() {
                return None;
            }
            Some(Self { class })
        }
    }

    /// Add a method.
    ///
    /// `types` is an Objective-C type encoding: return type, then `@:` for the
    /// implicit receiver and selector, then one code per argument. `B@:@@` is
    /// "returns BOOL, takes two objects".
    ///
    /// # Safety
    /// `imp`'s signature must match `types`, receiver and selector included.
    pub unsafe fn method(
        self,
        selector: &core::ffi::CStr,
        imp: *const c_void,
        types: &core::ffi::CStr,
    ) -> Self {
        unsafe {
            let ok = class_addMethod(self.class, sel(selector), imp, types.as_ptr());
            assert!(ok, "could not add {selector:?}");
        }
        self
    }

    /// Declare conformance to a protocol.
    ///
    /// # Safety
    /// The class must actually implement the protocol's required methods.
    pub unsafe fn conforms(self, protocol: &core::ffi::CStr) -> Self {
        unsafe {
            let p = objc_getProtocol(protocol.as_ptr());
            if !p.is_null() {
                class_addProtocol(self.class, p);
            }
        }
        self
    }

    /// Register the class with the runtime and return it.
    pub fn register(self) -> *mut c_void {
        unsafe { objc_registerClassPair(self.class) };
        self.class
    }
}

/// Look up a registered class by name, or null.
pub fn class_named(name: &core::ffi::CStr) -> *mut c_void {
    unsafe { objc_getClass(name.as_ptr().cast()) }
}

/// An Objective-C block with no captures.
///
/// A block is an object whose first word is an `isa` like any other; what makes
/// it callable is the `invoke` pointer at offset 16, which is passed the block
/// itself as its first argument. With nothing captured there is nothing to copy
/// or release, so the block can be a plain `static` tagged `BLOCK_IS_GLOBAL` —
/// `_NSConcreteGlobalBlock` is exactly the isa for that.
#[repr(C)]
pub struct GlobalBlock {
    isa: *const c_void,
    flags: i32,
    reserved: i32,
    invoke: *const c_void,
    descriptor: *const BlockDescriptor,
}

/// The size-and-copy record every block points at.
#[repr(C)]
pub struct BlockDescriptor {
    reserved: u64,
    size: u64,
}

// `Sync` because nothing here is ever written after construction.
unsafe impl Sync for GlobalBlock {}
unsafe impl Sync for BlockDescriptor {}

/// `BLOCK_IS_GLOBAL`.
const BLOCK_IS_GLOBAL: i32 = 1 << 28;

impl GlobalBlock {
    /// Wrap `invoke` as a block. It is called with the block as its first
    /// argument, then whatever the block's signature declares.
    ///
    /// # Safety
    /// `invoke` must match the signature the receiver expects, including the
    /// leading block pointer.
    pub unsafe fn new(invoke: *const c_void, descriptor: &'static BlockDescriptor) -> Self {
        let isa = unsafe { dlsym(RTLD_DEFAULT, c"_NSConcreteGlobalBlock".as_ptr()) };
        assert!(!isa.is_null(), "_NSConcreteGlobalBlock");
        GlobalBlock { isa, flags: BLOCK_IS_GLOBAL, reserved: 0, invoke, descriptor }
    }
}

impl BlockDescriptor {
    /// The descriptor for a block with no captures.
    pub const fn empty() -> Self {
        BlockDescriptor { reserved: 0, size: core::mem::size_of::<GlobalBlock>() as u64 }
    }
}

/// A block carrying one pointer.
///
/// [`GlobalBlock`] captures nothing, which is all a block needs when the
/// receiver only ever calls it — but a *completion* handler has to reach
/// something, and the something differs per call. One captured word is enough
/// for that: it is where the answer gets written.
///
/// `_NSConcreteStackBlock` with no copy/dispose helpers, so a receiver that
/// copies it gets a `memcpy` and the capture comes along by value. That is only
/// sound while the caller outlives the call, which is why this is used with a
/// wait and not handed to anything that keeps it.
#[repr(C)]
pub struct CapturingBlock {
    isa: *const c_void,
    flags: i32,
    reserved: i32,
    invoke: *const c_void,
    descriptor: *const BlockDescriptor,
    /// What the invoke function reads, at a fixed offset from the block.
    pub capture: *mut c_void,
}

impl CapturingBlock {
    /// Wrap `invoke` as a block carrying `capture`.
    ///
    /// # Safety
    /// `invoke` must match the signature the receiver expects, taking the block
    /// itself first; and the block must outlive every call to it.
    pub unsafe fn new(invoke: *const c_void, capture: *mut c_void) -> Self {
        static DESCRIPTOR: BlockDescriptor = BlockDescriptor {
            reserved: 0,
            size: core::mem::size_of::<CapturingBlock>() as u64,
        };
        let isa = unsafe { dlsym(RTLD_DEFAULT, c"_NSConcreteStackBlock".as_ptr()) };
        assert!(!isa.is_null(), "_NSConcreteStackBlock");
        CapturingBlock {
            isa,
            flags: 0,
            reserved: 0,
            invoke,
            descriptor: &DESCRIPTOR,
            capture,
        }
    }
}