Objective-C Runtime (Low-level ObjC interfacing)

This adds a new homemade package `pkg/objc` that wraps the Objective-C runtime so that Zig can interface with ObjC. This doesn't have 100% API coverage but all the hard problems (mainly `objc_msgSend`) are done. We can merge this now, start working on ObjC stuff, and expand APIs as we need them.

The `objc` package is of course only available to Mac builds.

README.md

@@ -75,16 +75,12 @@ on the [Zig downloads page](https://ziglang.org/download/).
 With Zig installed, a binary can be built using `zig build`:
 
 ```shell-session
-$ zig build -fstage1
+$ zig build
 ...
 
 $ zig-out/bin/ghostty
 ```
 
-**Important: you must specify the `-fstage1` flag.** Ghostty can't yet be
-built with the self-hosted Zig backend, so we have to use "stage1" (the
-C++ LLVM backend).
-
 This will build a binary for the currently running system (if supported).
 You can cross compile by setting `-Dtarget=<target-triple>`. For example,
 `zig build -Dtarget=aarch64-macos` will build for Apple Silicon macOS. Note

build.zig

@@ -11,6 +11,7 @@ const libxml2 = @import("vendor/zig-libxml2/libxml2.zig");
 const libuv = @import("pkg/libuv/build.zig");
 const libpng = @import("pkg/libpng/build.zig");
 const macos = @import("pkg/macos/build.zig");
+const objc = @import("pkg/objc/build.zig");
 const stb_image_resize = @import("pkg/stb_image_resize/build.zig");
 const utf8proc = @import("pkg/utf8proc/build.zig");
 const zlib = @import("pkg/zlib/build.zig");
@@ -199,6 +200,7 @@ fn addDeps(
 
     // Mac Stuff
     if (step.target.isDarwin()) {
+        step.addPackage(objc.pkg);
         step.addPackage(macos.pkg);
         _ = try macos.link(b, step, .{});
     }

pkg/objc/build.zig

@@ -0,0 +1,10 @@
+const std = @import("std");
+
+pub const pkg = std.build.Pkg{
+    .name = "objc",
+    .source = .{ .path = thisDir() ++ "/main.zig" },
+};
+
+fn thisDir() []const u8 {
+    return std.fs.path.dirname(@src().file) orelse ".";
+}

pkg/objc/c.zig

@@ -0,0 +1,4 @@
+pub usingnamespace @cImport({
+    @cInclude("objc/runtime.h");
+    @cInclude("objc/message.h");
+});

pkg/objc/class.zig

@@ -0,0 +1,74 @@
+const std = @import("std");
+const c = @import("c.zig");
+const objc = @import("main.zig");
+const MsgSend = @import("msg_send.zig").MsgSend;
+
+pub const Class = struct {
+    value: c.Class,
+
+    pub usingnamespace MsgSend(Class);
+
+    /// Returns the class definition of a specified class.
+    pub fn getClass(name: [:0]const u8) ?Class {
+        return Class{
+            .value = c.objc_getClass(name.ptr) orelse return null,
+        };
+    }
+
+    /// Returns a property with a given name of a given class.
+    pub fn getProperty(self: Class, name: [:0]const u8) ?objc.Property {
+        return objc.Property{
+            .value = c.class_getProperty(self.value, name.ptr) orelse return null,
+        };
+    }
+
+    /// Describes the properties declared by a class. This must be freed.
+    pub fn copyPropertyList(self: Class) []objc.Property {
+        var count: c_uint = undefined;
+        const list = @ptrCast([*c]objc.Property, c.class_copyPropertyList(self.value, &count));
+        if (count == 0) return list[0..0];
+        return list[0..count];
+    }
+};
+
+test "getClass" {
+    const testing = std.testing;
+    const NSObject = Class.getClass("NSObject");
+    try testing.expect(NSObject != null);
+    try testing.expect(Class.getClass("NoWay") == null);
+}
+
+test "msgSend" {
+    const testing = std.testing;
+    const NSObject = Class.getClass("NSObject").?;
+
+    // Should work with primitives
+    const id = NSObject.msgSend(c.id, objc.Sel.registerName("alloc"), .{});
+    try testing.expect(id != null);
+    {
+        const obj: objc.Object = .{ .value = id };
+        obj.msgSend(void, objc.sel("dealloc"), .{});
+    }
+
+    // Should work with our wrappers
+    const obj = NSObject.msgSend(objc.Object, objc.Sel.registerName("alloc"), .{});
+    try testing.expect(obj.value != null);
+    obj.msgSend(void, objc.sel("dealloc"), .{});
+}
+
+test "getProperty" {
+    const testing = std.testing;
+    const NSObject = Class.getClass("NSObject").?;
+
+    try testing.expect(NSObject.getProperty("className") != null);
+    try testing.expect(NSObject.getProperty("nope") == null);
+}
+
+test "copyProperyList" {
+    const testing = std.testing;
+    const NSObject = Class.getClass("NSObject").?;
+
+    const list = NSObject.copyPropertyList();
+    defer objc.free(list);
+    try testing.expect(list.len > 20);
+}

pkg/objc/main.zig

@@ -0,0 +1,17 @@
+const std = @import("std");
+
+pub const c = @import("c.zig");
+pub usingnamespace @import("class.zig");
+pub usingnamespace @import("object.zig");
+pub usingnamespace @import("property.zig");
+pub usingnamespace @import("sel.zig");
+
+/// This just calls the C allocator free. Some things need to be freed
+/// and this is how they can be freed for objc.
+pub inline fn free(ptr: anytype) void {
+    std.heap.c_allocator.free(ptr);
+}
+
+test {
+    std.testing.refAllDecls(@This());
+}

pkg/objc/msg_send.zig

@@ -0,0 +1,135 @@
+const std = @import("std");
+const assert = std.debug.assert;
+const c = @import("c.zig");
+const objc = @import("main.zig");
+
+/// Returns a struct that implements the msgSend function for type T.
+/// This is meant to be used with `usingnamespace` to add dispatch
+/// capability to a type that supports it.
+pub fn MsgSend(comptime T: type) type {
+    // 1. T should be a struct
+    // 2. T should have a field "value" that can be an "id" (same size)
+
+    return struct {
+        /// Invoke a selector on the target, i.e. an instance method on an
+        /// object or a class method on a class. The args should be a tuple.
+        pub fn msgSend(
+            target: T,
+            comptime Return: type,
+            sel: objc.Sel,
+            args: anytype,
+        ) Return {
+            // Build our function type and call it
+            const Fn = MsgSendFn(Return, @TypeOf(target.value), @TypeOf(args));
+            const msg_send_ptr = @ptrCast(std.meta.FnPtr(Fn), &c.objc_msgSend);
+            const result = @call(.{}, msg_send_ptr, .{ target.value, sel } ++ args);
+
+            // This is a special nicety: if the return type is one of our
+            // public structs then we wrap the msgSend id result with it.
+            // This lets msgSend magically work with Object and so on.
+            const is_pkg_struct = comptime is_pkg_struct: {
+                for (@typeInfo(objc).Struct.decls) |decl| {
+                    if (decl.is_pub and
+                        @TypeOf(@field(objc, decl.name)) == type and
+                        Return == @field(objc, decl.name))
+                    {
+                        break :is_pkg_struct true;
+                    }
+                }
+
+                break :is_pkg_struct false;
+            };
+
+            if (!is_pkg_struct) return result;
+            return .{ .value = result };
+        }
+    };
+}
+
+/// This returns a function body type for `obj_msgSend` that matches
+/// the given return type, target type, and arguments tuple type.
+///
+/// obj_msgSend is a really interesting function, because it doesn't act
+/// like a typical function. You have to call it with the C ABI as if you're
+/// calling the true target function, not as a varargs C function. Therefore
+/// you have to cast obj_msgSend to a function pointer type of the final
+/// destination function, then call that.
+///
+/// Example: you have an ObjC function like this:
+///
+///     @implementation Foo
+///     - (void)log: (float)x { /* stuff */ }
+///
+/// If you call it like this, it won't work (you'll get garbage):
+///
+///     objc_msgSend(obj, @selector(log:), (float)PI);
+///
+/// You have to call it like this:
+///
+///     ((void (*)(id, SEL, float))objc_msgSend)(obj, @selector(log:), M_PI);
+///
+/// This comptime function returns the function body type that can be used
+/// to cast and call for the proper C ABI behavior.
+fn MsgSendFn(
+    comptime Return: type,
+    comptime Target: type,
+    comptime Args: type,
+) type {
+    const argsInfo = @typeInfo(Args).Struct;
+    assert(argsInfo.is_tuple);
+
+    // Target must always be an "id". Lots of types (Class, Object, etc.)
+    // are an "id" so we just make sure the sizes match for ABI reasons.
+    assert(@sizeOf(Target) == @sizeOf(c.id));
+
+    // Build up our argument types.
+    const Fn = std.builtin.Type.Fn;
+    const args: []Fn.Param = args: {
+        var acc: [argsInfo.fields.len + 2]Fn.Param = undefined;
+
+        // First argument is always the target and selector.
+        acc[0] = .{ .arg_type = Target, .is_generic = false, .is_noalias = false };
+        acc[1] = .{ .arg_type = objc.Sel, .is_generic = false, .is_noalias = false };
+
+        // Remaining arguments depend on the args given, in the order given
+        for (argsInfo.fields) |field, i| {
+            acc[i + 2] = .{
+                .arg_type = field.field_type,
+                .is_generic = false,
+                .is_noalias = false,
+            };
+        }
+
+        break :args &acc;
+    };
+
+    // Copy the alignment of a normal function type so equality works
+    // (mainly for tests, I don't think this has any consequence otherwise)
+    const alignment = @typeInfo(fn () callconv(.C) void).Fn.alignment;
+
+    return @Type(.{
+        .Fn = .{
+            .calling_convention = .C,
+            .alignment = alignment,
+            .is_generic = false,
+            .is_var_args = false,
+            .return_type = Return,
+            .args = args,
+        },
+    });
+}
+
+test {
+    // https://github.com/ziglang/zig/issues/12360
+    if (true) return error.SkipZigTest;
+
+    const testing = std.testing;
+    try testing.expectEqual(fn (
+        u8,
+        objc.Sel,
+    ) callconv(.C) u64, MsgSendFn(u64, u8, @TypeOf(.{})));
+    try testing.expectEqual(fn (u8, objc.Sel, u16, u32) callconv(.C) u64, MsgSendFn(u64, u8, @TypeOf(.{
+        @as(u16, 0),
+        @as(u32, 0),
+    })));
+}

pkg/objc/object.zig

@@ -0,0 +1,20 @@
+const std = @import("std");
+const c = @import("c.zig");
+const objc = @import("main.zig");
+const MsgSend = @import("msg_send.zig").MsgSend;
+
+pub const Object = struct {
+    value: c.id,
+
+    pub usingnamespace MsgSend(Object);
+};
+
+test {
+    const testing = std.testing;
+    const NSObject = objc.Class.getClass("NSObject").?;
+
+    // Should work with our wrappers
+    const obj = NSObject.msgSend(objc.Object, objc.Sel.registerName("alloc"), .{});
+    try testing.expect(obj.value != null);
+    obj.msgSend(void, objc.sel("dealloc"), .{});
+}

pkg/objc/property.zig

@@ -0,0 +1,27 @@
+const std = @import("std");
+const c = @import("c.zig");
+const objc = @import("main.zig");
+
+pub const Property = extern struct {
+    value: c.objc_property_t,
+
+    /// Returns the name of a property.
+    pub fn getName(self: Property) [:0]const u8 {
+        return std.mem.sliceTo(c.property_getName(self.value), 0);
+    }
+};
+
+test {
+    // Critical properties because we ptrCast C pointers to this.
+    const testing = std.testing;
+    try testing.expect(@sizeOf(Property) == @sizeOf(c.objc_property_t));
+    try testing.expect(@alignOf(Property) == @alignOf(c.objc_property_t));
+}
+
+test {
+    const testing = std.testing;
+    const NSObject = objc.Class.getClass("NSObject").?;
+
+    const prop = NSObject.getProperty("className").?;
+    try testing.expectEqualStrings("className", prop.getName());
+}

pkg/objc/sel.zig

@@ -0,0 +1,30 @@
+const std = @import("std");
+const c = @import("c.zig");
+
+// Shorthand, equivalent to Sel.registerName
+pub inline fn sel(name: [:0]const u8) Sel {
+    return Sel.registerName(name);
+}
+
+pub const Sel = struct {
+    value: c.SEL,
+
+    /// Registers a method with the Objective-C runtime system, maps the
+    /// method name to a selector, and returns the selector value.
+    pub fn registerName(name: [:0]const u8) Sel {
+        return Sel{
+            .value = c.sel_registerName(name.ptr),
+        };
+    }
+
+    /// Returns the name of the method specified by a given selector.
+    pub fn getName(self: Sel) [:0]const u8 {
+        return std.mem.sliceTo(c.sel_getName(self.value), 0);
+    }
+};
+
+test {
+    const testing = std.testing;
+    const s = Sel.registerName("yo");
+    try testing.expectEqualStrings("yo", s.getName());
+}