使用 FFI 时如何创建“C 块”？

Question

我在 OS X 上使用CoreFoundation 框架，但我不知道如何在 Rust 中映射这个函数：

void CFRunLoopPerformBlock(CFRunLoopRef fl, CFTypeRef mode, void (^block)(void));

最后一个参数是void(^block)(void)——如何创建这种类型的参数？

Answer 1

简短的，可能有用的答案：有 block 板条箱，看起来它可以完成这项工作。

简短而无益的回答：据我所知，Rust 不支持 Apple 的块扩展。没有等效的 Rust 类型，假设您要调用一个需要一个块的 API。

答案更长，没有什么帮助：根据我从some Clang documentation on the Apple Block ABI 收集到的信息，void(^)(void) 的大小与常规指针相同。

因此，我的建议如下：将块视为不透明的指针大小的值。要调用它，请用 C 编写一个函数来为您调用它。

以下内容未经测试（我没有 Mac），但至少应该让您朝着正确的方向前进。另外，我正在标记这个社区 wiki，以便可以对其进行测试的任何人都可以在需要时修复它。

在锈中：

// These are the "raw" representations involved.  I'm not using std::raw
// because that's not yet stabilised.
#[deriving(Copy, Clone)]
struct AppleBlock(*const ());

#[deriving(Copy, Clone)]
struct RustClosure(*const(), *const());

// Functions that we need to be written in C:
extern "C" {
    fn rust_closure_to_block(closure_blob: RustClosure) -> AppleBlock;
    fn block_release(block_blob: AppleBlock);
}

// The function that the C code will need.  Note that this is *specific* to
// FnMut() closures.  If you wanted to generalise this, you could write a
// generic version and pass a pointer to that to `rust_closure_to_block`.
extern "C" fn call_rust_closure(closure_blob: RustClosure) {
    let closure_ref: &FnMut() = unsafe { mem::transmute(closure_blob) };
    closure_ref();
}

// This is what you call in order to *temporarily* turn a closure into a
// block.  So, you'd use it as:
//
//     with_closure_as_block(
//         || do_stuff(),
//         |block| CFRunLoopPerformBlock(fl, mode, block)
//     );
fn with_closure_as_block<C, B, R>(closure: C, body: B) -> R
where C: FnMut(), B: FnOnce(block_blob) -> R {
    let closure_ref: &FnMut() = &closure;
    let closure_blob: RustClosure = unsafe { mem::transmute(closure_ref) };
    let block_blob = unsafe { rust_closure_to_block(closure_blob) };
    let r = body(block_blob);
    unsafe { block_release(block_blob) };
    r
}

在 C 中：

typedef struct AppleBlock {
    void *ptr;
} AppleBlock;

typedef struct RustClosure {
    void *ptr;
    void *vt;
} RustClosure;

void call_rust_closure(RustClosure closure_blob);

AppleBlock rust_closure_to_block(RustClosure closure_blob) {
    return (AppleBlock)Block_copy(^() {
        call_rust_closure(closure_blob);
    });
}

// I'm not using Block_release directly because I don't know if or how
// blocks change name mangling or calling.  You might be able to just
// use Block_release directly from Rust.
void block_release(AppleBlock block) {
    Block_release((void (^)(void))block);
}