再presenting空函数指针斯威夫特C函数 [英] Representing NULL Function Pointers to C Functions in Swift

问题描述

考虑私营尚未排序的详细记录可可的C函数 _NSLogCStringFunction（）和 _NSSetLogCStringFunction（） 。 _NSLogCStringFunction（）返回一个函数指针，为使用背后的幕后的Objective-C运行C函数的NSLog（）和 _NSSetLogCStringFunction（）允许开发人员指定记录自己的C函数。在这两种功能的更多信息可以在发现这个堆栈溢出问题并的此的WebObjects支持文章。

Consider the private-yet-sort-of-documented Cocoa C functions _NSLogCStringFunction() and _NSSetLogCStringFunction(). _NSLogCStringFunction() returns a function pointer to the C function used by the Objective-C runtime behind-the-scenes for NSLog(), and _NSSetLogCStringFunction() allows developers to specify their own C function for logging. More information on both of these functions can be found in this Stack Overflow question and this WebObjects support article.

在C，我可以在一个空函数指针传递给 _NSSetLogCStringFunction（）：

In C, I can pass in a NULL function pointer to _NSSetLogCStringFunction():

extern void _NSSetLogCStringFunction(void(*)(const char*, unsigned, BOOL));

_NSSetLogCStringFunction(NULL); // valid

不过，我遇到了一些问题，当我尝试这样做纯粹斯威夫特：

However, I'm running into some issues when I try to do this in pure Swift:

/// Represents the C function signature used under-the-hood by NSLog
typealias NSLogCStringFunc = (UnsafePointer<Int8>, UInt32, Bool) -> Void

/// Sets the C function used by NSLog
@_silgen_name("_NSSetLogCStringFunction")
func _NSSetLogCStringFunction(_: NSLogCStringFunc) -> Void

_NSSetLogCStringFunction(nil) // Error: nil is not compatible with expected argument type 'NSLogCStringFunc' (aka '(UnsafePointer<Int8>, UInt32, Bool) -> ()')

如果我试图绕过这个编译时通过 unsafeBitCast 警告，我的程序刚刚与 EXC_BAD_INSTRUCTION 崩溃（如预期，因为签名是错误的）：

If I try bypass this compile-time warning with unsafeBitCast, my program just crashes with EXC_BAD_INSTRUCTION (as expected, since the signature is wrong):

let nullPtr: UnsafePointer<Void> = nil
let nullFuncPtr = unsafeBitCast(nullPtr, NSLogCStringFunc.self)
_NSSetLogCStringFunction(nullFuncPtr) // crash

我怎样重新present一个 NULL 函数指针（无效*）或（无效（*）（为const char *，无符号，布尔）） / （UnsafePointer＆LT; INT8＆gt;中UInt32的，布尔） - GT;虚空在斯威夫特？

推荐答案

在（Objective-）C申报的雨燕映射

The Swift mapping of the (Objective-)C declaration

extern void _NSSetLogCStringFunction(void(*)(const char*, unsigned, BOOL));

是

public func _NSSetLogCStringFunction(_: (@convention(c) (UnsafePointer<Int8>, UInt32, ObjCBool) -> Void)!)

最简单的解决办法是把Objective-C的的extern
声明成一个Objective-C头文件，并包括
从桥接报头。

The easiest solution would be to put the Objective-C extern declaration into an Objective-C header file and include that from the bridging header.

替代地，在纯夫特它应

typealias NSLogCStringFunc = @convention(c) (UnsafePointer<Int8>, UInt32, ObjCBool) -> Void

@_silgen_name("_NSSetLogCStringFunction")
func _NSSetLogCStringFunction(_: NSLogCStringFunc!) -> Void

在任一情况下，该函数的参数是一个隐式展开可选
你可以用称之为零。例如：

In either case, the function parameter is an implicitly unwrapped optional, and you can call it with nil. Example:

func myLogger(message: UnsafePointer<Int8>, _ length: UInt32, _ withSysLogBanner: ObjCBool) -> Void {
    print(String(format:"myLogger: %s", message))
}

_NSSetLogCStringFunction(myLogger) // Set NSLog hook.
NSLog("foo")
_NSSetLogCStringFunction(nil) // Reset to default.
NSLog("bar")

输出：

myLogger: foo
2016-04-28 18:24:05.492 prog[29953:444704] bar

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