Swift：创建指针数组以调用C函数。

Question

背景

我正在使用Swift调用libxlsxwriter C库的特定函数。文档在此处：https://libxlsxwriter.github.io/worksheet_8h.html#a62bf44845ce9dcc505bf228999db5afa

该函数组装了一个"富文本字符串"（相当于AttributedString，在不同的范围应用不同的格式/样式），并将其写入Excel工作表中的特定单元格。在C语言中，函数的工作方式如下：

lxw_format *bold = workbook_add_format(workbook);
format_set_bold(bold);
 
lxw_format *italic = workbook_add_format(workbook);
format_set_italic(italic);
 
lxw_rich_string_tuple fragment11 = {.format = NULL,   .string = "This is "     };
lxw_rich_string_tuple fragment12 = {.format = bold,   .string = "bold"         };
lxw_rich_string_tuple fragment13 = {.format = NULL,   .string = " and this is "};
lxw_rich_string_tuple fragment14 = {.format = italic, .string = "italic"       };
 
lxw_rich_string_tuple *rich_string1[] = {&fragment11, &fragment12,
                                         &fragment13, &fragment14, NULL};
 
worksheet_write_rich_string(worksheet, CELL("A1"), rich_string1, NULL);

问题

我有一个lxw_rich_string_tuple结构体的数组，但我不清楚如何将它转换为worksheet_write_rich_string()接受的指针数组：

// The worksheet object and `lxw_format` objects are already existent. 
// This array contains multiple well-formed `lxw_rich_string_tuple` structs, which I can see in the debugger.
//
var rawTuples: [lxw_rich_string_tuple] =  ...


// Parameters: 
//    - the worksheet on which to write this string
//    - the row of the cell in which to write
//    - the column of the cell in which to write
//    - a null-terminated array of pointers to `lxw_rich_string_tuple` structs
//    - an optional format object to use, null in this case.
//
worksheet_write_rich_string(worksheet, 0, 1, ?, NULL);

问题出在问号?上。我尝试了各种withUnsafeBytes、withUnsafeMutableBytes 以及UnsafeMutableRawPointer().bindMemory(to:capacity:)，但我无法弄清楚Swift中应该如何实现C语言中非常简单的操作。谢谢。

我的尝试

代码没有编译错误，但会出现Bad Access异常：

let argsSize: Int = rawTuples.count

rawTuples.withUnsafeMutableBufferPointer { rawTuplesPointer in

    let ptr = UnsafeMutableRawPointer(rawTuplesPointer.baseAddress!).bindMemory(to: lxw_rich_string_tuple.self, capacity: argsSize)
    var tuplePointers: [UnsafeMutablePointer<lxw_rich_string_tuple>?] = []

    for i in 0 ..< argsSize
    {
        let tp: UnsafeMutablePointer<lxw_rich_string_tuple>? = ptr + (i * MemoryLayout<lxw_rich_string_tuple>.stride)           
        tuplePointers.append(tp)
    }
    tuplePointers.append(nil)

    tuplePointers.withUnsafeBufferPointer { tpPointer in

        let m = UnsafeMutablePointer(mutating: tpPointer.baseAddress)
        worksheet_write_rich_string(lxw_worksheet, cell.row, cell.col, m, nil)

    }
}

- Bryan

Answer 1

试试这个：

rawTuples.withUnsafeMutableBufferPointer { p in
    guard let arrBaseAddress = p.baseAddress else { return }

    var pointersToEachArrayElement: [UnsafeMutablePointer<_>?] = 
        Array(arrBaseAddress ..< arrBaseAddress.advanced(by: p.count))
    pointersToEachArrayElement.append(nil)
    pointersToEachArrayElement.withUnsafeMutableBufferPointer { q in
        // use q.baseAddress in the call to worksheet_write_rich_string
    }
}

这个想法与你的尝试类似——创建一个指向每个数组元素的指针数组。但与你的尝试不同的是，我避免了调用指针类型的初始化函数（我认为这是不应该做的），而是尽可能使用withXXX函数。

你也可以考虑只编写Objective-C包装器来调用C函数和lxw_rich_string_tuple。有时候C函数并不能很方便地桥接到Swift中，而且这并不是我第一次经历这样的事情，不幸的是:(

- Sweeper