swift - Swift 中 CoreText CTFrameGetLineOrigins 的问题

Question

我有以下代码在 Objective-C 中运行

NSArray *lines = (NSArray *)CTFrameGetLines((__bridge CTFrameRef)columnFrame);
CGPoint origins[[lines count]];
CTFrameGetLineOrigins((__bridge CTFrameRef)columnFrame, CFRangeMake(0, 0), origins);

但是当移植到 Swift 时，编译器会提示 Cannot convert the expression´s ´Void´to type ´CMutablePointer<CGPoint>在 CTFrameGetLineOrigins线

let nsLinesArray: NSArray = CTFrameGetLines(ctFrame) // Use NSArray to bridge to Array
let ctLinesArray = nsLinesArray as Array
var originsArray: Array<CGPoint> = CGPoint[]()
//var originsArray: NSMutableArray = NSMutableArray.array()
let range: CFRange = CFRangeMake(0, 0)
CTFrameGetLineOrigins(ctFrame, range, originsArray)

我必须在 CGFrameGetLines 中使用 NSArray函数，然后转换为 Swift 数组，并检查 ctLinesArray，显示 CTLine正确检索对象

我尝试使用NSMutableArray对于 originsArray，结果相同。

知道缺少什么吗？

Answer 1

您必须添加地址运算符 &将指针传递给 originsArray 的开始到函数:

CTFrameGetLineOrigins(ctFrame, range, &originsArray)

引用:Interacting with C APIs 在《使用 Swift 与 Cocoa 和 Objective-C》一书中:

C Mutable Pointers

When a function is declared as taking a CMutablePointer<Type> argument, it can accept any of the following:

• ...
• An in-out Type[] value, which is passed as a pointer to the start of the array, and lifetime-extended for the duration of the call.

来自 Expressions在《Swift 编程语言》一书中:

In addition to the standard library operators listed above, you use & immediately before the name of a variable that’s being passed as an in-out argument to a function call expression.

一个补充(@eharo2 已经弄清楚了)，originsArray必须有空间 必要数量的元素，可以通过以下方式实现

var originsArray = CGPoint[](count:ctLinesArray.count, repeatedValue: CGPointZero)