如何在Swift中正确初始化UnsafePointer？

发布时间：2020-12-14 05:51:59 所属栏目：百科来源：网络整理

导读：我正在尝试使用CTFontCreatePathForGlyph(font：CTFont？，glyph：CGGlyph，transform：CConstPointer CGAffineTransform)： let myFont = CTFontCreateWithName("Helvetica",12,nil)let myGlyph = CTFontGetGlyphWithName(myFont,"a")let myTransform = CGA

我正在尝试使用CTFontCreatePathForGlyph(font：CTFont？，glyph：CGGlyph，transform：CConstPointer< CGAffineTransform>)：

let myFont = CTFontCreateWithName("Helvetica",12,nil)
let myGlyph = CTFontGetGlyphWithName(myFont,"a")
let myTransform = CGAffineTransformIdentity

但是如何正确地将myTransform传递给CTFontCreatePathForGlyph？

我已经尝试创建一个myTransformPointer来传递给这样的功能：

var myTransformPointer: UnsafePointer<CGAffineTransform> = UnsafePointer().initialize(newvalue: myTransform)

但是我收到这个错误：

Playground execution failed: error: <REPL>:20:76: error: '()' is not convertible to 'UnsafePointer<CGAffineTransform>'
var myTransformPointer: UnsafePointer<CGAffineTransform> = UnsafePointer().initialize(newvalue: myTransform)

所以我尝试明确地命名类型：

var myTransformPointer: UnsafePointer<CGAffineTransform> = UnsafePointer<CGAffineTransform>().initialize(newvalue: myTransform)

然后我得到一个不同的错误：

Playground execution failed: error: <REPL>:20:95: error: could not find an overload for 'init' that accepts the supplied arguments
var myTransformPointer: UnsafePointer<CGAffineTransform> = UnsafePointer<CGAffineTransform>().initialize(newvalue: myTransform)
                                                       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

自动完成建议这应该工作？

最简单的解决方案是使用withUnsafePointer函数：

let myFont = CTFontCreateWithName("Helvetica","a")
var myTransform = CGAffineTransformIdentity

var path = withUnsafePointer(&myTransform) { (pointer: UnsafePointer<CGAffineTransform>) -> (CGPath) in
    return CTFontCreatePathForGlyph(myFont,myGlyph,pointer)
}

初始化不是构造函数。您将不得不使用UnsafePointer< T> .alloc分配一个新内存，然后初始化然后释放。功能withUnsafePointer为您做的一切。

请注意，myTransform不能是常量(var not let)，否则不能将其用于inout参数(& myTransform)。

（编辑：李大同）

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