如何提取 UnsafePointer<cgfloat> 来自 UnsafePointer<cgpoint> - Swift</cgpoint></cgfloat>

Question

我正在学习 Swift 中的指针。

例如，这段代码从一个CGPoint数组开始，创建一个UnsafePointer ，然后将所有 x 值提取到一个CGFloat数组中：

import Foundation

let points = [CGPoint(x:1.2, y:3.33), CGPoint(x:1.5, y:1.21), CGPoint(x:1.48, y:3.97)]
print(points)

let ptr = UnsafePointer(points)
print(ptr)

func xValues(buffer: UnsafePointer<CGPoint>, count: Int) -> [CGFloat]? {
    return UnsafeBufferPointer(start: buffer, count: count).map { $0.x }
}

let x = xValues(buffer: ptr, count: points.count)
print(x)

而预期的 output 是：

[Foundation.CGPoint(x: 1.2, y: 3.33), Foundation.CGPoint(x: 1.5, y: 1.21), Foundation.CGPoint(x: 1.48, y: 3.97)]
0x0000556d6b818aa0
Optional([1.2, 1.5, 1.48])

现在我想让 xValues function 直接返回UnsafePointer<CGFloat> ，而不是通过[CGFloat] 。

我该怎么做，这可能吗？

Answer 1

像这样的 output 指针是不安全的。 正如评论中提到的，您应该使用withUnsafeBufferPointer方法来访问底层缓冲区：

let points = [
  CGPoint(x:1.2, y:3.33), 
  CGPoint(x:1.5, y:1.21), 
  CGPoint(x:1.48, y:3.97)
]

let xValues = points.withUnsafeBufferPointer { buffer in
  return buffer.map { $0.x }
}

如果您需要指向CGFloat数组的指针，只需使用与上述相同的方法：

xValues.withUnsafeBufferPointer { buffer in 
  // Do things with UnsafeBufferPointer<CGFloat>
}

一个很好的 Swift 指针教程在这里。

---

编辑

这是一个工作示例：

let points = [
    CGPoint(x:1.2, y:3.33), 
    CGPoint(x:1.5, y:1.21),
    CGPoint(x:1.48, y:3.97)
]

// Create, init and defer dealoc
let ptr = UnsafeMutablePointer<CGFloat>.allocate(capacity: points.count)
ptr.initialize(repeating: 0.0, count: points.count)
defer {
    ptr.deallocate()
}

// Populate pointer
points.withUnsafeBufferPointer { buffer in
    for i in 0..<buffer.count {
        ptr.advanced(by: i).pointee = buffer[i].x
    }
}

// Do things with UnsafeMutablePointer<CGFloat>, for instance:
let buffer = UnsafeBufferPointer(start: ptr, count: points.count)

for (index, value) in buffer.enumerated() {
    print("index: \(index), value: \(value)")
}