objective-c - Swift 3 中的 withMemoryRebound 不适用于 Objective-C 结构指针

Question

我一直在尝试连接一个 Objective-C 库，该库仅公开指向 C 结构数组的指针以及该数组中的计数。

我已经对结构使用了 withMemoryRebound 并设置了容量，正如我在此处和其他地方的其他帖子中看到的那样。然而，我在 Swift 中得到的数组有垃圾数据。作为解决方法，我向库中添加了一个访问器方法。但我真的不想修改这个库。

以下是测试应用程序的组件，用于演示问题和预期结果:

圆.h:

#import <Foundation/Foundation.h>

typedef struct {
    float x, y;         /**< Position in pixels. */
    float radius;       /**< Radius in pixels. */
} circle;

@interface CircleGenerator : NSObject {
}

@property (nonatomic)      size_t              numCircles;
@property (nonatomic, readonly) const circle*  circles;

- (id)init;
// Added as a workaround:
- (void)fetchCircle:(size_t) i circle:(circle *) c;

@end

圆.m:

#import "Circle.h"

@interface CircleGenerator ()

@end

const circle myCircles[] = {
    {.x=2467.38184, .y=338.520477, .radius=140.096466},
    {.x=438.689453, .y=1879.302, .radius=116.18354}
};

@implementation CircleGenerator

- (id)init {
    _numCircles = 2;
    _circles = myCircles;
    return self;
}

// Added as a workaround:
- (void)fetchCircle:(size_t) i circle:(circle *) c {
    *c = _circles[i];
}

@end

ViewController.swift:

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        let circleGen = CircleGenerator()!

        var circlesPtr = UnsafePointer(circleGen.circles)
        withUnsafePointer(to: &circlesPtr) {
            $0.withMemoryRebound(to: circle.self, capacity: circleGen.numCircles, {
                for i in 0..<circleGen.numCircles {
                    print($0[i])
                }
            })
        }
        print("\nShould be:")
        var nextCircle:circle = circle(x: 0, y: 0, radius: 0)
        for i in 0..<circleGen.numCircles {
            circleGen.fetchCircle(i, circle: &nextCircle)
            print(nextCircle)
        }
    }    
}

输出:

circle(x: 6.70394638e-38, y: 1.40129846e-45, radius: 2.47778904e+18)
circle(x: 4.59163468e-41, y: 6.7002974e-38, radius: 1.40129846e-45)

<小时/>

应该是:

circle(x: 2467.38184, y: 338.520477, radius: 140.096466)
circle(x: 438.689453, y: 1879.302, radius: 116.18354)

此外，$0.pointee 和 $.pointee.x 等显示第一个元素的值不正确。

我是否有错误，或者我应该向 Apple 报告此问题吗？

Answer 1

circleGen.circles已经是类型化指针( UnsafePointer<circle> )，无需使用 withUnsafePointer() :

for i in 0..<circleGen.numCircles {
    let circle = circleGen.circles[i]
    print(circle)
}

输出:

circle(x: 2467.38184, y: 338.520477, radius: 140.096466)
circle(x: 438.689453, y: 1879.302, radius: 116.18354)

或者创建一个UnsafeBufferPointer这是一个“类似数组”的集合:

let circles = UnsafeBufferPointer(start: circleGen.circles, count: circleGen.numCircles)
for circle in circles {
    print(circle)
}

但是如果您好奇为什么您的代码不起作用:您正在将指针重新绑定(bind)到 circlesPtr变量本身，不是存储在该变量中的指针。您将得到正确的结果

let circlesPtr = UnsafePointer(circleGen.circles)!
circlesPtr.withMemoryRebound(to: circle.self, capacity: circleGen.numCircles, {
    for i in 0..<circleGen.numCircles {
        print($0[i])
    }
})