无法将NSData Objective-C代码转换为Swift [英] Trouble converting NSData Objective-C code to Swift

问题描述

在将Objective-C片段转换为使用 NSData 和 CoreBluetooth 的Swift时遇到了问题.我看过这个问题和一个与其他在Swift中处理 NSData 的人，但是没有成功.

I've been having issues converting an Objective-C snippet to Swift that uses NSData and CoreBluetooth. I have looked at this question and a couple others dealing with NSData in Swift but haven't had any success.

Objective-C代码段:

- (CGFloat) minTemperature
{
    CGFloat result = NAN;
    int16_t value = 0;

    // characteristic is a CBCharacteristic
    if (characteristic) { 
        [[characteristic value] getBytes:&value length:sizeof (value)];
        result = (CGFloat)value / 10.0f;
    }
    return result;
}

到目前为止，我在Swift中所拥有的(不起作用):

What I have so far in Swift (not working):

func minTemperature() -> CGFloat {
    let bytes = [UInt8](characteristic?.value)
    let pointer = UnsafePointer<UInt8>(bytes)
    let fPointer = pointer.withMemoryRebound(to: Int16.self, capacity: 2) { return $0 }
     value = Int16(fPointer.pointee)

    result = CGFloat(value / 10) // not correct value

    return result
}

这里的逻辑看起来不对吗?谢谢！

Does the logic look wrong here? Thanks!

推荐答案

一个错误

let fPointer = pointer.withMemoryRebound(to: Int16.self, capacity: 2) { return $0 }

因为回弹指针 $ 0 仅在闭包内部有效，并且必须不能传递到外面.此外，对于单个 Int16 值.另一个问题是

because the rebound pointer $0 is only valid inside the closure and must not be passed to the outside. Also the capacity should be 1 for a single Int16 value. Another problem is the integer division in

result = CGFloat(value / 10)

会截断结果(如The4kman观察到的).

which truncates the result (as already observed by the4kman).

不必从数据创建 [UInt8] 数组，可以改用 Data 的 withUnsafeBytes()方法.

Creating an [UInt8] array from the data is not necessary, the withUnsafeBytes() method of Data can be used instead.

最后，如果没有，您可以返回 nil (而不是"not a number")特征值给出:

Finally you could return nil (instead of "not a number") if no characteristic value is given:

func minTemperature() -> CGFloat? {
    guard let value = characteristic?.value else {
        return nil
    }
    let i16val = value.withUnsafeBytes { (ptr: UnsafePointer<Int16>) in
        ptr.pointee
    }
    return CGFloat(i16val) / 10.0
}

这篇关于无法将NSData Objective-C代码转换为Swift的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！