目标C到Swift代码转换，似乎无法在Swift中正确处理字节 [英] Objective C to Swift code conversion, can't seem to treat bytes correctly in Swift

问题描述

我正在开发一款简单的Swift蓝牙心率监测器iOS App。我找到了这个伟大的教程，它有目标C码。我已将它转换为Swift，我从心率监测器获取数据。我的问题是我似乎无法在Swift中正确访问和转换字节数据。

I'm working on a simple Swift bluetooth heart rate monitor iOS App. I found this great tutorial which has objective C code. I've converted it to Swift and I'm getting data from my heart rate monitor. My problem is that I can't seem to correctly access and convert the byte data in Swift.

这是Objective C代码：

Here's the Objective C code:

// Instance method to get the heart rate BPM information
- (void) getHeartBPMData:(CBCharacteristic *)characteristic error:(NSError *)error
{
    // Get the Heart Rate Monitor BPM
    NSData *data = [characteristic value];      // 1
    const uint8_t *reportData = [data bytes];
    uint16_t bpm = 0;

    if ((reportData[0] & 0x01) == 0) {          // 2
        // Retrieve the BPM value for the Heart Rate Monitor
        bpm = reportData[1];
    }
    else {
        bpm = CFSwapInt16LittleToHost(*(uint16_t *)(&reportData[1]));  // 3
    }
    // Display the heart rate value to the UI if no error occurred
    if( (characteristic.value)  || !error ) {   // 4
        self.heartRate = bpm;
        self.heartRateBPM.text = [NSString stringWithFormat:@"%i bpm", bpm];
        self.heartRateBPM.font = [UIFont fontWithName:@"Futura-CondensedMedium" size:28];
        [self doHeartBeat];
        self.pulseTimer = [NSTimer scheduledTimerWithTimeInterval:(60. / self.heartRate) target:self selector:@selector(doHeartBeat) userInfo:nil repeats:NO];
    }
    return;
}

这是Swift代码：

func peripheral(peripheral: CBPeripheral!,
    didUpdateValueForCharacteristic characteristic: CBCharacteristic!,
    error: NSError!) -> String
{

    // Get the Heart Rate Monitor BPM
    var data = characteristic.value
    var reportData = data.bytes
    var bpm : UInt16
    var rawByte : UInt8
    var outputString = ""
    rawByte = UInt8(reportData[0])
    bpm = 0

    if ((rawByte & 0x01) == 0) {          // 2
        // Retrieve the BPM value for the Heart Rate Monitor
        bpm = UInt16( reportData[4] )
    }
    else {
        bpm = CFSwapInt16LittleToHost(UInt16(reportData[1]))
    }

    outputString = String(bpm)
    return outputString
}

推荐答案

const uint8_t *reportData = [data bytes];

转换为

let reportData = UnsafePointer<UInt8>(data.bytes)

然后 reportData 的类型为 UnsafePointer< UInt8> ，您可以在（目标 - ）C中访问它：

Then reportData has the type UnsafePointer<UInt8> and you can access it as in (Objective-)C:

if (reportData[0] & 0x01) == 0 { ... }

接下来，

bpm = reportData[1];

在Swift中几乎相同。你必须明确地从 UInt8 转换为
UInt16 ，因为 - 与（目标 - ）C - Swift不同不能在类型之间隐式转换：

is almost identical in Swift. You have to convert explicitly from UInt8 to UInt16 because – unlike (Objective-)C – Swift does not implicitly convert between types:

bpm = UInt16(reportData[1]) 

汇总：

func getHeartBPMData(characteristic: CBCharacteristic!) {
    let data = characteristic.value
    let reportData = UnsafePointer<UInt8>(data.bytes)
    var bpm : UInt16
    if (reportData[0] & 0x01) == 0 {
        bpm = UInt16(reportData[1])
    } else {
        bpm = UnsafePointer<UInt16>(reportData + 1)[0]
        bpm = CFSwapInt16LittleToHost(bpm)
    }

    // ...
}

请注意，大多数变量都可以声明为常量，其中 let ，而不是
VAR 。而不是

Note that most of your variables can be declared as constants with let, instead of var. Instead of

bpm = CFSwapInt16LittleToHost(bpm)

您也可以使用 littleEndian：构造函数，对于所有整数类型，可以使用
：

you can alternatively use the littleEndian: constructor which is available for all integer types:

bpm = UInt16(littleEndian: bpm)

---

Swift 3/4更新：

func getHeartBPMData(characteristic: CBCharacteristic) {
    guard  let reportData = characteristic.value else {
        return 
    }

    let bpm : UInt16
    if (reportData[0] & 0x01) == 0 {
        bpm = UInt16(reportData[1])
    } else {
        bpm = UInt16(littleEndian: reportData.subdata(in: 1..<3).withUnsafeBytes { $0.pointee } )
    }

    // ...
}

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