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ios - Swift 3 如何解析数据

转载 作者:行者123 更新时间:2023-11-28 12:29:49 25 4
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我的 Swift 3 代码有一个基础 Data字节集合(从 SQlite BLOB 读取)。数据的内部内容有多个这样结构的 block :

 {
UINT32 count; // number of points in this trkseg
UINT32 colour; // RGB colour of the line drawn for this trkseq
Double lat; // latitude of 1st point
Double long; // longitude of 1st point
Coord point[count-1] // array of points (2nd to last points)
}

typedef struct {
Float lat // difference in latitude of this point from the lat of the 1st point
Float long // difference in longitude of this point from the lat of the 1st point
} Coord;

这在 C 和 Java 中很容易解析。不幸的是,我无法找出使用 Swift 3 解析此数据的最佳方法。我不是在问您如何解析这个确切的数据布局,而只是询问使用 Swift 3 解析此类原始数据的建议和最佳实践。来自网络搜索和 Apple文档我很困惑!

*** 答案 - 感谢 Martin R 让我走上正轨。我在这里添加一些代码来展示我是如何解决这个问题的,以防它对其他人有帮助。正如马丁所说,有很多方法可以解决这个问题。我的解决方案确保 blob 数据,无论主机字节顺序如何,都将始终正确解析网络端顺序(大端)。

/// Parse the SQLite data blob to add GPS track segments
///
/// - Parameter data: GPS track information
private func addTracks(_ data: Data) {

// The data holds compressed GPX data. It has multiple track segments.
// It has a block of binary data per track segment with this structure:
// {
// UINT32 count; // number of points in this trkseg
// UINT32 colour; // RGB colour of the line drawn for this trkseq
// Double lat; // latitude of 1st point
// Double long; // longitude of 1st point
// Coord point[count-1] // array of points (2nd to last points)
// }
//
// typedef struct {
// Float lat // difference in latitude of this point from the lat of the 1st point
// Float long // difference in longitude of this point from the lat of the 1st point
// } Coord;

var dataCount = data.count // number of data bytes

var pointCount = 0 // counts coordinates per trkseg
var colour:UInt = 0
var lat:Double = 0.0
var long:Double = 0.0
var bigEndian = true
var i = 0

// From http://codereview.stackexchange.com/questions/114730/type-to-byte-array-conversion-in-swift
if (NSHostByteOrder() == NS_LittleEndian) {
bigEndian = false
}

while (dataCount >= 40) {
pointCount = Int(self.uint32Value(data: data.subdata(in: i..<i+4), isBigEndian: bigEndian))
i = i+4

if (pointCount < 2 || ((pointCount-1)*8 + 24 > dataCount)) {
print("ERROR, pointCount=\(pointCount)")
break
}
colour = UInt(self.uint32Value(data: data.subdata(in: i..<i+4), isBigEndian: bigEndian))
i = i+4
let firstLat = self.doubleValue(data: data.subdata(in: i..<i+8), isBigEndian: bigEndian)
i = i+8
let firstLong = self.doubleValue(data: data.subdata(in: i..<i+8), isBigEndian: bigEndian)
i = i+8
print("pointCount=\(pointCount) colour=\(colour) firstLat=\(firstLat) firstLong=\(firstLong)")

for _ in 1..<pointCount {
lat = firstLat - Double(self.floatValue(data: data.subdata(in: i..<i+4), isBigEndian: bigEndian))
i = i+4
long = firstLong - Double(self.floatValue(data: data.subdata(in: i..<i+4), isBigEndian: bigEndian))
i = i+4
print("lat=\(lat) long=\(long)")
}
dataCount = dataCount - 24 - (pointCount-1)*8;
}
}

private func floatValue(data: Data, isBigEndian: Bool) -> Float {
if (isBigEndian) {
return Float(bitPattern: UInt32(littleEndian: data.withUnsafeBytes { $0.pointee } ))
}
else {
return Float(bitPattern: UInt32(bigEndian: data.withUnsafeBytes { $0.pointee }))
}
}

private func doubleValue(data: Data, isBigEndian: Bool) -> Double {

if (isBigEndian) {
return Double(bitPattern: UInt64(littleEndian: data.withUnsafeBytes { $0.pointee } ))
}
else {
return Double(bitPattern: UInt64(bigEndian: data.withUnsafeBytes { $0.pointee } ))
}
}

private func uint32Value(data: Data, isBigEndian: Bool) -> UInt32 {

if (isBigEndian) {
return data.withUnsafeBytes{ $0.pointee }
}
else {
let temp: UInt32 = data.withUnsafeBytes{ $0.pointee }
return temp.bigEndian
}
}

最佳答案

一种可能的方法是使用

public func withUnsafeBytes<ResultType, ContentType>(_ body: (UnsafePointer<ContentType>) throws -> ResultType) rethrows -> ResultType

访问和取消引用数据中字节的方法。占位符类型 ContentType 可以从上下文中推断出来:

let color: UInt32 = data.subdata(in: 0..<4).withUnsafeBytes { $0.pointee }
// ...
let lat: Double = data.subdata(in: 8..<16).withUnsafeBytes { $0.pointee }
// ...

从 Swift 4 开始,您可以使用下标来提取数据:

let color: UInt32 = data[0..<4].withUnsafeBytes { $0.pointee }
// ...
let lat: Double = data[8..<16].withUnsafeBytes { $0.pointee }
// ...

如果所有字段的类型都正确对齐,那么您可以使用

public func load<T>(fromByteOffset offset: Int = default, as type: T.Type) -> T

来自 UnsafeRawPointer:

data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) -> Void in
let rawPointer = UnsafeRawPointer(bytes)
let color = rawPointer.load(fromByteOffset: 0, as: UInt32.self)
// ...
let lat = rawPointer.load(fromByteOffset: 8, as: Double.self)
// ...
}

在较低级别上,您可以使用 memcpy 再次使用任意对齐的数据:

var color: UInt32 = 0
var lat: Double = 0
data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) -> Void in
memcpy(&color, bytes, 4)
// ...
memcpy(&lat, bytes + 8, 8)
// ...
}

我可能会使用第一种方法,除非性能是一个问题在哪里你可以使用第二个或第三个,取决于是否所有字段都保证与其类型对齐或不对齐。

关于ios - Swift 3 如何解析数据,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/42325431/

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