[英]Swift 3 how to parse DATA
我的Swift 3代碼有一個字節的Foundation Data集合(從SQlite BLOB中讀取)。 數據的內部內容具有以下結構的多個塊:
{
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;
這在C和Java中非常容易解析。 不幸的是,我無法找到使用Swift 3解析此數據的最佳方法。我不是問您如何解析此確切的數據布局,而只是想提供有關使用Swift 3解析此類原始數據的建議和最佳實踐。來自網絡搜索和Apple我留下的文檔非常困惑!
***答案-感謝Martin R使我走上正軌。 我在此處添加一些代碼,以說明如何解決此問題,以防其他人遇到問題。 正如馬丁所說,有很多解決方法。 我的解決方案可確保始終正確解析網絡尾數順序(大尾數)的Blob數據,而與主機的尾數無關。
/// 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
}
}
一種可能的方法是使用
public func withUnsafeBytes<ResultType, ContentType>(_ body: (UnsafePointer<ContentType>) throws -> ResultType) rethrows -> ResultType
訪問和取消引用數據中字節的方法。 可以從上下文中推斷出占位符類型ContentType
:
let color: UInt32 = data.subdata(in: 0..<4).withUnsafeBytes { $0.pointee }
// ...
let lat: Double = data.subdata(in: 8..<16).withUnsafeBytes { $0.pointee }
// ...
從Swift 4開始,您可以使用下標提取數據:
let color: UInt32 = data[0..<4].withUnsafeBytes { $0.pointee }
// ...
let lat: Double = data[8..<16].withUnsafeBytes { $0.pointee }
// ...
如果所有字段都針對其類型正確對齊,則可以使用
public func load<T>(fromByteOffset offset: Int = default, as type: T.Type) -> T
來自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)
// ...
}
在較低的級別上,您可以使用memcpy
,它再次適用於任意對齊的數據:
var color: UInt32 = 0
var lat: Double = 0
data.withUnsafeBytes { (bytes: UnsafePointer<UInt8>) -> Void in
memcpy(&color, bytes, 4)
// ...
memcpy(&lat, bytes + 8, 8)
// ...
}
我可能會使用第一種方法,除非性能是一個問題,您可以使用第二種或第三種方法,具體取決於是否保證所有字段的類型都對齊。
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