我对 guard let 语句有疑问,它的行为很奇怪。整个代码如下。 Else 语句 block guard let data = readData, let size = sizeOfData else ...
在方法 readActivity(subdata: Data)
中被错误执行,即使 readData
和 sizeOfData
不是 nil
。
代码
import Foundation
enum ActivityDataReaderError: Error {
case activityIsReadingOtherCentral
case bluetooth(Error?)
case staleData
}
protocol ActivityDataReaderDelegate: class {
func didReadActivity(data: Data)
func didFailToReadActivity(error: ActivityDataReaderError)
}
final class ActivityDataReader {
private var sizeOfData: Int?
private var isOtherDeviceReading: Bool {
// 0xFFFF
return sizeOfData == 65535
}
private var readData: Data?
var isEmpty: Bool {
return sizeOfData == nil
}
weak var delegate: ActivityDataReaderDelegate?
static func timestampValue(_ timestamp: UInt32) -> Data {
var value = timestamp
return Data(buffer: UnsafeBufferPointer(start: &value, count: 1))
}
func reset() {
readData = nil
sizeOfData = nil
NSLog("reset() -- \(Thread.current)")
}
func readActivity(data: Data?, error: Error? = nil) {
guard let data = data else {
delegate?.didFailToReadActivity(error: .bluetooth(error))
return
}
let isFirstChunk = readData == nil
if isFirstChunk {
let sizeData = data.subdata(in: 0..<2)
sizeOfData = sizeData.withUnsafeBytes { $0.pointee }
guard !isOtherDeviceReading else {
delegate?.didFailToReadActivity(error: .activityIsReadingOtherCentral)
return
}
NSLog(String("readActivity() Size of data: \(String(describing: sizeOfData))"))
let subdata = data.subdata(in: 2..<data.count)
readActivity(subdata: subdata)
} else {
readActivity(subdata: data)
}
}
private func readActivity(subdata: Data) {
if let lastReadData = readData {
readData = lastReadData + subdata
} else {
readData = subdata
}
guard let data = readData, let size = sizeOfData else {
NSLog("WTF? data:\(String(describing: readData)), "
+ "sizeOfData: \(String(describing: sizeOfData)), "
+ "thread: \(Thread.current)")
assertionFailure("WTF")
return
}
NSLog("subdata: \(String(describing: subdata)), "
+ "totalReadBytes: \(data.count), "
+ "size: \(size)")
if data.count == size {
delegate?.didReadActivity(data: data)
reset()
}
}
}
测试
由于 assertionFailure("WTF")
而有时会通过有时会崩溃的测试。
class ActivityDataServiceReaderTests: XCTestCase {
var service: ActivityDataReader?
override func setUp() {
super.setUp()
service = ActivityDataReader()
}
override func tearDown() {
service = nil
super.tearDown()
}
func testBufferIsNotEmpty() {
NSLog("testBufferIsNotEmpty thread: \(Thread.current)")
guard let service = service else { fatalError() }
let firstDataBytes = [UInt8.min]
let data1 = Data(bytes: [7, 0] + firstDataBytes)
service.readActivity(data: data1)
XCTAssertFalse(service.isEmpty)
service.reset()
XCTAssertTrue(service.isEmpty)
}
}
崩溃时的控制台日志
2018-10-25 14:53:30.033573+0200 GuardBug[84042:11188210] WTF? data:Optional(1 bytes), sizeOfData: Optional(7), thread: <NSThread: 0x600003399d00>{number = 1, name = main}
环境
- Xcode10
- swift 4.1 与遗留构建系统
- swift 4.2
在我看来,在方法 readActivity(subdata: Data)
的 guard let else block 中执行 else block 中的代码是不可能的。一切都在主线程上运行。我错过了什么吗?怎么可能有时测试通过有时崩溃?
感谢您的帮助。
编辑:
更窄的guard let + data.withUnsafeBytes问题:
func testGuardLet() {
let data = Data(bytes: [7, 0, UInt8.min])
let sizeData = data.subdata(in: 0 ..< 2)
let size: Int? = sizeData.withUnsafeBytes { $0.pointee }
guard let unwrappedSize = size else {
NSLog("failure: \(size)")
XCTFail()
return
}
NSLog("success: \(unwrappedSize)")
}
日志:
2018-10-25 16:32:19.497540+0200 GuardBug[90576:11351167] failure: Optional(7)
最佳答案
let size: Int? = sizeData.withUnsafeBytes { $0.pointee }
读取数据被向下转换为 Optional Int(8 字节长)但 sizeData 本身只有 2 字节长。我不知道它有时是如何工作的,但解决方案 - 似乎工作正常 - 是以休闲方式使用方法 withUnsafeBytes
:
let size = sizeData.withUnsafeBytes { (pointer: UnsafePointer<UInt16>) in pointer.pointee }
返回值不是可选的,并且具有正确的类型 UInt16(2 个字节长)。
关于ios - 展开可选值(由 data.withUnsafeBytes(_ :)) sometimes does not work with guard let 返回,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/52989902/