在Swift 4中从InputStream读取n个字节

我的建议是不要反对网络I/O的异步特性。 每当 Stream.Event.hasBytesAvailable 事件 已发出信号。如果缓冲区包含足够的数据(4个长度字节加上 预期的消息长度),然后处理数据并将其删除。否则什么也不做 等待更多数据。

以下(未测试的)代码旨在演示。 它只显示与此特定问题相关的部分。 为简洁起见,省略了初始化、事件处理程序等。

class MessageReader {

    var buffer = Data(count: 1024) // Must be large enough for largest message + 4
    var bytesRead = 0 // Number of bytes read so far

    // Called from `handleInput` with a complete message.
    func processMessage(message: Data) {
        // ...
    }

    // Called from event handler if `Stream.Event.hasBytesAvailable` is signalled.
    func handleInput(istr: InputStream) {
        assert(bytesRead < buffer.count)

        // Read from input stream, appending to previously read data:
        let maxRead = buffer.count - bytesRead
        let amount = buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
            istr.read(p + bytesRead, maxLength: maxRead)
        }
        guard amount > 0 else {
            // handle EOF or read error ...
            fatalError()
        }
        bytesRead += amount

        while bytesRead >= 4 {
            // Read message size:
            let messageSize = buffer.withUnsafeBytes { (p: UnsafePointer<UInt32>) in
                Int(UInt32(bigEndian: p.pointee))
            }
            let totalSize = 4 + messageSize
            guard totalSize <= buffer.count else {
                // Handle buffer too small for message situation ...
                fatalError()
            }

            if bytesRead < totalSize {
                break // Not enough data to read message.
            }

            // Buffer contains complete message now. Process it ...
            processMessage(message: buffer[4 ..< totalSize])

            // ... and remove it from the buffer:
            if totalSize < bytesRead {
                // Move remaining data to the front:
                buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
                    _ = memmove(p, p + totalSize, bytesRead - totalSize)
                }
            }
            bytesRead -= totalSize
        }
    }
}

灵感来自Martin R( https://stackoverflow.com/a/48344040/3827381 -非常感谢!)我想出了这个解决方案:

var buffer = Data(count: 4096)
var offset = 0 // the index of the first byte that can be overridden
var readState = 0
var missingMsgBytes = 0
var msg = ""

func handleInput(_ istr: InputStream) {
    assert(buffer.count >= 5, "buffer must be large enough to contain length info (4 bytes) and at least one payload byte => min 5 bytes buffer required")
    assert(offset < buffer.count, "offset \(offset) is not smaller than \(buffer.count)")

    let toRead = buffer.count - offset
    let read = buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in istr.read(p + offset, maxLength: toRead) }
    guard read > 0 else {
        self.errorHandler(NetworkingError.InputError("Input Stream received \(read) bytes which is smaller than 0 => Network error"))
        return
    }
    offset += read
    var msgStart = 0
    var msgEnd = 0

    if readState == 0 {
        if offset < 4 {
            return
        }
        missingMsgBytes = buffer[0...3].withUnsafeBytes { (p: UnsafePointer<UInt32>) in Int(UInt32(bigEndian: p.pointee)) }
        msgStart = 4
        msgEnd = offset
        readState = 1
    } else {
        msgStart = 0
        msgEnd = offset
    }

    var fullMessageRead = false

    if readState == 1 {
        let payloadRead = msgEnd - msgStart
        if payloadRead <= missingMsgBytes {
            assert(msgEnd > msgStart, "msgEnd (\(msgEnd) <= msgStart \(msgStart). This should not happen")
            if msgEnd > msgStart {
                msg += String(data: buffer[msgStart..<msgEnd], encoding: .utf8)!
                missingMsgBytes -= payloadRead
                offset = 0
            }
            fullMessageRead = (missingMsgBytes == 0)
        } else { // read more than was missing
            msg += String(data: buffer[msgStart..<msgStart+missingMsgBytes], encoding: .utf8)!
            fullMessageRead = true
            buffer.withUnsafeMutableBytes { (p: UnsafeMutablePointer<UInt8>) in
                _ = memmove(p, p + missingMsgBytes, read - missingMsgBytes) // dst, src, number
            }
            offset = read-missingMsgBytes
        }
    }

    if fullMessageRead {
        handleMessage(msg)
        readState = 0
         msg = ""
        missingMsgBytes = 0
    }
}

此解决方案能够读取任意大小的消息。缓冲区大小仅决定一次可以读取的量=>缓冲区越大,应用程序运行速度越快。

我测试了大约一个小时的代码,它没有崩溃。旧代码在1-2分钟后崩溃。现在似乎终于起作用了。

但是,当我想提高编程知识时,我想问一下,我的代码中是否有一些不必要的复杂内容,或者是否有人看到了一个可能仍然会导致应用程序崩溃或读取错误数据的错误?