swift - 闭包捕获上下文 Swift

Question

当我尝试更改闭包中的变量时出现此错误:

A C function pointer cannot be formed from a closure that captures context

是否有变通办法，或者仍然可以更改闭包中的变量？

我的代码:

let callback: @convention(c) (readStream: CFWriteStream!, event: CFStreamEventType, data: UnsafeMutablePointer<Void>) -> Void = {
    (readStream, event, data) -> Void in
    switch event {
    case CFStreamEventType.ErrorOccurred:
        self.isError = true
        break
    case CFStreamEventType.EndEncountered:
        self.isRunLoop = false
        break
    case CFStreamEventType.HasBytesAvailable:
        break
    case CFStreamEventType.OpenCompleted:
        break
    case CFStreamEventType.CanAcceptBytes:
        self.bytesWritten = CFWriteStreamWrite(readStream, self.buffer, self.leftOverSize)
        break
    default:
        break
    }
}

let registeredEvents: CFOptionFlags =
    CFStreamEventType.CanAcceptBytes.rawValue |
    CFStreamEventType.HasBytesAvailable.rawValue |
    CFStreamEventType.ErrorOccurred.rawValue |
    CFStreamEventType.EndEncountered.rawValue |
    CFStreamEventType.None.rawValue

var context = CFStreamClientContext(version: CFIndex(0), info: nil, retain: nil, release: nil, copyDescription: nil)
let stream = CFWriteStreamCreateWithFTPURL(nil, uploadURL).takeUnretainedValue()

CFWriteStreamSetClient(stream, registeredEvents, callback, &context)

Answer 1

我假设您想使用此回调将其作为第三个参数 (clientCB) 传递给 CFWriteStreamSetClient .

由于该参数具有以下类型定义

typedef void (*CFWriteStreamClientCallBack) ( CFWriteStreamRef stream, CFStreamEventType eventType, void *clientCallBackInfo );

您只能使用全局函数或不从周围上下文捕获任何变量(如 self)的闭包。

在这种情况下你可以做的是将 self 传递给 CFStreamClientContext 结构的 info 字段( 的第 4 个参数>CFWriteStreamSetClient)，并使用该信息在闭包中重建 self:

let callback: @convention(c) (readStream: CFWriteStream!, event: CFStreamEventType, data: UnsafeMutablePointer<Void>) -> Void = {
    (readStream, event, data) -> Void in
    // assuming your class name is Client
    let client = unsafeBitCast(data.memory, Client.self)
    switch event {
    case CFStreamEventType.ErrorOccurred:
        client.isError = true
    case CFStreamEventType.EndEncountered:
        client.isRunLoop = false
    case CFStreamEventType.HasBytesAvailable:
        break
    case CFStreamEventType.OpenCompleted:
        break
    case CFStreamEventType.CanAcceptBytes:
        client.bytesWritten = CFWriteStreamWrite(readStream, client.buffer, client.leftOverSize)
    default:
        break
    }
}

var context = CFStreamClientContext(version: 0, info: unsafeBitCast(self, UnsafeMutablePointer<Void>.self), retain: nil, release: nil, copyDescription: nil)
CFWriteStreamSetClient(stream, 0, callback, &context)

注意。与 Objective-C 一样，您需要确保 self 在流被销毁之前没有被销毁，否则如果流接收到新事件，您可能会崩溃。