问iPhone：如何确定线程是否是主线程？

func generateRandomData(count:Int) -> NSData
{
    var array = Array<UInt8>(count: count, repeatedValue: 0)

    arc4random_buf(&array, UInt(count))
    return NSData(bytes: array, length: count)
}

let randomData = generateRandomData(256 * 1024)

let stream = NSInputStream(data: randomData)
stream.open() // IMPORTANT

var readBuffer = Array<UInt8>(count: 16 * 1024, repeatedValue: 0)

var totalBytesRead = 0

while (totalBytesRead < randomData.length)
{
    let numberOfBytesRead = stream.read(&readBuffer, maxLength: readBuffer.count)

    // Do something with the data

    totalBytesRead += numberOfBytesRead
}

extension NSInputStream
{
    func readInt32() -> Int
    {
        var readBuffer = Array<UInt8>(count:sizeof(Int32), repeatedValue: 0)

        var numberOfBytesRead = self.read(&readBuffer, maxLength: readBuffer.count)

        return Int(readBuffer[0]) << 24 |
            Int(readBuffer[1]) << 16 |
            Int(readBuffer[2]) << 8 |
            Int(readBuffer[3])
    }
}

   let oldPointer = UnsafePointer<()>
   let newPointer = oldPointer + 10

   let castPointer = UnsafePointer<MyStruct>(oldPointer)