“Variable ‘characteristic’ was never mutated; consider changing to
‘let’ constant”

for var characteristic:CBCharacteristic in service.characteristics ?? [] {
    print(str)
    _selectedPeripheral!.writeValue(str.dataUsingEncoding(NSUTF8StringEncoding)!,forCharacteristic: characteristic,type: CBCharacteristicWriteType.WithoutResponse)
}

当我改变为“let”时，有一个错误：

‘let’ pattern cannot appear nested in an already immutable context

为什么它推荐我的变化，后来标记为一个错误？