我正在尝试测量 ftp 下载期间的实际传输速度，下载本身正在运行，流连接在运行循环中。在事件开始时使用 CFTimeGetCurrent 在 NSStreamEventHasBytesAvailable 中完成测量，最后，在将数据写入文件后，使用 (double)previousTimestamp-CFAbsoluteTimeGetCurrent 计算经过的时间，但我得到的时间绝对不合理。在模拟器和设备上测试过，有人能启发我吗？
代码：
switch (eventCode) {
case NSStreamEventOpenCompleted: {
} break;
case NSStreamEventHasBytesAvailable: {
if ([[self.fileStream propertyForKey:NSStreamFileCurrentOffsetKey] intValue]==0) {
previousTimestamp = CFAbsoluteTimeGetCurrent();
}
NSInteger bytesRead;
uint8_t buffer[32768];
bytesRead = [self.networkStream read:buffer maxLength:sizeof(buffer)];
if (bytesRead == -1)
{
[self _stopReceiveWithStatus:@"Err"];
}
else if (bytesRead == 0)
{
[self _stopReceiveWithStatus:nil];
}
else
{
[self completition:bytesRead];
NSInteger bytesWritten;
NSInteger bytesWrittenSoFar;
// Write to the file.
bytesWrittenSoFar = 0;
do {
bytesWritten = [self.fileStream write:&buffer[bytesWrittenSoFar] maxLength:bytesRead - bytesWrittenSoFar];
assert(bytesWritten != 0);
if (bytesWritten == -1) {
[self _stopReceiveWithStatus:@"File err"];
break;
} else {
bytesWrittenSoFar += bytesWritten;
}
} while (bytesWrittenSoFar != bytesRead);
[self downloadSpeedSave:bytesRead :previousTimestamp-CFAbsoluteTimeGetCurrent()];
previousTimestamp = CFAbsoluteTimeGetCurrent();