NSThread waking up - objective-c

Question

I would like to know how I can achieve the following in objective-c,
I am doing communication with FTDI232R modem using serial communication,So I am using POSIX call to open,write and read from the path of the modem(dev/tty/nameOfModem).
The POSIX calls are synchronous calls so while read I don't want to block my main thread hence I am thinking to do read call in separate thread.
I don't want this secondary thread to run continuously but wake up only when there is something to read and after read is completed it should sleep.I went through the documentation and read about providing an input source to the NSRunLoop and adding that runloop to the secondary thread,but couldn't figure out how to do it.
Thank you in advance for all your help.

Answer 1

You typically have a BOOL to indicate your run state, and then a date to run until. When doing socket-y things, I tend to do something like:
NSDate *beforeDate = [NSDate dateWithTimeIntervalSinceNow:.1];
while (self.isActive && [[NSRunLoop currentRunLoop] runMode: NSRunLoopCommonModes beforeDate:beforeDate]) {
beforeDate = [NSDate dateWithTimeIntervalSinceNow:.1];
}
And then when you disconnect from your modem, you can set isActive to NO to let the runloop spin down.
While not exactly what you want, Apple's docs on threading with NSOperation might be interesting for you to skim.

Answer 2

You should probably use GCD dispatch sources for this. Here's example code copied directly out of that article:
dispatch_source_t ProcessContentsOfFile(const char* filename)
{
// Prepare the file for reading.
int fd = open(filename, O_RDONLY);
if (fd == -1)
return NULL;
fcntl(fd, F_SETFL, O_NONBLOCK); // Avoid blocking the read operation
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_source_t readSource = dispatch_source_create(DISPATCH_SOURCE_TYPE_READ,
fd, 0, queue);
if (!readSource)
{
close(fd);
return NULL;
}
// Install the event handler
dispatch_source_set_event_handler(readSource, ^{
size_t estimated = dispatch_source_get_data(readSource) + 1;
// Read the data into a text buffer.
char* buffer = (char*)malloc(estimated);
if (buffer)
{
ssize_t actual = read(fd, buffer, (estimated));
Boolean done = MyProcessFileData(buffer, actual); // Process the data.
// Release the buffer when done.
free(buffer);
// If there is no more data, cancel the source.
if (done)
dispatch_source_cancel(readSource);
}
});
// Install the cancellation handler
dispatch_source_set_cancel_handler(readSource, ^{close(fd);});
// Start reading the file.
dispatch_resume(readSource);
return readSource;
}