dispatch_semaphore_wait() can't receive the semaphore - ios

Question

my code is :
- (NSString*)run:(NSString*)command{
_semaphore = dispatch_semaphore_create(0);
// Create and start timer
NSTimer *timer = [NSTimer timerWithTimeInterval:0.5f
target:self
selector:#selector(getState:)
userInfo:nil
repeats:YES];
NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
[runLoop addTimer:timer forMode:NSRunLoopCommonModes];
[runLoop run];
//and it stuck there
// Wait until signal is called
dispatch_semaphore_wait(_semaphore, DISPATCH_TIME_FOREVER);
return _state;
}
- (void)getState:(NSTimer*)time{
// Send the url-request.
NSURLSessionDataTask* task =
[_session dataTaskWithRequest:_request
completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
if (!error) {
NSLog(#"result: %#", data);
} else {
_state = #"error";
NSLog(#"received data is invalid.");
}
if (![_state isEqualToString:#"inProgress"]) {
dispatch_semaphore_signal(_semaphore);
// Stop timer
[timer invalidate];
}
}];
[task resume];
}
after run the code
[runLoop run];
it had nothing happened!
so, what's wrong with the code?
Calling dispatch_semaphore_wait will block the thread until dispatch_semaphore_signal is called. This means that signal must be called from a different thread, since the current thread is totally blocked. Further, you should never call wait from the main thread, only from background threads.
is that helpful?

Answer 1

A couple of observations:
Your use of the semaphore is unnecessary. The run won't return until the timer is invalidated.
The documentation for run advises that you don't use that method as you've outlined, but rather use a loop with runMode:beforeDate: like so:
_shouldKeepRunning = true;
while (_shouldKeepRunning && [runLoop runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]]) {
// this is intentionally blank
}
Then, where you invalidate the timer, you can set _shouldKeepRunning to false.
This notion of spinning up a run loop to run a timer is, with no offense intended, a bit dated. Nowadays if I really needed a timer running on a background thread, I'd use a dispatch timer, like outlined the first half of https://stackoverflow.com/a/23144007/1271826. Spinning up a runloop for something like this is an inefficient pattern.
But let's step back and look at what you're trying to achieve, I assume you're trying to poll some server about some state and you want to be notified when it's no longer in an "in progress" state. If so, I'd adopt an asynchronous pattern, such as completion handlers, and do something like:
- (void)checkStatus:(void (^ _Nonnull)(NSString *))completionHandler {
// Send the url-request.
NSURLSessionDataTask* task = [_session dataTaskWithRequest:_request completionHandler:^(NSData *data, NSURLResponse *response, NSError *error) {
NSString *state = ...
if (![state isEqualToString:#"inProgress"]) {
completionHandler(#"inProgress");
} else {
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
[self checkStatus:completionHandler];
});
}
}];
[task resume];
}
and then call it like so:
[self checkStatus:^(NSString *state) {
// can look at `state` here
}];
// but not here, because the above is called asynchronously (i.e. later)
That completely obviates the need for run loop. I also eliminate the timer pattern in favor of the "try again x seconds after prior request finished", because you can have timer problems if one request wasn't done by the time the next timer fires. (Yes, I know you could solve that by introducing additional state variable to keep track of whether you're currently in a request or not, but that's silly.)