dispatch_semaphore_wait blocked my network request callback - ios

Question

-(void)init{
self.sema = dispatch_semaphore_create(1)
}
-(void)main{
//sending one message is fine
[self preSendMessage:#"hi"];
//deadlock happen when sending multiple msg in a short time.
for(int i = 0; i < 10; i++){
[self preSendMessage:#"yo"];
}
}
- (void)preSendMessage:(NSString*)msg
{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_semaphore_wait(self.sema, DISPATCH_TIME_FOREVER);
RACSignal* signal = [self sendMessage:msg];
#weakify(self);
[signal subscribeNext:^(id x) {
} error:^(NSError *error) {
#strongify(self);
dispatch_semaphore_signal(self.sema);
} completed:^{
#strongify(self);
dispatch_semaphore_signal(self.sema);
}];
});
}
Some informations:
In sendMessage function, I am using AFNetworking.
When I remove the semaphore logic, it works fine, I need this because need to control the sequence of sending msgs.
[Update] my semaphore is init with value 1, so It would run at the first time
My Target is to ensure the code inside preSendMessage execute when the previous one is completed/error

Answer 1

Edit: you should use dispatch_semaphore_create(0)
You should put dispatch_semaphore_wait(self.sema, DISPATCH_TIME_FOREVER); after dispatch_async(....)
Something like this:
- (void)preSendMessage:(NSString*)msg{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
RACSignal* signal = [self sendMessage:msg];
#weakify(self);
[signal subscribeNext:^(id x) {
} error:^(NSError *error) {
#strongify(self);
dispatch_semaphore_signal(self.sema);
} completed:^{
#strongify(self);
dispatch_semaphore_signal(self.sema);
}];
});
dispatch_semaphore_wait(self.sema, DISPATCH_TIME_FOREVER);
}

Answer 2

You are calling dispatch_semaphore_wait in the wrong order. Your code subscription does not get executed, because your semaphore is in a wait state. You should call your dispatch_semaphore_wait right after dispatching to background thread.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
RACSignal* signal = [self sendMessage:msg];
#weakify(self);
[signal subscribeNext:^(id x) {
} error:^(NSError *error) {
#strongify(self);
dispatch_semaphore_signal(self.sema);
} completed:^{
#strongify(self);
dispatch_semaphore_signal(self.sema);
}];
});
dispatch_semaphore_wait(self.sema, DISPATCH_TIME_FOREVER);

Answer 3

You tell your app to wait for the semaphore, so your app waits for the semaphore. It doesn't execute any code after the wait call.
You need to set up everything, including the callbacks that will signal, and then you wait for the signal.