Obj-C synchronize an async method which using block callbacks - objective-c

Question

I have an async method called getCount: which goes to a web URL, counts some stuff, and invokes a callback with the count when it's done.
I have another method which is synchronous and needs to take those results, puts them into a message, and returns that message. Here are the two together:
- (NSString *)describe {
__block bool gotCount = NO;
[self getCount:^(int count) {
NSLog(#"Got the count: %i", count);
_count = count; // _count is an ivar of the object with this method.
gotCount = YES;
}];
// Pause here until the count has been fetched.
while (!gotCount) {
[NSThread sleepForTimeInterval:0.05];
}
return [NSString stringWithFormat:#"The count is %i", _count];
}
My callback is never called when I try this. It never prints
Got the count 0
or any other value for count in this scenario.
If I comment out the while loop, that message does get printed out. So I know that the getCount: method works, there's just something wrong with my loop waiting for it to arrive.
I need getCount: to remain asynchronous (there's other places it gets used where that's more important) and I need describe to remain synchronous. How can I handle this?

Answer 1

one possible thing: if your describe method is in the main thread then you call getCount method also from the main thread and all web callbacks are in the main thread. BUT you block the main thread with the thread sleep -> you can not get call back from the web to get a count.
Edited:
try to call getCount method from another thread. use e.g.
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[self getCount:^(int count) {
NSLog(#"Got the count: %i", count);
_count = count; // _count is an ivar of the object with this method.
gotCount = YES;
}];
});
Edited 2:
I tried this code and it works fine -> something probably wrong with the threads in your getCount method.
- (NSString *)describe {
__block bool gotCount = NO;
__block NSInteger _count;
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[NSThread sleepForTimeInterval:5.00];
_count = 5;
gotCount = YES;
});
// Pause here until the count has been fetched.
while (!gotCount) {
[NSThread sleepForTimeInterval:0.05];
}
return [NSString stringWithFormat:#"The count is %li", _count];
}

Answer 2

a way that would work but is QUITE the hack (which apple employed in older APIs on the mac all the time) would be to run the runloop while waiting:
note: this relies on the callback being on the same queue as the describe method
see: JSON async request [SAME ISSUE]
a self contained WORKING example:
#import <Foundation/Foundation.h>
#interface T : NSObject
- (NSString *)describe;
#end
#implementation T {
int _count;
}
- (void)getCount:(void (^)(int c)) handler {
dispatch_async(dispatch_get_global_queue(0,0), ^ {
sleep(5);
dispatch_sync(dispatch_get_main_queue(), ^{
handler(55);
});
});
}
- (NSString *)describe {
__block bool gotCount = NO;
[self getCount:^(int count) {
NSLog(#"Got the count: %i", count);
_count = count; // _count is an ivar of the object with this method.
gotCount = YES;
}];
// Pause here until the count has been fetched.
while (!gotCount) {
[[NSRunLoop currentRunLoop] runUntilDate:[NSDate dateWithTimeIntervalSinceNow:0.1]];
}
return [NSString stringWithFormat:#"The count is %i", _count];
}
#end
int main(int argc, char *argv[]) {
#autoreleasepool {
T *t = [T new];
NSLog(#"describe: %#", [t describe]);
}
}