Alexander-Ignition/gcd_pefomance_anti-pattern.md

## gcd_pefomance_anti-pattern.md

      
    Raw
  

              gcd_pefomance_anti-pattern.md
            
          
    Grand Central Dispatch Performance Anti-Pattern

WWDC18 What's New in LLVM at 16:48
Performance implications: slowdown and hangs
__block NSString *taskName = nil;
dispatch_semaphore_t sema = dispatch_semaphore_create(0); 

[self.connection.remoteObjectProxy requestCurrentTaskName:^(NSString *task) {
    taskName = task;
    dispatch_semaphore_signal(sema); 
}];

dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER); 
return taskName;

Blocks current thread by the execution on a different queue
The task queue usually has lower priority, leads to priority inversion
Spawns useless threads

If available, use synchronous version of the API
__block NSString *taskName = nil;
id remoteObjectProxy = [self.connection synchronousRemoteObjectProxyWithErrorHandler: ^(NSError *error) { 
    NSLog(@“Error: %@“, error); 
}]; 
[remoteObjectProxy requestCurrentTaskName:^(NSString *task) {
    taskName = task; 
}];
return taskName;
Alternatively, use the API in an asynchronous manner
[self.connection.remoteObjectProxy requestCurrentTaskName:^(NSString *task) { 
    completionHandler(task);
}];
Enabling check in build settings CLANG_ANALYZER_GCD_PERFORMANCE