DispatchQueue.main.sync returning exc_bad_instruction Swift 3

Question

I want to display an ActivityIndicatorView in my app, but when I call the sync method from the main thread, the app crashes with the error: exc_bad_instruction (code=exc_i386_invop subcode=0x0) I'm using xcode 8.0 and swift 3

Can someone please help me?

 func POST(endpoint:NSString!,body:NSString!,vc:UIViewController? = nil)->NetworkResult{     let result = NetworkResult()     DispatchQueue.main.sync {         self.displayActivityIndicator(viewController: vc)     }     let urlStr = self.url.appending(endpoint as String).appending(self.getHashAutenticacao() as String)     print(urlStr)     let request = NSMutableURLRequest(url: URL(string: urlStr)!, cachePolicy: NSURLRequest.CachePolicy.reloadIgnoringLocalCacheData, timeoutInterval: 20)     print(request.debugDescription)     request.setValue("application/json", forHTTPHeaderField: "Accept")     request.setValue("application/json", forHTTPHeaderField: "Content-Type")     request.httpMethod = "POST"     request.httpBody = body.data(using: String.Encoding.utf8.rawValue, allowLossyConversion: true)      // send the request     var data: NSData!     do {         data = try NSURLConnection.sendSynchronousRequest(request as URLRequest, returning: &self.response) as NSData!     } catch let error1 as NSError {         self.error = error1         data = nil     }     if let httpResponse = self.response as? HTTPURLResponse {         result.resultCode = httpResponse.statusCode          if httpResponse.statusCode == 200{             if data != nil{                 if data.length > 0{                     let json = (try! JSONSerialization.jsonObject(with: data as Data, options: JSONSerialization.ReadingOptions.mutableContainers))                     if let jsonArray:NSArray = json as? NSArray{                         result.data = jsonArray                     }else{                         if let jsonDict:NSDictionary = json as? NSDictionary{                             result.data = [jsonDict]                         }                     }                 }             }         }     }else {         result.message = self.error!.debugDescription as NSString?     }      DispatchQueue.main.sync {         self.hideActivityIndicator(viewController: vc)     }     return result } 

Answer 1

What you are trying to do here is to launch the main thread synchronously from a background thread before it exits. This is a logical error.

You should do it like this:

DispatchQueue.global().async(execute: {     print("teste")     DispatchQueue.main.sync{         print("main thread")     } }) 

Answer 2

Maybe it's because you are trying to DispatchQueue.main.sync from the Main thread. You can check if you are already in the main thread like:

if Thread.isMainThread {   // do stuff } else {   DispatchQueue.main.sync {     // do stuff   } }