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
}

Corissa

what is -displayActivityIndicator(_:) method doing? – Zooplasty

Do not load data form a server synchronously, it causes bad user experience. Get used to the asynchronous pattern. – Armindaarming

I just noticed this too, i have a method that can return from main or secondary thread (which probably is not such a good idea but it makes sense for my case). I think is something new because i never worried about dispatching main thread from main thread till now. – Godparent

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")
    }
})

Unshackle

@RodrigoCosta what are you trying to achieve with DispatchQueue.main.sync { self.displayActivityIndicator(viewController: vc) }? – Unshackle

The method POST needs to return data to the caller, the method displayActivityIndicator, displays an UIActivityIndicatorView into vc.view. I can't use async because i need data returned from the post. – Corissa

The method POST needs to return data to the caller, the method displayActivityIndicator, displays an UIActivityIndicatorView into vc.view. I can't use async because i need data returned from the post. – Corissa

@RodrigoCosta it will never work. Read the accepted answer to this question: #31776614. It is the proper way to do it. – Unshackle

@RodrigoCosta welcome. I would appreciate, if you accept it. – Unshackle

Why does DispatchQueue.main.sync mean I launch it from a background thread? – Putout

This answer makes no sense. The code after the DispatchQueue.global().async(execute: { block is still going to run before the print("main thread") code is run. Just use DispatchQueue.main.async and don't bother with the global background queue. – Robers

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
  }
}

Merriott

Thanks, saved me some time. I find it kind of strange though that calling DispatchQueue.main.sync on the main thread leads to an error. – Drawshave

There is nothing strange. You put block of code to be executed on main thread. So, if it already on main thread, that's obvious error..) – Chandra

Answer 3

If you want to read the value from the main thread, here is a handy function:

func syncMain<T>(_ closure: () -> T) -> T {
    if Thread.isMainThread {
        return closure()
    } else {
        return DispatchQueue.main.sync(execute: closure)
    }
}

Usage:

// we are in BG or Main, does not matter
let value = syncMain {
    // we are in Main for sure
    return ...
}

Owenism

Answer 4

According to the Apple documentation given below. https://developer.apple.com/documentation/dispatch/dispatchqueue/1452870-sync

Calling the sync function in the current target queue(main thread in this case) may lead to a deadlock state. Because of this, an error will be thrown. So you have to change the thread or you have to call asynchronously.

Suggestion: You can use a global thread to execute the function.

DispatchQueue.global().sync {
    self.displayActivityIndicator(viewController: vc)
}

Or also you can use the async function in the main thread.

DispatchQueue.main.async {
    self.displayActivityIndicator(viewController: vc)
}

Fascist

Your first option is bad because you can't run UI code on a background thread. – Robers

Got it! What about second option @HangarRash? – Fascist

The 2nd option is the correct one. It will work whether the current thread is on the main queue or not. – Robers