Main Thread Checker 问题解决

1. without a return value

https://developer.apple.com/documentation/code_diagnostics/main_thread_checker

NSURLSessionTask *task = [[NSURLSession sharedSession] dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
    dispatch_async(dispatch_get_main_queue(), ^{ // Correct
        self.label.text = [NSString stringWithFormat:@"%lu bytes downloaded", data.length];
    });
}];
[task resume];

　　

let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
   if let data = data {
      DispatchQueue.main.async { // Correct
         self.label.text = "\(data.count) bytes downloaded"
      }
   }
}
task.resume()

　　

2. with a return value

https://stackoverflow.com/questions/45832155/how-do-i-refactor-my-code-to-call-appdelegate-on-the-main-thread

static var realDelegate: AppDelegate?;

static var appDelegate: AppDelegate {
    if Thread.isMainThread{
        return UIApplication.shared.delegate as! AppDelegate;
    }
    let dg = DispatchGroup();
    dg.enter()
    DispatchQueue.main.async{
        realDelegate = UIApplication.shared.delegate as? AppDelegate;
        dg.leave();
    }
    dg.wait();
    return realDelegate!;
}