Project9- GCD(Grand Central Dispatch)

1. performSelector

//在子线程中执行
performSelector(inBackground: #selector(fetchJSON), with: nil)
//在主线程中执行
tableView.performSelector(onMainThread: #selector(UITableView.reloadData), with: nil, waitUntilDone: false)

2. async()

//开辟一个qos队列，在其中执行
DispatchQueue.global(qos: .userInitiated).async { [unowned self] in
}
//在主线程
DispatchQueue.main.async { [unowned self] in
    self.tableView.reloadData()
}