Swift Mac App：使用调度队列加快执行速度

I'm a swift newbie and I'm working on a Swift Mac App as a demo project.The app stores stock symbols in a sqlite table, fetches the stock price, calculates value and then finally displays the results in a table view. I'm looking for ways to improve execution speed when fetching data to populate my table view. So I used Dispatch Queue as shown below. The problem is that the Stock Price and Stock Value columns (calculated in the async closure) are always empty. What am i doing wrong? The function getStocksData returns a NSMutableArray which is the datasource for my table viewI'm a swift newbie and I'm working on a Swift M