ios - Firebase 下载完成后如何直接执行代码？

Question

我有一些代码需要在 Firebase 完成下载我事先指定它下载的内容后立即执行。问题是此代码始终在下载完成之前运行。

if currentVersionNumber < newVersionNumber {
            print("Feed: button donwloading cards")
            //self.databaseButton.setTitle("Downloading New Cards...", for: .normal)
            ref.observe(.value, with: { snapshot in
                print("Feed: Checking for new cards from firebase")
                for item in snapshot.children {
                    // Download in memory with a maximum allowed size of 1MB (1 * 1024 * 1024 bytes)
                    cardRef.data(withMaxSize: 1 * 1024 * 1024) { (data, error) -> Void in
                        if (error != nil) {
                            // Uh-oh, an error occurred!
                            print("Feed: error occured")
                            print(error)
                        } else {
                            // Data for "images/island.jpg" is returned
                            cards.imageName = data!
                            print("Feed: downloaded \(cards.name)")
                        }
                    }
                    // add to updated list of cards

                    updateCards.append(cards);
                }
            })

        } else {
             print("Feed: cards are up to date. \(currentVersionNumber)")
        }

    })

此代码从 Firebase 数据库下载我想要的项目，但会在完成之前运行它之后的任何代码。如何才能让我可以选择在下载完成后立即执行代码块？

Answer 1

发生这种情况是因为来自 Firebase 的所有更新都发生在后台线程中，而您的代码是在主线程上执行的。要处理此问题，请在一个函数内调用您的 firebase 方法，该函数具有一个闭包，该闭包会在您的 firebase 下载完成后立即调用。

例如:

在你的viewDidLoad中:

override func viewDidLoad() {

   super.viewDidLoad()

   fetchData {

       //do whatever action you wish to perform on download completion
       mainTableView.reloadData()
   }
}

func fetchData(andOnCompletion completion:@escaping ()->()){

   ref.observe(.value, with: { snapshot in
            print("Feed: Checking for new cards from firebase")
            for item in snapshot.children {
                // Download in memory with a maximum allowed size of 1MB (1 * 1024 * 1024 bytes)
                cardRef.data(withMaxSize: 1 * 1024 * 1024) { (data, error) -> Void in
                    if (error != nil) {
                        // Uh-oh, an error occurred!
                        print("Feed: error occured")
                        print(error)
                    } else {
                        // Data for "images/island.jpg" is returned
                        cards.imageName = data!
                        print("Feed: downloaded \(cards.name)")
                    }
                }
                // add to updated list of cards

                updateCards.append(cards);
            }
         //call the block when done processing          
     completion()
  })
}