在 Swift 3 中访问闭包外的数组

Question

我有这个闭包，我用它来填充我的数组和字典。但是，当我尝试在函数之外使用它时，它是空的。我知道这个闭包在异步线程上工作，所以假设我在填充之前尝试访问该变量是否正确？结果，我得到一个空数组。这是我的代码。

class HomeCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout, UISearchBarDelegate, UIGestureRecognizerDelegate {

var entries = [String: DiaryEntry]()
var entryIDS =  [String]()

var searchController: UISearchController!

override func viewDidLoad() {
    super.viewDidLoad()

    // Register cell classes
    self.collectionView!.register(DiaryCell.self, forCellWithReuseIdentifier: "homeCell")
    collectionView?.backgroundColor = UIColor.white
    navigationController?.hidesBarsOnSwipe = true

    if let userID = FIRAuth.auth()?.currentUser?.uid {
        FirebaseService.service.getUserEntriesRef(uid: userID).observe(.value, with: { [weak weakSelf = self] (snapshot) in
            let enumerator = snapshot.children
            while let entry = enumerator.nextObject() as? FIRDataSnapshot {
                weakSelf?.entryIDS.append(entry.key)
                weakSelf?.entries[entry.key] = DiaryEntry(snapshot: entry)
            }
            weakSelf?.entryIDS.reverse()
            weakSelf?.collectionView?.reloadData()
        })
        print("Entries: \(entryIDS.count) ")
    }
    // Do any additional setup after loading the view.
}

处理这种多线程执行的最佳方法是什么？

Answer 1

使用Dispatch Groups 跟踪您完成将所有元素附加到数组的时间，然后进行notify 回调，当它们全部添加时会自动调用。

class HomeCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout, UISearchBarDelegate, UIGestureRecognizerDelegate {

var entries = [String: DiaryEntry]()
var entryIDS =  [String]()

let dispatchGroup = DispatchGroup()

var searchController: UISearchController!

override func viewDidLoad() {
    super.viewDidLoad()

    // Register cell classes
    self.collectionView!.register(DiaryCell.self, forCellWithReuseIdentifier: "homeCell")
    collectionView?.backgroundColor = UIColor.white
    navigationController?.hidesBarsOnSwipe = true
    self.dispatchGroup.enter()
    if let userID = FIRAuth.auth()?.currentUser?.uid {
        FirebaseService.service.getUserEntriesRef(uid: userID).observe(.value, with: { [weak weakSelf = self] (snapshot) in
            let enumerator = snapshot.children
            while let entry = enumerator.nextObject() as? FIRDataSnapshot {
                weakSelf?.entryIDS.append(entry.key)
                weakSelf?.entries[entry.key] = DiaryEntry(snapshot: entry)
            }
            self.dispatchGroup.leave()
        })
        self.dispatchGroup.notify(queue: DispatchQueue.main, execute: {
             print("Entries: \(entryIDS.count) ")
             weakSelf?.entryIDS.reverse()
             weakSelf?.collectionView?.reloadData()
        })
    }
    // Do any additional setup after loading the view.
}

我还建议只使用self 而不是weakSelf?。

Answer 2

我遵循 Raywenderlich 及其团队的编码标准（针对 Swift）。如果我要重写你的代码来拥有一个强大的自我，它会是这样的：

class HomeCollectionViewController: UICollectionViewController, UICollectionViewDelegateFlowLayout, UISearchBarDelegate, UIGestureRecognizerDelegate {

    var entries = [String: DiaryEntry]()
    var entryIDS =  [String]()

    var searchController: UISearchController!

    override func viewDidLoad() {
        super.viewDidLoad()

        // Register cell classes
        self.collectionView!.register(DiaryCell.self, forCellWithReuseIdentifier: "homeCell")
        collectionView?.backgroundColor = UIColor.white
        navigationController?.hidesBarsOnSwipe = true

        if let userID = FIRAuth.auth()?.currentUser?.uid {
            FirebaseService.service.getUserEntriesRef(uid: userID).observe(.value, with: {
                [weak self] (snapshot) in

                guard let strongSelf = self else {
                    return
                }

                let enumerator = snapshot.children
                while let entry = enumerator.nextObject() as? FIRDataSnapshot {
                    strongSelf.entryIDS.append(entry.key)
                    strongSelf.entries[entry.key] = DiaryEntry(snapshot: entry)
                }
                strongSelf.entryIDS.reverse()
                strongSelf.collectionView?.reloadData()
            })
            print("Entries: \(entryIDS.count) ")
        }
        // Do any additional setup after loading the view.
}

我希望这有效。在连接到任何 API（例如 Firebase 和 Alamofire）时，我也在编写这样的代码。

Answer 3

我刚刚遇到了您的问题，同时寻找相同问题的解决方案，我终于设法弄清楚了。所以，我会尽量回答这对我们后面的许多人会有所帮助。

问题是数组只存在于闭包内部。因此，解决方案是在 viewDidLoad 之外创建一个数组，并在拥有完整数组后使用它进行设置，然后使用 didSet 设置 entryIDS

var entryIDS =  [String]() {
    didSet {
        //something
     }
}