ios - 在 Swift 3 中访问闭包外的数组

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？

---

Best Answer-推荐答案

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?。

关于ios - 在 Swift 3 中访问闭包外的数组，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/42015954/