使用DispatchSemaphore wait()冻结应用程序

问题描述

我创建了一个getFriends函数，该函数从firestore中读取用户的好友列表，并将每个好友放入LocalUser对象(这是我的自定义用户类)中，以便在表格视图中显示该好友列表.我需要DispatchSemaphore.wait()，因为仅在调用for循环内的完成处理程序时才需要for循环进行迭代.加载视图时，该应用程序冻结.我知道问题是在主线程中调用了semaphore.wait().但是，通过阅读DispatchQueue-tutorials，我仍然不了解如何解决此问题.另外:您看到实现我想要做的任何更简单的方法了吗?

I have created a function getFriends that reads a User's friendlist from firestore and puts each friend in a LocalUser object (which is my custom user class) in order to display the friendlist in a tableview. I need the DispatchSemaphore.wait() because I need the for loop to iterate only when the completion handler inside the for loop is called.When loading the view, the app freezes. I know that the problem is that semaphore.wait() is called in the main thread. However, from reading DispatchQueue-tutorials I still don't understand how to fix this in my case.Also: do you see any easier ways to implement what I want to do?

这是我对viewDidLoad()中的函数的调用:

This is my call to the function in viewDidLoad():

    self.getFriends() { (friends) in
        self.foundFriends = friends
        self.friendsTable.reloadData()
    }

函数getFriends:

And the function getFriends:

let semaphore = DispatchSemaphore(value: 0)

    func getFriends(completion: @escaping ([LocalUser]) -> ()) {
    var friendsUID = [String : Any]()
    database.collection("friends").document(self.uid).getDocument { (docSnapshot, error) in
        if error != nil {
            print("Error:", error!)
            return
        }
        friendsUID = (docSnapshot?.data())!
        var friends = [LocalUser]()
        let friendsIdents = Array(friendsUID.keys)

        for (idx,userID) in friendsIdents.enumerated() {
            self.getUser(withUID: userID, completion: { (usr) in
                var tempUser: LocalUser
                tempUser = usr
                friends.append(tempUser)
                self.semaphore.signal()
            })
            if idx == friendsIdents.endIndex-1 {
                print("friends at for loop completion:", friends.count)
                completion(friends)
            }
            self.semaphore.wait()
        }
    }
}

friendsUID是一个字典，每个朋友的uid为键，而true为值.由于我只需要密钥，因此将它们存储在数组friendsIdents中.函数getUser在firestore中搜索传递的uid，并创建相应的LocalUser(usr).最终将其添加到friends数组中.

friendsUID is a dict with each friend's uid as a key and true as the value. Since I only need the keys, I store them in the array friendsIdents. Function getUser searches the passed uid in firestore and creates the corresponding LocalUser (usr). This finally gets appended in friends array.

推荐答案

几乎永远不要在主线程上有semaphore.wait().除非您期望等待<10毫秒.

You should almost never have a semaphore.wait() on the main thread. Unless you expect to wait for < 10ms.

相反，请考虑将您的朋友列表处理分派到后台线程.后台线程可以执行对数据库/api和wait()的调度，而不会阻塞主线程.

Instead, consider dispatching your friends list processing to a background thread. The background thread can perform the dispatch to your database/api and wait() without blocking the main thread.

如果您需要触发任何UI工作，只需确保从该线程使用DispatchQueue.main.async {}.

Just make sure to use DispatchQueue.main.async {} from that thread if you need to trigger any UI work.

let semaphore = DispatchSemaphore(value: 0)

func getFriends(completion: @escaping ([LocalUser]) -> ()) {
    var friendsUID = [String : Any]()
    database.collection("friends").document(self.uid).getDocument { (docSnapshot, error) in
        if error != nil {
            print("Error:", error!)
            return
        }


    DispatchQueue.global(qos: .userInitiated).async {
        friendsUID = (docSnapshot?.data())!
        var friends = [LocalUser]()
        let friendsIdents = Array(friendsUID.keys)

        for (idx,userID) in friendsIdents.enumerated() {
            self.getUser(withUID: userID, completion: { (usr) in
                var tempUser: LocalUser
                tempUser = usr
                friends.append(tempUser)
                self.semaphore.signal()
            })
            if idx == friendsIdents.endIndex-1 {
                print("friends at for loop completion:", friends.count)
                completion(friends)
            }
            self.semaphore.wait()
        }
        // Insert here a DispatchQueue.main.async {} if you need something to happen
        // on the main queue after you are done processing all entries
    }
}

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