如何构造代码以处理异步Firebase快照? [英] How to structure code to deal with asynchronous Firebase snapshot?

问题描述

我有一个无法解决的问题.JS中有很多问题，但我不太了解.

I have an problem that I can not solve. A lot of questions are in JS and I don't really understand them.

我将Firebase用作我的IOS应用程序和Swift的数据库.这是我的情况:

I'm using Firebase as my database for my IOS app and Swift. Here is my situation:

我有一个类文件，其中包含可以检索数据库中值的函数.我在某些 viewControllers 中调用这些函数.

I have a class file that contains functions that can retrieve values in my database. I'm calling these functions in some viewControllers.

这些函数检索的值将立即在这些 viewControllers 中使用.

The values retrieved by these functions are immediately used in these viewControllers.

我的问题是我的应用程序由于类文件函数返回的nil值而崩溃.发生这种情况是由于异步Firebase快照.

My problem is that my app crash because of nil values returned by the class file functions. This happen because of the asynchronous Firebase snapshot.

假定包含值的变量在分配值之前已使用=>我的应用程序崩溃，或打印的值为nil.

The variables assumed to contain the values are used before their value is assigned => My app crash, or printed values are nil.

然后我的问题很简单:如何构建代码以避免出现此问题?我已经尝试完成了，但这对我不起作用:函数仍然是异步的.

Then my question is simple: How can I structure my code to avoid this issue? I already tried completions, but that's not working for me: functions are still asynchronous.

这是我在类文件中的功能之一:

func initAverageMark(completionHandler: @escaping (_ mark: Double) -> ()) {

    let userRef = ref.child("users").child((user?.uid)!).child("mark")

    userRef.observeSingleEvent(of: .value, with: { (snapshot) -> Void in
        if let mark: Double = snapshot.value as? Double {
            completionHandler(mark)
        }
    }) { (error) in
        print(error.localizedDescription)
    }
}

我的其中一个viewController代码:

private var totalAsks: Double!

override func viewDidLoad() {
    super.viewDidLoad()

    initInfos()
}

func initInfos() {
    mainUser().initTotalAsks{ total in
        self.totalAsks = total
}
    initLabels()
}

func initLabels() {
    totalAsksLabel.text = " \(totalAsks!)" // it crashs here
}

推荐答案

假定您要在viewController中将某些标签或某些内容设置为mark的值，就可以这样做.

Assuming you'd want to set some label or something in your viewController to the value of mark you'd do it like this.

mainUser().initTotalAsks { mark in
    self.totalAsksLabel.text = " \(mark)"
}

编辑

或者，如果您绝对要使用该Double.

Edit

Or if you absolutely want to use that Double.

private var totalAsks: Double? = nil {
    didSet {
        initLabels()
    }
}

override func viewDidLoad() {
    super.viewDidLoad()

    initInfos()
}

func initInfos() {
    mainUser().initTotalAsks{ total in
        self.totalAsks = total
    }
}

func initLabels() {
     guard totalAsks != nil else {
         return
     }
     totalAsksLabel.text = " \(totalAsks!)"
}

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