只有在从Firebase下载数据后才能执行操作 [英] How to perform an action only after data are downloaded from Firebase

问题描述

我试图找出如何触发火力地堡的observeSingleEvent后才能完成其工作的作用。

背后的原因是，我需要做的以下操作：

• 据我需要从数据库中获取一些数据的用户一个输入


• 当这些数据被完全下载，我需要处理它们一点点，用于检索其它数据被显示给用户

$ p $ lt; code> let listOfTags = [String]（）
var outputArray = [string]（）

如果listOfTags.count！= 0 {
为listOfTags中的标签{

ref.child（tags）。child（tag ）.observeSingleEvent（：.value，其中：{（snapshot）in

if temp = snapshot.children.allObjects as？[FIRDataSnapshot] {
for temp in {
outputArray.append（elem.k EY）
}
}

// ...
}）{（错误）在
打印（error.localizedDescription）
}
}

//这里我想在循环完成后执行一个操作
//完成并且outputArray被所有的值赋值
doSomethingHere（）

}

但是我注意到函数doSomethingHere （）在outputArray仍然是EMPTY时立即执行！所以它什么也没做。然后我的问题是，我怎样才能构造程序，使得这个函数在下载过程完成后执行？也许observeSingleEvent是不是正确的Firebase方法使用？对于这个特定的情况，我不想观察任何改变（因为这个列表并没有经常改变），我只需要下载数据，稍后用这些数据做一些其他的事情。

Firebase函数中的闭包就是这样做的 - 当数据（快照）有效时，闭包内的代码将执行。

移动doSomethingHere到火力地堡闭包内，它会以正确的顺序执行 - 在这种情况下

 如果listOfTags.count = 0 {$！ 
 $ b ref.child（tags）。child（tag）.observeSingleEvent（of：.value，with：{（snapshot）in 
 
如果让temp = snapshot.children.allObjects as？[FIRDataSnapshot] {
为临时元素{
 outputArray.append（elem。 key）
} 
 doSomethingHere（）//在此处填充数组
} 
}）
} 
} 
  

请注意，您也可以删除这个

 如果listOfTags.count！= 0 
  

好像listOfTags是0，for循环将不会执行。

I'm trying to figure out how to trigger an action only after the observeSingleEvent in Firebase has finished its work.

The reason behind is that I need to do the following operations:

• According to one input from the user I need to get some data from the database
• When these data are completely downloaded, I need to process them a little bit and use to retrieve other data to be shown to the user

Currently I'm using this code:

    let listOfTags = [String]()
    var outputArray = [String]()

    if listOfTags.count != 0 {
        for tag in listOfTags {

            ref.child("tags").child(tag).observeSingleEvent(of: .value, with: { (snapshot) in

                if let temp = snapshot.children.allObjects as? [FIRDataSnapshot] {
                    for elem in temp {
                        outputArray.append(elem.key)
                    }
                }

                // ...
            }) { (error) in
                print(error.localizedDescription)
            }
        }

        // HERE I WANT TO DO AN ACTION "ONLY" AFTER THE FOR LOOP IS
        // COMPLETED AND THE "outputArray" IS FILLED WITH ALL VALUES
        doSomethingHere()

    }

But I noticed that the function doSomethingHere() is executed immediately when the outputArray is still EMPTY! So it does nothing.

Then my question is, how can I structure the program such that this function is executed after the downloading process is completed? Maybe observeSingleEvent is not the correct Firebase method to be used? For this specific case I don't want to observe any change (because this list is not changing so often), I just need to download data and do something else with these data later ..

解决方案

Firebase is asynchronous so to make code execute in a defined order, you need to 'wait' for firebase to return data.

The closures that go with Firebase functions do just that - the code within the closure executes when the data (snapshot) is valid.

Move doSomethingHere() to within the Firebase closure and it will execute in the correct order - in this case

if listOfTags.count != 0 {
    for tag in listOfTags {

        ref.child("tags").child(tag).observeSingleEvent(of: .value, with: { (snapshot) in

            if let temp = snapshot.children.allObjects as? [FIRDataSnapshot] {
                for elem in temp {
                    outputArray.append(elem.key)
                }
                doSomethingHere() //array is populated at this point
            }
        })
    }
}

Note you can also remove this

if listOfTags.count != 0

as if listOfTags is 0, the for loop will not execute.

这篇关于只有在从Firebase下载数据后才能执行操作的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！