我如何在 Firebase 中使用完成处理程序

Question

我无法理解如何正确使用完成处理程序，尤其是与 firebase 代码相关的问题。据我了解，完成处理程序在 Fire Base 异步完成它时执行，对吧。因此，为了了解它是如何工作的，我只想使用完成处理程序在使用 FB 收集数据时打印 "DONE"。

我试过下面的代码，但是我显然不明白，因为它不起作用。

//PUTTING THE FB CODE IN FUNCTION
func test(completion: @escaping (String) -> Void) {

   databaseRef.child(dataRecieverStringRecipeView!).observeSingleEvent(of: .value, with: {(snapshot) in
        for item in snapshot.children.allObjects as! [DataSnapshot] {
            let thisItem = item.value as! NSDictionary

            let tempRecipe = Recipe()
            tempRecipe.fbKey = item.key
            tempRecipe.recipeHeaderObject = (thisItem["recipeHeaderFirebase"] as! String)
            tempRecipe.recipeTextObject = (thisItem["recipeIngredientsTextFirebase"] as! String)
            tempRecipe.recipeImageObject = (thisItem["recipeImageFirebase"] as! String)

            self.recipeClassArray.append(tempRecipe) //ADD OBJECT TO ARRAY
        }

    }) //END LOOP
       completion("DONE")

 }//COMPLETION
test()//???Not sure what to do here...

Answer 1

完成处理程序基本上是 WHEN 语句。

当事件发生时（例如收到 api 调用的结果），触发 completion()。您也可以通过此完成传递值。

func test(completion: @escaping (_ message: String) -> Void) {

   databaseRef.child(dataRecieverStringRecipeView!).observeSingleEvent(of: .value, with: {(snapshot) in
        //you are inside a completion handler. this code executes WHEN you've received a snapshot object

        // so you loop through and process the items
        for item in snapshot.children.allObjects as! [DataSnapshot] {
            let thisItem = item.value as! NSDictionary

            let tempRecipe = Recipe()
            tempRecipe.fbKey = item.key
            tempRecipe.recipeHeaderObject = (thisItem["recipeHeaderFirebase"] as! String)
            tempRecipe.recipeTextObject = (thisItem["recipeIngredientsTextFirebase"] as! String)
            tempRecipe.recipeImageObject = (thisItem["recipeImageFirebase"] as! String)

            self.recipeClassArray.append(tempRecipe) //ADD OBJECT TO ARRAY
        }

        // and its done here
        completion("DONE")

    })

 }

 // Now to use it...
 // You need to pass in a completion handler varaible to your test function when calling it
 test(completion: { message in
     // WHEN you get a callback from the completion handler,
        print(message)

 })

Answer 2

您需要向 test() 方法传递闭包，如下所示：

test(completion: (status: String) -> Void { print(status) })

或者像这样：

func completion(status: String) { print(status) } test(completion: completion)

您也可以使用尾随闭包语法，因为您的闭包是最后一个参数：

test() { (status) in print(status) }

我如何在 Firebase 中使用完成处理程序

How do I use completion handlers in Fire Base

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completionhandler

swift