问题描述
我有一个简单的功能，可以从Firebase加载数据.
func loadFromFireBase() -> Array<Song>? {
var songArray:Array<Song> = []
ref.observe(.value, with: { snapshot in
//Load songArray
})
if songArray.isEmpty {
return nil
}
return songArray
}
当前，即使有要加载的数据，此函数也始终返回nil.之所以这样做，是因为它永远不会执行执行完成功能块，在函数返回之前，该功能不会在此完成数组的加载.我正在寻找一种方法，使函数仅在调用完成块后才返回，但不能将return放入完成块中.
(关于该问题的变化经常出现在SO上.我永远找不到一个好的，全面的答案，因此下面是尝试提供这种答案的尝试)
您不能那样做. Firebase是异步的.其功能采用完成处理程序并立即返回.您需要重写loadFromFirebase函数以获取完成处理程序.
我在Github上有一个名为 Async_demo 的示例项目，该链接是一个正在运行的(Swift 3)应用程序，用于说明该技术.
其中的关键部分是函数downloadFileAtURL，该函数具有完成处理程序并进行异步下载:
typealias DataClosure = (Data?, Error?) -> Void
/**
This class is a trivial example of a class that handles async processing. It offers a single function, `downloadFileAtURL()`
*/
class DownloadManager: NSObject {
static var downloadManager = DownloadManager()
private lazy var session: URLSession = {
return URLSession.shared
}()
/**
This function demonstrates handling an async task.
- Parameter url The url to download
- Parameter completion: A completion handler to execute once the download is finished
*/
func downloadFileAtURL(_ url: URL, completion: @escaping DataClosure) {
//We create a URLRequest that does not allow caching so you can see the download take place
let request = URLRequest(url: url,
cachePolicy: .reloadIgnoringLocalAndRemoteCacheData,
timeoutInterval: 30.0)
let dataTask = URLSession.shared.dataTask(with: request) {
//------------------------------------------
//This is the completion handler, which runs LATER,
//after downloadFileAtURL has returned.
data, response, error in
//Perform the completion handler on the main thread
DispatchQueue.main.async() {
//Call the copmletion handler that was passed to us
completion(data, error)
}
//------------------------------------------
}
dataTask.resume()
//When we get here the data task will NOT have completed yet!
}
}
上面的代码使用Apple的URLSession类从远程服务器异步下载数据.创建dataTask时，您传入一个完成处理程序，该处理程序在数据任务完成(或失败)时被调用.但是请注意:您的完成处理程序在后台线程上被调用.
那很好，因为如果您需要执行诸如解析大型JSON或XML结构之类的耗时的处理，则可以在完成处理程序中进行操作而不会导致应用程序的UI冻结.但是，结果是，如果不将这些UI调用发送到主线程，就无法在数据任务完成处理程序中进行UI调用.上面的代码使用对DispatchQueue.main.async() {}的调用在主线程上调用整个完成处理程序.
返回OP的代码:
我发现以闭包作为参数的函数很难读取，因此我通常将闭包定义为类型别名.
从@ Raghav7890的答案中重新编写代码以使用typealias:
typealias SongArrayClosure = (Array<Song>?) -> Void
func loadFromFireBase(completionHandler: @escaping SongArrayClosure) {
ref.observe(.value, with: { snapshot in
var songArray:Array<Song> = []
//Put code here to load songArray from the FireBase returned data
if songArray.isEmpty {
completionHandler(nil)
}else {
completionHandler(songArray)
}
})
}
我已经很长时间没有使用Firebase了(然后只修改了别人的Firebase项目)，所以我不记得它是在主线程还是在后台线程上调用它的完成处理程序.如果它在后台线程上调用完成处理程序，那么您可能希望将对完成处理程序的调用包装在对主线程的GCD调用中.
基于对 的回答>这个问题 ，听起来像Firebase在后台线程上进行网络调用，但在主线程上调用它的侦听器.
在这种情况下，您可以忽略以下有关Firebase的代码，但是对于那些阅读此线程以寻求其他种类的异步代码帮助的人来说，以下是重写代码以在主线程上调用完成处理程序的方法:
typealias SongArrayClosure = (Array<Song>?) -> Void
func loadFromFireBase(completionHandler:@escaping SongArrayClosure) {
ref.observe(.value, with: { snapshot in
var songArray:Array<Song> = []
//Put code here to load songArray from the FireBase returned data
//Pass songArray to the completion handler on the main thread.
DispatchQueue.main.async() {
if songArray.isEmpty {
completionHandler(nil)
}else {
completionHandler(songArray)
}
}
})
}
I have a simple function loading data from Firebase.
func loadFromFireBase() -> Array<Song>? {
var songArray:Array<Song> = []
ref.observe(.value, with: { snapshot in
//Load songArray
})
if songArray.isEmpty {
return nil
}
return songArray
}
Currently this function returns nil always, even though there is data to load. It does this because it doesn't ever get to the perform the completion block where it loads the array before the function returns. I'm looking for a way to make the function only return once the completion block has been called but I can't put return in the completion block.
(Variations on this question come up constantly on SO. I can never find a good, comprehensive answer, so below is an attempt to provide such an answer)
You can't do that. Firebase is asynchronous. Its functions take a completion handler and return immediately. You need to rewrite your loadFromFirebase function to take a completion handler.
I have a sample project on Github called Async_demo (link) that is a working (Swift 3) app illustrating this technique.
The key part of that is the function downloadFileAtURL, which takes a completion handler and does an async download:
typealias DataClosure = (Data?, Error?) -> Void
/**
This class is a trivial example of a class that handles async processing. It offers a single function, `downloadFileAtURL()`
*/
class DownloadManager: NSObject {
static var downloadManager = DownloadManager()
private lazy var session: URLSession = {
return URLSession.shared
}()
/**
This function demonstrates handling an async task.
- Parameter url The url to download
- Parameter completion: A completion handler to execute once the download is finished
*/
func downloadFileAtURL(_ url: URL, completion: @escaping DataClosure) {
//We create a URLRequest that does not allow caching so you can see the download take place
let request = URLRequest(url: url,
cachePolicy: .reloadIgnoringLocalAndRemoteCacheData,
timeoutInterval: 30.0)
let dataTask = URLSession.shared.dataTask(with: request) {
//------------------------------------------
//This is the completion handler, which runs LATER,
//after downloadFileAtURL has returned.
data, response, error in
//Perform the completion handler on the main thread
DispatchQueue.main.async() {
//Call the copmletion handler that was passed to us
completion(data, error)
}
//------------------------------------------
}
dataTask.resume()
//When we get here the data task will NOT have completed yet!
}
}
The code above uses Apple's URLSession class to download data from a remote server asynchronously. When you create a dataTask, you pass in a completion handler that gets invoked when the data task has completed (or failed.) Beware, though: Your completion handler gets invoked on a background thread.
That's good, because if you need to do time-consuming processing like parsing large JSON or XML structures, you can do it in the completion handler without causing your app's UI to freeze. However, as a result you can't do UI calls in the data task completion handler without sending those UI calls to the main thread. The code above invokes the entire completion handler on the main thread, using a call to DispatchQueue.main.async() {}.
Back to the OP's code:
I find that a function with a closure as a parameter is hard to read, so I usually define the closure as a typealias.
Reworking the code from @Raghav7890's answer to use a typealias:
typealias SongArrayClosure = (Array<Song>?) -> Void
func loadFromFireBase(completionHandler: @escaping SongArrayClosure) {
ref.observe(.value, with: { snapshot in
var songArray:Array<Song> = []
//Put code here to load songArray from the FireBase returned data
if songArray.isEmpty {
completionHandler(nil)
}else {
completionHandler(songArray)
}
})
}
I haven't used Firebase in a long time (and then only modified somebody else's Firebase project), so I don't remember if it invokes it's completion handlers on the main thread or on a background thread. If it invokes completion handlers on a background thread then you may want to wrap the call to your completion handler in a GCD call to the main thread.
Edit:
Based on the answers to this SO question, it sounds like Firebase does it's networking calls on a background thread but invokes it's listeners on the main thread.
In that case you can ignore the code below for Firebase, but for those reading this thread for help with other sorts of async code, here's how you would rewrite the code to invoke the completion handler on the main thread:
typealias SongArrayClosure = (Array<Song>?) -> Void
func loadFromFireBase(completionHandler:@escaping SongArrayClosure) {
ref.observe(.value, with: { snapshot in
var songArray:Array<Song> = []
//Put code here to load songArray from the FireBase returned data
//Pass songArray to the completion handler on the main thread.
DispatchQueue.main.async() {
if songArray.isEmpty {
completionHandler(nil)
}else {
completionHandler(songArray)
}
}
})
}
这篇关于从功能返回之前，等待Firebase加载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！