在Kotlin协程中等待Java 5的Futures而不阻塞线程。

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英文:

Await Java 5 Futures in Kotlin Coroutines without blocking the Thread

问题

Sure, here's the translated code:

  1. // 从中取得一个`suspend`函数，我希望从中返回Java 5 `Future`的结果。Future对象来自另一个库[Firebase Cloud Firestore- Admin SDK for Java](https://firebase.google.com/docs/firestore/)，并提供阻塞调用`get()`来检索所述future的结果。
  3. // 我的函数看起来像这样-
  4. suspend fun getPrefix(messageCreateEvent: MessageCreateEvent): String {
  6.     val snapshot = db.collection("prefixes")
  7.         .document(messageCreateEvent.guildId.get().asString())
  8.         .get() //This returns a future
  9.         .get() //Retrieves the future's result (Blocks thread; IDE gives warning)
  11.     //Return the prefix
  12.     return if (snapshot.exists())
  13.         snapshot.getString("prefix") ?: DEFAULT_PREFIX
  14.     else DEFAULT_PREFIX
  15. }
  17. ## 我考虑过的解决方案 ##
  18. 我首先考虑的是在`kotlinx.coroutine`中寻找扩展来连接这些futures。虽然这些扩展是存在的，但它们只适用于`CompletionStatge`。所以我决定将future包装到其中一个中-
  20. ```kotlin
  21. val snapshot = CompleteableFuture.supplyAsync {
  22.     db.collection("prefixes")
  23.         .document(messageCreateEvent.guildId.get().asString())
  24.         .get() // This returns a future
  25.         .get() // Get the result
  26. }.await()

我经验有限，不确定这是否是正确的解决方案。我在一个编程社区中提出了我的问题，其中一个人建议我使用Deferred-

  1. val deferred = CompletableDeferred<DocumentSnapshot>()
  2. val future = db.collection("prefixes")
  3.     .document(messageCreateEvent.guildId.get().asString())
  4.     .get()
  6. future.addListener(
  7.     Runnable { deferred.complete(future.get()) },
  8.     ForkJoinPool.commonPool()
  9. )
  10.             
  11. val snapshot = deferred.await()

我已经花了很多时间搜索将futures与协程连接的方法，甚至在SO上都没有类似的问题。尽管如此，如果这个问题被标记为重复，我也不会感到惊讶。

  2. Please let me know if you need any further assistance!
  4. <details>
  5. <summary>英文:</summary>
  7. I have a `suspend` function from which I want to return the result of a Java 5 `Future`. The future object comes from another library [Firebase Cloud Firestore- Admin SDK for Java](https://firebase.google.com/docs/firestore/) and provides a blocking call `get()` to retrieve the result of the said future.
  9. My function looks like this-
  11. ```kotlin
  12. suspend fun getPrefix(messageCreateEvent: MessageCreateEvent): String {
  14.     val snapshot = db.collection(&quot;prefixes&quot;)
  15.         .document(messageCreateEvent.guildId.get().asString())
  16.         .get() //This returns a future
  17.         .get() //Retrieves the future&#39;s result (Blocks thread; IDE gives warning)
  19.     //Return the prefix
  20.     return if (snapshot.exists())
  21.         snapshot.getString(&quot;prefix&quot;) ?: DEFAULT_PREFIX
  22.     else DEFAULT_PREFIX
  23. }

Solutions I have considered

The first thing that I considered was to look in kotlinx.coroutine for extensions to bridge the futures. While the extensions exist, they do only for CompletionStatge. So I decided to wrap the future into one ()-

  1. val snapshot = CompleteableFuture.supplyAsync {
  2.     db.collection(&quot;prefixes&quot;)
  3.         .document(messageCreateEvent.guildId.get().asString())
  4.         .get() // This returns a future
  5.         .get() // Get the result
  6. }.await()

I am quite inexperienced and not sure if this is was proper solution. I queried my question on a programming community, where a person recommended me to use a Deferred-

  1. val deferred = CompletableDeferred&lt;DocumentSnapshot&gt;()
  2. val future = db.collection(&quot;prefixes&quot;)
  3.     .document(messageCreateEvent.guildId.get().asString())
  4.     .get()
  6. future.addListener(
  7.     Runnable { deferred.complete(future.get()) },
  8.     ForkJoinPool.commonPool()
  9. )
  10.             
  11. val snapshot = deferred.await()

I've give it quite a time to search for a way to bridge futures to co-routines, there isn't even a similar question on SO. Through, I wouldn't be surprised if this question gets a duplicate mark.

答案1

得分: 3

The key to this problem is the suspendCoroutine function. The other non-obvious bit is that to add a callback to the ApiFuture, you use a static method on ApiFutures.

Here's an extension function that implements await() on an ApiFuture.

  1. /**
  2.  * Function to convert an ApiFuture into a coroutine return.
  3.  */
  4. suspend fun <F : Any?, R : Any?> ApiFuture<F>.await(
  5.     successHandler: (F) -> R,
  6. ): R {
  7.     return suspendCoroutine { cont ->
  8.         ApiFutures.addCallback(this, object : ApiFutureCallback<F> {
  9.             override fun onFailure(t: Throwable?) {
  10.                 cont.resumeWithException(t ?: IOException("Unknown error"))
  11.             }
  13.             override fun onSuccess(result: F) {
  14.                 cont.resume(successHandler(result))
  15.             }
  16.         }, Dispatchers.IO.asExecutor())
  17.     }
  18. }
  20. /**
  21.  * inline function to retrieve a document as a POJO from a DocumentReference
  22.  */
  23. suspend inline fun <reified T: Any> DocumentReference.toObject(): T? {
  24.     return get().await<DocumentSnapshot, T?> {
  25.         it.toObject(T::class.java)
  26.     }
  27. }

Now you can write things like:

  1. suspend fun getUser(id: String): User? {
  2.     return db.collection("users").document(id).toObject()
  3. }
英文:

The key to this problem is the suspendCoroutine function. The other non-obvious bit is that to add a callback to the ApiFuture you use a static method on ApiFutures.

Here's an extension function that implements await() on an ApiFuture.

  1.         /**
  2.          * Function to convert an ApiFuture into a coroutine return.
  3.          */
  4.         suspend fun &lt;F : Any?, R : Any?&gt; ApiFuture&lt;F&gt;.await(
  5.             successHandler: (F) -&gt; R,
  6.         ): R {
  7.             return suspendCoroutine { cont -&gt;
  8.                 ApiFutures.addCallback(this, object : ApiFutureCallback&lt;F&gt; {
  9.                     override fun onFailure(t: Throwable?) {
  10.                         cont.resumeWithException(t ?: IOException(&quot;Unknown error&quot;))
  11.                     }
  13.                     override fun onSuccess(result: F) {
  14.                         cont.resume(successHandler(result))
  15.                     }
  16.                 }, Dispatchers.IO.asExecutor())
  17.             }
  18.         }
  20. /**
  21.  * inline function to retrieve a document as a POJO from a DocumentReference
  22. */
  24. suspend inline fun &lt;reified T: Any&gt;DocumentReference.toObject(): T? {
  25.             return get().await&lt;DocumentSnapshot, T?&gt; {
  26.                 it.toObject(T::class.java)
  27.             }
  28.         }

Now you can write things like:

  1.     suspend fun getUser(id: String): User? {
  2.         return db.collection(&quot;users&quot;).document(id).toObject()
  4.     }

答案2

得分: 0

你可以使用 kotlinx-coroutines-guava 来实现这个功能。

首先，使用 ApiFutureToListenableFuture 将 ApiFuture 转换为 ListenableFuture，然后使用 .await 来等待可监听的未来的完成，而无需阻塞线程。

  1. ApiFutureToListenableFuture(this).await()
英文:

You can use kotlinx-coroutines-guava to do this

First convert ApiFuture to ListenableFuture using ApiFutureToListenableFuture then use .await to await completion of the listenableFuture without blocking a thread.

  1. ApiFutureToListenableFuture(this).await()

huangapple
  • 本文由 发表于 2020年8月12日 16:12:21
  • 转载请务必保留本文链接：https://go.coder-hub.com/63372461.html
  • concurrency
  • google-cloud-firestore
  • java
  • kotlin
  • kotlin-coroutines
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