【发布时间】：2015-04-08 18:08:45
【问题描述】：
在this blog 中，他给出了回调地狱的this（复制/粘贴以下代码）示例。但是，没有提到如何通过使用响应式扩展来消除该问题。
所以这里 F3 取决于 F1 完成，而 F4 和 F5 取决于 F2 完成。
- 想知道 Rx 中的等效功能是什么。
- 如何在 Rx 中表示 F1、F2、F3、F4 和 F5 都应该异步拉取？
注意：我目前正试图围绕 Rx 进行思考，所以在问这个问题之前我没有尝试解决这个例子。
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
public class CallbackB {
/**
* Demonstration of nested callbacks which then need to composes their responses together.
* <p>
* Various different approaches for composition can be done but eventually they end up relying upon
* synchronization techniques such as the CountDownLatch used here or converge on callback design
* changes similar to <a href="https://github.com/Netflix/RxJava">Rx</a>.
*/
public static void run() throws Exception {
final ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());
/* the following are used to synchronize and compose the asynchronous callbacks */
final CountDownLatch latch = new CountDownLatch(3);
final AtomicReference<String> f3Value = new AtomicReference<String>();
final AtomicReference<Integer> f4Value = new AtomicReference<Integer>();
final AtomicReference<Integer> f5Value = new AtomicReference<Integer>();
try {
// get f3 with dependent result from f1
executor.execute(new CallToRemoteServiceA(new Callback<String>() {
@Override
public void call(String f1) {
executor.execute(new CallToRemoteServiceC(new Callback<String>() {
@Override
public void call(String f3) {
// we have f1 and f3 now need to compose with others
System.out.println("intermediate callback: " + f3 + " => " + ("f4 * f5"));
// set to thread-safe variable accessible by external scope
f3Value.set(f3);
latch.countDown();
}
}, f1));
}
}));
// get f4/f5 after dependency f2 completes
executor.execute(new CallToRemoteServiceB(new Callback<Integer>() {
@Override
public void call(Integer f2) {
executor.execute(new CallToRemoteServiceD(new Callback<Integer>() {
@Override
public void call(Integer f4) {
// we have f2 and f4 now need to compose with others
System.out.println("intermediate callback: f3" + " => " + (f4 + " * f5"));
// set to thread-safe variable accessible by external scope
f4Value.set(f4);
latch.countDown();
}
}, f2));
executor.execute(new CallToRemoteServiceE(new Callback<Integer>() {
@Override
public void call(Integer f5) {
// we have f2 and f5 now need to compose with others
System.out.println("intermediate callback: f3" + " => " + ("f4 * " + f5));
// set to thread-safe variable accessible by external scope
f5Value.set(f5);
latch.countDown();
}
}, f2));
}
}));
/* we must wait for all callbacks to complete */
latch.await();
System.out.println(f3Value.get() + " => " + (f4Value.get() * f5Value.get()));
} finally {
executor.shutdownNow();
}
}
public static void main(String[] args) {
try {
run();
} catch (Exception e) {
e.printStackTrace();
}
}
private static final class CallToRemoteServiceA implements Runnable {
private final Callback<String> callback;
private CallToRemoteServiceA(Callback<String> callback) {
this.callback = callback;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call("responseA");
}
}
private static final class CallToRemoteServiceB implements Runnable {
private final Callback<Integer> callback;
private CallToRemoteServiceB(Callback<Integer> callback) {
this.callback = callback;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(40);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call(100);
}
}
private static final class CallToRemoteServiceC implements Runnable {
private final Callback<String> callback;
private final String dependencyFromA;
private CallToRemoteServiceC(Callback<String> callback, String dependencyFromA) {
this.callback = callback;
this.dependencyFromA = dependencyFromA;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(60);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call("responseB_" + dependencyFromA);
}
}
private static final class CallToRemoteServiceD implements Runnable {
private final Callback<Integer> callback;
private final Integer dependencyFromB;
private CallToRemoteServiceD(Callback<Integer> callback, Integer dependencyFromB) {
this.callback = callback;
this.dependencyFromB = dependencyFromB;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(140);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call(40 + dependencyFromB);
}
}
private static final class CallToRemoteServiceE implements Runnable {
private final Callback<Integer> callback;
private final Integer dependencyFromB;
private CallToRemoteServiceE(Callback<Integer> callback, Integer dependencyFromB) {
this.callback = callback;
this.dependencyFromB = dependencyFromB;
}
@Override
public void run() {
// simulate fetching data from remote service
try {
Thread.sleep(55);
} catch (InterruptedException e) {
e.printStackTrace();
}
callback.call(5000 + dependencyFromB);
}
}
private static interface Callback<T> {
public void call(T value);
}
}
【问题讨论】：
-
我不是 Rx 大师，但来自他提供的一个会话：您可以通过编写 Observables 来避免回调地狱。如果@benjchristensen 在附近-他也许可以提供更多详细信息。
-
@alfasin 我对 RxJava 的了解足够多，知道应该通过组合 observables 来解决它。这里的问题是如何组成:-)
-
在这种情况下 - 我会将问题从“如何避免回调地狱？”到“如何组成 observables？” ;)
-
@alfasin 重新措辞了这个问题……你有答案吗？
-
@EntryLevelDev 是的。改变了它。谢谢。