谜之RxJava （一） ―― 最基本的观察者模式
jopen
9年前
最近在Android界，最火的framework大概就是RxJava了。
扔物线大大之前写了一篇文章 《给 Android 开发者的 RxJava 详解》，在我学习RxJava的过程中受益匪浅。经过阅读这篇文章后，我们来看下RxJava的源码，揭开它神秘的面纱。
这里准备分几篇文章写，为了能让自己有个喘口气的机会。
先来上个最最简单的，经典的Demo。
Demo
Observable.create(new Observable.OnSubscribe<String>() { @Override public void call(Subscriber<? super String> subscriber) { subscriber.onNext("hello"); } }).subscribe(new Subscriber<String>() { @Override public void onCompleted() { } @Override public void onError(Throwable e) { } @Override public void onNext(String s) { Log.d("rx", s); } }); 这段代码产生的最终结果就是在Log里会出现hello。
看下这段代码的具体流程吧。
这里有2个函数create和subscribe，我们看看create里面看了啥。
OnSubscribe对象
public final static <T> Observable<T> create(OnSubscribe<T> f) { return new Observable<T>(hook.onCreate(f)); } // constructor protected Observable(OnSubscribe<T> f) { this.onSubscribe = f; } 这里的hook是一个默认实现，里面不做任何事，就是返回f。我们看见create只是给Observable的onSubscribe赋值了我们定义的OnSubscribe。
Subscriber对象
来看下subscribe这个函数做了什么事
public final Subscription subscribe(Subscriber<? super T> subscriber) { return Observable.subscribe(subscriber, this); } private static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) { // validate and proceed if (subscriber == null) { throw new IllegalArgumentException("observer can not be null"); } if (observable.onSubscribe == null) { throw new IllegalStateException("onSubscribe function can not be null."); /* * the subscribe function can also be overridden but generally that's not the appropriate approach * so I won't mention that in the exception */ } // new Subscriber so onStart it subscriber.onStart(); /* * See https://github.com/ReactiveX/RxJava/issues/216 for discussion on "Guideline 6.4: Protect calls * to user code from within an Observer" */ // if not already wrapped if (!(subscriber instanceof SafeSubscriber)) { // assign to `observer` so we return the protected version subscriber = new SafeSubscriber<T>(subscriber); } // The code below is exactly the same an unsafeSubscribe but not used because it would add a sigificent depth to alreay huge call stacks. try { // allow the hook to intercept and/or decorate hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber); return hook.onSubscribeReturn(subscriber); } catch (Throwable e) { // special handling for certain Throwable/Error/Exception types Exceptions.throwIfFatal(e); // if an unhandled error occurs executing the onSubscribe we will propagate it try { subscriber.onError(hook.onSubscribeError(e)); } catch (OnErrorNotImplementedException e2) { // special handling when onError is not implemented ... we just rethrow throw e2; } catch (Throwable e2) { // if this happens it means the onError itself failed (perhaps an invalid function implementation) // so we are unable to propagate the error correctly and will just throw RuntimeException r = new RuntimeException("Error occurred attempting to subscribe [" + e.getMessage() + "] and then again while trying to pass to onError.", e2); // TODO could the hook be the cause of the error in the on error handling. hook.onSubscribeError(r); // TODO why aren't we throwing the hook's return value. throw r; } return Subscriptions.unsubscribed(); } } 我们看到，这里我们的subscriber被SafeSubscriber包裹了一层。
if (!(subscriber instanceof SafeSubscriber)) { // assign to `observer` so we return the protected version subscriber = new SafeSubscriber<T>(subscriber); } 然后开始执行工作流
hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber); return hook.onSubscribeReturn(subscriber);
默认的hook只是返回我们之前定义的onSubscribe，这里调用的call方法就是我们在外面定义的
new Observable.OnSubscribe<String>() { @Override public void call(Subscriber<? super String> subscriber) { subscriber.onNext("hello"); } }) 我们调用传入的subscriber对象的onNext方法，这里的subscriber是SafeSubscriber
在SafeScriber中
public void onNext(T args) { try { if (!done) { actual.onNext(args); } } catch (Throwable e) { // we handle here instead of another method so we don't add stacks to the frame // which can prevent it from being able to handle StackOverflow Exceptions.throwIfFatal(e); // handle errors if the onNext implementation fails, not just if the Observable fails onError(e); } } actual就是我们自己定义的subscriber。 原来SafeSubscriber只是为了帮我们处理好异常，以及防止工作流的重复。
这是RxJava最最基本的工作流，让我们认识到他是怎么工作的。之后我们来讲讲其中的细节和其他神奇的内容。