谜之RxJava （一） —— 最基本的观察者模式
2015-11-26 20:55 330 0
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。
