RxJava源码分析
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又好长一段时间没写博客了。今天来个较简短的博客来强势回归哈哈。上篇博客写了RN的源码分析,出乎意料的就有两个出版社联系我要不要出书,可见RN的火热。本来也想写RN的,但因为最近接触了挺多Android的东东,还是想先总结总结先。RN就放后面啦。
什么是RxJava
接触RxJava也一段时间了,一直想写下关于它的文章,RxJava是用来实现异步框架的,类似于AsyncTask。这里推荐看下给Android开发者的RxJava详解这篇文章,写得很好。
RxJava能够很简洁的完成异步操作,在逻辑很复杂的情况下,RxJava的优势就很明显。
RxJava的使用
先看下RxJava完成网络请求的代码:
Observable.create(new Observable.OnSubscribe<Integer>(){ @Override public void call(Subscriber<? super Integer> subscriber) { Log.e("rxjava","observable call"); subscriber.onNext(1); } }).map(new Func1<Integer, String>() { @Override public String call(Integer integer) { return "===>" + integer; } }).subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(new Action1<String>() { @Override public void call(String s) { } } );
链式编程风格。简单讲下上面代码。
* Observable:被观察者,有变化时通知观察者。
* Observer:观察者,监测被观察者,有变化就做相应的触发。
* OnSubscribe:Observable中的成员变量。
首先OnSubscribe的call方法中实现业务逻辑,如网络请求等,可以在子线程中执行。subscribeOn(Schedulers.io())就会在内部实现是是用一个无数量上限的线程池,在线程中实现相应操作。observeOn(AndroidSchedulers.mainThread())用于指定观察者的执行环境,这里指定的是主纯程。执行非常简单吧。
在这里给出我自己写的Demo地址,使用RxJava和Retrofit一起进行网络请求。Demo中也对RxJava与Retrofit做了封装。可以看下。
代码下载
源码分析
进入本篇博客的重点。讲下RxJava的源码。主要是了解下它的思想,很值得学习。以下面那段代码进行分析。
Observable.create(new Observable.OnSubscribe<Integer>(){ @Override public void call(Subscriber<? super Integer> subscriber) { Log.e("rxjava","observable call"); subscriber.onNext(1); } }).map(new Func1<Integer, String>() { @Override public String call(Integer integer) { return "===>" + integer; } }).subscribe(new Action1<String>() { @Override public void call(String s) { } } );
首先看create:
public static <T> Observable<T> create(OnSubscribe<T> f) { return new Observable<T>(hook.onCreate(f));}
这里的f就是我们定义的OnSubscribe。这里简称On1。hook.onCreate(f)的代码很简单如下:
public <T> OnSubscribe<T> onCreate(OnSubscribe<T> f) { return f;}
这里只是直接把f返回,而f在这里是On1。然后就新建一了个被观察者,这里简称Ob1。看下Observable的构造方法:
protected Observable(OnSubscribe<T> f) { this.onSubscribe = f;}
直接把On1赋给 Ob1里的onSubscribe变量。即Ob1.onSubscribe就是On1。记住啦。接下来,看下Map的代码,从上面分析可知,它是调用的Ob1.map()。
public final <R> Observable<R> map(Func1<? super T, ? extends R> func) { return lift(new OperatorMap<T, R>(func));}
这里实例化了OperatorMap,并将Func1作为参数传入。
public final class OperatorMap<T, R> implements Operator<R, T> { final Func1<? super T, ? extends R> transformer; public OperatorMap(Func1<? super T, ? extends R> transformer) { this.transformer = transformer; } @Override public Subscriber<? super T> call(final Subscriber<? super R> o) { MapSubscriber<T, R> parent = new MapSubscriber<T, R>(o, transformer); o.add(parent); return parent; } static final class MapSubscriber<T, R> extends Subscriber<T> { final Subscriber<? super R> actual; final Func1<? super T, ? extends R> mapper; boolean done; public MapSubscriber(Subscriber<? super R> actual, Func1<? super T, ? extends R> mapper) { this.actual = actual; this.mapper = mapper; } @Override public void onNext(T t) { R result; try { result = mapper.call(t); } catch (Throwable ex) { Exceptions.throwIfFatal(ex); unsubscribe(); onError(OnErrorThrowable.addValueAsLastCause(ex, t)); return; } actual.onNext(result); }}
其中transformer就是存的Func1。看下看下lift的代码:
public final <R> Observable<R> lift(final Operator<? extends R, ? super T> operator) { return new Observable<R>(new OnSubscribeLift<T, R>(onSubscribe, operator));}
回顾下这里的operator就是上面的OperatorMap,其中transformer就是我们定义的Func1。主要是调用OnSubscribeLift。
public final class OnSubscribeLift<T, R> implements OnSubscribe<R> { static final RxJavaObservableExecutionHook hook = RxJavaPlugins.getInstance().getObservableExecutionHook(); final OnSubscribe<T> parent; final Operator<? extends R, ? super T> operator; public OnSubscribeLift(OnSubscribe<T> parent, Operator<? extends R, ? super T> operator) { this.parent = parent; this.operator = operator; } @Override public void call(Subscriber<? super R> o) { try { Subscriber<? super T> st = hook.onLift(operator).call(o); try { // new Subscriber created and being subscribed with so 'onStart' it st.onStart(); parent.call(st); } catch (Throwable e) { // localized capture of errors rather than it skipping all operators // and ending up in the try/catch of the subscribe method which then // prevents onErrorResumeNext and other similar approaches to error handling Exceptions.throwIfFatal(e); st.onError(e); } } catch (Throwable e) { Exceptions.throwIfFatal(e); // if the lift function failed all we can do is pass the error to the final Subscriber // as we don't have the operator available to us o.onError(e); } }}
首先先说明,这个类很重要。这里parent就是On1,operator就是OperatorMap。很好理解,我们先定义的On1再定义的Func1,所以叫On1为parent,好记。这里我们把生成的OnSubscribeList类简称为lift。多啰嗦下,lift的的parent是On1,operator是OperatorMap(存着Func1),记住,调用是在.subscribe方法中,这此之前,这些call方法都还不会运行。那什么时候运行呢。别急,让我们来看看subscribe方法。上面最后又生成了一个Observable。我们叫它Ob2。
protected Observable(OnSubscribe<T> f) { this.onSubscribe = f;}
可知,它把lift作为参数存入了Ob2的onSubscribe中。所以Ob2.onSubscribe=lift。好终于到最后的subscribe了,来看看它的源码:
public final Subscription subscribe(final Action1<? super T> onNext) { if (onNext == null) { throw new IllegalArgumentException("onNext can not be null"); } Action1<Throwable> onError = InternalObservableUtils.ERROR_NOT_IMPLEMENTED; Action0 onCompleted = Actions.empty(); return subscribe(new ActionSubscriber<T>(onNext, onError, onCompleted));}
简单,主要是生成了ActionSubscriber。这里OnNext就是我们定义的Action1,好,把Action1作为参数传到ActionSubscriber生成一个Subscriber,我们把生成的这个Subscriber称为Sub1。看下它的代码:
public final class ActionSubscriber<T> extends Subscriber<T> { final Action1<? super T> onNext; final Action1<Throwable> onError; final Action0 onCompleted; public ActionSubscriber(Action1<? super T> onNext, Action1<Throwable> onError, Action0 onCompleted) { this.onNext = onNext; this.onError = onError; this.onCompleted = onCompleted; } @Override public void onNext(T t) { onNext.call(t); } @Override public void onError(Throwable e) { onError.call(e); } @Override public void onCompleted() { onCompleted.call(); }}
没啥,但从这我们就知道,如果调用sub1的onNext,就是调用Action1的call方法。继续跟subscribe方法,它把sub1作为参数传了进去。
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 significant depth to already 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); // in case the subscriber can't listen to exceptions anymore if (subscriber.isUnsubscribed()) { RxJavaPluginUtils.handleException(hook.onSubscribeError(e)); } else { // if an unhandled error occurs executing the onSubscribe we will propagate it try { subscriber.onError(hook.onSubscribeError(e)); } catch (Throwable e2) { Exceptions.throwIfFatal(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 OnErrorFailedException("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(); }}
这里的subscriber就是上面的sub1。主要调用onSubscribeStart,它的两个参数,observable其实就是Ob2,Ob2.onSubscribe当然知道是谁啦,就是lift。看看这个方法的实现:
public <T> OnSubscribe<T> onSubscribeStart(Observable<? extends T> observableInstance, final OnSubscribe<T> onSubscribe) { // pass through by default return onSubscribe;}
OK,返回onSubscribe。那就是lift,则上面的代码等价于:
lift.call(sub1)
看OnSubscribeLift的call方法。
@Overridepublic void call(Subscriber<? super R> o) { try { Subscriber<? super T> st = hook.onLift(operator).call(o); try { // new Subscriber created and being subscribed with so 'onStart' it st.onStart(); parent.call(st); } catch (Throwable e) { // localized capture of errors rather than it skipping all operators // and ending up in the try/catch of the subscribe method which then // prevents onErrorResumeNext and other similar approaches to error handling Exceptions.throwIfFatal(e); st.onError(e); } } catch (Throwable e) { Exceptions.throwIfFatal(e); // if the lift function failed all we can do is pass the error to the final Subscriber // as we don't have the operator available to us o.onError(e); }}
o是sub1,list中的operator是operatorMap,parent是On1,这个我们上面己经分析过,忘记的可以回去看看。看下hook.OnLift方法:
public <T, R> Operator<? extends R, ? super T> onLift(final Operator<? extends R, ? super T> lift) { return lift;}
lift是传进来的operatorMap,onLift返回lift。所以相当于是调用了operatorMap.call(sub1),看它OperatorMap中的call方法:
public Subscriber<? super T> call(final Subscriber<? super R> o) { MapSubscriber<T, R> parent = new MapSubscriber<T, R>(o, transformer); o.add(parent); return parent;}
这里的transformer就是Func1,创建了一个MapSubscriber,这里我们叫它Sub2。o是Sub1。看下MapSubscriber的代码:
static final class MapSubscriber<T, R> extends Subscriber<T> { final Subscriber<? super R> actual; final Func1<? super T, ? extends R> mapper; boolean done; public MapSubscriber(Subscriber<? super R> actual, Func1<? super T, ? extends R> mapper) { this.actual = actual; this.mapper = mapper; } @Override public void onNext(T t) { R result; try { result = mapper.call(t); } catch (Throwable ex) { Exceptions.throwIfFatal(ex); unsubscribe(); onError(OnErrorThrowable.addValueAsLastCause(ex, t)); return; } actual.onNext(result); } @Override public void onError(Throwable e) { if (done) { RxJavaPluginUtils.handleException(e); return; } done = true; actual.onError(e); } @Override public void onCompleted() { if (done) { return; } actual.onCompleted(); } @Override public void setProducer(Producer p) { actual.setProducer(p); }}
Ok,那么actual就是Sub1,mapper就是transformer,其实就是Func1,记住啦。回到OnSubscribeLift中的代码,接下来调用以下代码:
parent.call(st);
好,st就是我们创建的Sub2。parent就是On1。那么调用On1.call方法就回到了我们自定义的方法中:
public void call(Subscriber<? super Integer> subscriber) { Log.e("rxjava","observable call"); subscriber.onNext(1); }
在例子中,我们调用的就是Sub2的onNext方法。看下Sub2的onNext方法,在MapSubscriber中:
@Overridepublic void onNext(T t) { R result; try { result = mapper.call(t); } catch (Throwable ex) { Exceptions.throwIfFatal(ex); unsubscribe(); onError(OnErrorThrowable.addValueAsLastCause(ex, t)); return; } actual.onNext(result);}
oK,那result就为“===>1”。最后调用actual的onNext方法。其实就是Sub1的onNext方法,这在上面己经说过,调用的就是Action的call,如下:
@Override public void onNext(T t) { onNext.call(t); }
那最后就回到我们定义的Action的call方法了,t为“===>1”。
new Action1<String>() { @Override public void call(String s) { } }
这样调用就结束了。
总结
上面流程看起来有点复杂,先看下面这张图:
个人把它分为三个步骤。
- 从左边开始,从上到下每一次lift都会新建一个Observable,一个OnSubscribe和Operator从上面分析过程中,lift就是OnSubscribe。而opeatorMap就是这里的Operator。在没有调用subscribe方法之前,它们的call是不会调用的。直到调用到subscribe。
- 从下到上,创建Suscriber,每创建一个就调用OnSubscribe的call方法,接着调用Operator的Call方法,层层往上。之所以能层层往上是因为Suscriber中有parent变量。直到调用On1中的方法。
- 第三阶段,从上到下调用各个Subscriber,如这个例子就是先On1再Func1再Action1。从而完成整个过程 。
篇幅关系,没有分析RxJava的线程切换流程。但这里说明下,线程切换跟上文分析的一样,同样是用Lift来切换。被观察者的创建的对象OperatorSubscribeOn是一个OnSubscribe,而观察者创建的OperatorObserveOn对象是一个Operator。所以它们同样是执行上面的那些流程。之前subscribeOn只有一次有效,是因为上面第二阶段一直往上但又没执行,直到第一个声明subscribeOn之后再开始调用On1,所以明显只有一个有效。而之所以observeOn每个都有它之上的代码有效,是因为第三阶段往下执行,每执行完Subscriber如遇observeOn就会线程切换。所以是每次有效。
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