RxJava2 源码浅析
ReactiveX
历史：
ReactiveX是Reactive Extensions的缩写，一般简写为Rx，最初是LINQ的一个扩展，由微软的架构师Erik Meijer领导的团队开发，在2012年11月开源，Rx是一个编程模型，目标是提供一致的编程接口，帮助开发者更方便的处理异步数据流，Rx库支持.NET、JavaScript和C++，Rx近几年越来越流行了，现在已经支持几乎全部的流行编程语言了，Rx的大部分语言库由ReactiveX这个组织负责维护，比较流行的有RxJava/RxJS/Rx.NET，社区网站是 reactivex.io
定义：
ReactiveX.io给的定义是，Rx是一个使用可观察数据流进行异步编程的编程接口，ReactiveX结合了观察者模式、迭代器模式和函数式编程的精华。
ReactiveX不仅仅是一个编程接口，它是一种编程思想的突破，它影响了许多其它的程序库和框架以及编程语言。
这就是数据流？
RxJava2定义
a library for composing asynchronous and event-based programs by using observable sequences.(一个在 Java VM 上使用可观测的序列来组成异步的、基于事件的程序的库）
大致流程
上代码：
Observable.
create(new ObservableOnSubscribe<String>() {
@Override
public void subscribe(ObservableEmitter<String> emitter) throws Exception {
emitter.onNext("test First");
Log.e("TAG", "subScribe test First");
emitter.onNext("test Second");
Log.e("TAG", "subScribe test Second");
emitter.onComplete();
Log.e("TAG", "subScribe onComplete");
}
}).
subscribe(new Observer<String>() {
@Override
public void onSubscribe(Disposable d) {
Log.e("TAG", "onSubscribe");
mDisposable = d;
}
@Override
public void onNext(String s) {
Log.e("TAG", "onNext");
Log.e("TAG", s);
if (s.equals("test First")) {
mDisposable.dispose();
}
}
@Override
public void onError(Throwable e) {
Log.e("TAG", "onError");
}
@Override
public void onComplete() {
Log.e("TAG", "onComplete");
}
});
刚开始学习RxJava 时，这段代码给我最直观的感受就是，这不就是自己调用自己吗。ObservableEmitter<String> emitter 这个就是下面的subscribe(new Observer<String>()）。对吧，我觉得大家应该都是这样的感受吧...
追踪一下源码：点击create()方法进去看一下：
@SchedulerSupport(SchedulerSupport.NONE)
public static <T> Observable<T> create(ObservableOnSubscribe<T> source) {
ObjectHelper.requireNonNull(source, "source is null");
return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));
}
public static <T> Observable<T> onAssembly(Observable<T> source) {
Function<Observable, Observable> f = onObservableAssembly;
if (f != null) {
return apply(f, source);
}
return source;
}
看了代码onObservableAssembly为null,所以create方法之后这个对象被包装成new ObservableCreate<T>(source)，source是外面传进来的。
关键字：io.reactivex.internal.operators.observable.ObservableCreate
继续看下一个操作符：subscribe()，点进去看一下
@SchedulerSupport(SchedulerSupport.NONE)
@Override
public final void subscribe(Observer<? super T> observer) {
ObjectHelper.requireNonNull(observer, "observer is null");
try {
observer = RxJavaPlugins.onSubscribe(this, observer);//observer原样返回，没改动
ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");
subscribeActual(observer);//关键点
} catch (NullPointerException e) { // NOPMD
throw e;
} catch (Throwable e) {
Exceptions.throwIfFatal(e);
// can't call onError because no way to know if a Disposable has been set or not
// can't call onSubscribe because the call might have set a Subscription already
RxJavaPlugins.onError(e);
NullPointerException npe = new NullPointerException("Actually not, but can't throw other exceptions due to RS");
npe.initCause(e);
throw npe;
}
}
// RxJavaPlugins.onSubscribe
public static <T> Observer<? super T> onSubscribe(Observable<T> source, Observer<? super T> observer) {
BiFunction<Observable, Observer, Observer> f = onObservableSubscribe; //f 为null
if (f != null) {
return apply(f, source, observer);
}
return observer;
}
上面说过了经过create()方法或这个对象已经是ObservableCreate了，那么最终会调用的就是subscribeActual(observer) 看一下ObservableCreate这个类的代码：
@Override
protected void subscribeActual(Observer<? super T> observer) {
CreateEmitter<T> parent = new CreateEmitter<T>(observer);
observer.onSubscribe(parent);
try {
source.subscribe(parent);
} catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
parent.onError(ex);
}
}
static final class CreateEmitter<T>
extends AtomicReference<Disposable>
implements ObservableEmitter<T>, Disposable {
CreateEmitter(Observer<? super T> observer) {
this.observer = observer;
}
@Override
public void onNext(T t) {
if (t == null) {
onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
return;
}
if (!isDisposed()) {
observer.onNext(t);
}
}
}
看一下，这里会把observer包装成一个CreateEmitter对象，然后source是Observable.create(new ObservableOnSubscribe<String>())传进来的ObservableOnSubscribe对象。然后会调用observer.onSubscribe(parent);source.subscribe(parent);终于清晰了...可以回答上面的问题了，其实Observer和ObservableEmitter可以看成是一个对象，只是对observer做了个包装...
Scheduler 线程变换(subscribeOn 和 observeOn)
说到线程变换即线程间通信，因为我是学Android，所以第一印象就是Handler，然后就是Future。看了源码后发现RxJava用的是Future，ScheduledExecutorService，Runnable，二AndroidScheduler就是用Handler的，因为需要切换到Android中的UI线程。
subscribeOn(Schedulers.newThread())
点进去看一下：
public final class ObservableSubscribeOn<T> extends AbstractObservableWithUpstream<T, T> {
final Scheduler scheduler;
public ObservableSubscribeOn(ObservableSource<T> source, Scheduler scheduler) {
super(source);
this.scheduler = scheduler;
}
@Override
public void subscribeActual(final Observer<? super T> s) {
final SubscribeOnObserver<T> parent = new SubscribeOnObserver<T>(s);
s.onSubscribe(parent);
parent.setDisposable(scheduler.scheduleDirect(new Runnable() {
@Override
public void run() {
source.subscribe(parent);
}
}));
}
}
传进来的是Schedulers.newThread()，点击Schedulers.newThread() 点进去
发现最终返回的是NewThreadScheduler
关键字：io.reactivex.internal.schedulers.NewThreadScheduler
io.reactivex.internal.schedulers.NewThreadWorker(真正做线程调度的类)
发现有scheduler.scheduleDirect(new Runnable()）点击进去，最终调用
public Disposable scheduleDirect(Runnable run, long delay, TimeUnit unit) {
final Worker w = createWorker();
final Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
w.schedule(new Runnable() {
@Override
public void run() {
try {
decoratedRun.run();
} finally {
w.dispose();
}
}
}, delay, unit);
return w;
}
最终还是w.schedule(new Runnable()),w就是NewThreadWorker，找到这个类看一下schedule方法,最终会调用：
public ScheduledRunnable scheduleActual(final Runnable run, long delayTime, TimeUnit unit, DisposableContainer parent) {
Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
ScheduledRunnable sr = new ScheduledRunnable(decoratedRun, parent);
if (parent != null) {
if (!parent.add(sr)) {
return sr;
}
}
Future<?> f;
try {
if (delayTime <= 0) {
f = executor.submit((Callable<Object>)sr);
} else {
f = executor.schedule((Callable<Object>)sr, delayTime, unit);
}
sr.setFuture(f);
} catch (RejectedExecutionException ex) {
parent.remove(sr);
RxJavaPlugins.onError(ex);
}
return sr;
}
看到future和executor了，这里就是线程切换
observeOn(AndroidSchedulers.mainThread())
ObservableObserveOn 最终调用的是HandlerScheduler和HandlerWorker
HandlerWorker:
@Override
public Disposable schedule(Runnable run, long delay, TimeUnit unit) {
if (run == null) throw new NullPointerException("run == null");
if (delay < 0) throw new IllegalArgumentException("delay < 0: " + delay);
if (unit == null) throw new NullPointerException("unit == null");
if (disposed) {
return Disposables.disposed();
}
run = RxJavaPlugins.onSchedule(run);
ScheduledRunnable scheduled = new ScheduledRunnable(handler, run);
Message message = Message.obtain(handler, scheduled);
message.obj = this; // Used as token for batch disposal of this worker's runnables.
handler.sendMessageDelayed(message, unit.toMillis(delay));
// Re-check disposed state for removing in case we were racing a call to dispose().
if (disposed) {
handler.removeCallbacks(scheduled);
return Disposables.disposed();
}
return scheduled;
}
发现是用Handler来做线程切换，Handler管理的Looper是Looper.getMainLooper()，所以把消息发送到了主线程。
