RxJava是一个在Java虚拟机上的响应式扩展，通过使用可观察的序列将异步和基于事件的程序组合起来的一个库。
它扩展了观察者模式来支持数据/事件序列，并且添加了操作符，这些操作符允许你声明性地组合序列，同时抽象出要关注的问题：比如低级线程、同步、线程安全和并发数据结构等。
RxJava相信大家都非常了解吧，今天分享一下RxJava的消息发送和线程源码的分析。最后并分享一个相关demo,让大家更加熟悉我们天天都在用的框架。
消息订阅发送
首先让我们看看消息订阅发送最基本的代码组成：
Observable observable = Observable.create(new ObservableOnSubscribe<String>() {
@Override
public void subscribe(ObservableEmitter<String> emitter) throws Exception {
emitter.onNext("Jack1");
emitter.onNext("Jack2");
emitter.onNext("Jack3");
emitter.onComplete();
}
});
Observer<String> observer = new Observer<String>() {
@Override
public void onSubscribe(Disposable d) {
Log.d(TAG, "onSubscribe");
}
@Override
public void onNext(String s) {
Log.d(TAG, "onNext : " + s);
}
@Override
public void onError(Throwable e) {
Log.d(TAG, "onError : " + e.toString());
}
@Override
public void onComplete() {
Log.d(TAG, "onComplete");
}
};
observable.subscribe(observer);
代码很简单,observable为被观察者,observer为观察者,然后通过observable.subscribe(observer),把观察者和被观察者关联起来。被观察者发送消息(emitter.onNext("内容")),观察者就可以在onNext()方法里回调出来。
我们先来看Observable，创建是用Observable.create()方法进行创建，源码如下：
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> T requireNonNull(T object, String message) {
if (object == null) {
throw new NullPointerException(message);
}
return object;
}
public static <T> Observable<T> onAssembly(@NonNull Observable<T> source) {
Function<? super Observable, ? extends Observable> f = onObservableAssembly;
if (f != null) {
return apply(f, source);
}
return source;
}
可以看出，create()方法里最主要的还是创建用ObservableOnSubscribe传入创建了一个ObservableCreate对象并且保存而已。
public final class ObservableCreate<T> extends Observable<T> {
final ObservableOnSubscribe<T> source;
public ObservableCreate(ObservableOnSubscribe<T> source) {
this.source = source;
}
}
接着是创建Observer，这比较简单只是单纯创建一个接口对象而已
public interface Observer<T> {
void onSubscribe(@NonNull Disposable d);
void onNext(@NonNull T t);
void onError(@NonNull Throwable e);
void onComplete();
}
订阅发送消息
observable.subscribe(observer)的subscribe方法如下：
public final void subscribe(Observer<? super T> observer) {
ObjectHelper.requireNonNull(observer, "observer is null");
try {
observer = RxJavaPlugins.onSubscribe(this, observer);
ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");
subscribeActual(observer);
} catch (NullPointerException e) { // NOPMD
throw e;
} catch (Throwable e) {
Exceptions.throwIfFatal(e);
RxJavaPlugins.onError(e);
NullPointerException npe = new NullPointerException("Actually not, but can't throw other exceptions due to RS");
npe.initCause(e);
throw npe;
}
}
//ObjectHelper.requireNonNull()方法
public static <T> T requireNonNull(T object, String message) {
if (object == null) {
throw new NullPointerException(message);
}
return object;
}
//RxJavaPlugins.onSubscribe()方法
public static <T> Observer<? super T> onSubscribe(@NonNull Observable<T> source, @NonNull Observer<? super T> observer) {
BiFunction<? super Observable, ? super Observer, ? extends Observer> f = onObservableSubscribe;
if (f != null) {
return apply(f, source, observer);
}
return observer;
}
从上面源码可以看出requireNonNull()只是做非空判断而已，而RxJavaPlugins.onSubscribe()也只是返回最终的观察者而已。所以关键代码是抽象方法subscribeActual(observer);那么subscribeActual对应哪个代码段呢？
还记得Observable.create()创建的ObservableCreate类吗，这就是subscribeActual()具体实现类，源码如下：
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);
}
}
从上面的代码可以看出，首先创建了一个CreateEmitter对象并传入observer，然后回到observer的onSubscribe()方法，而source就是我们之前创建ObservableCreate传入的ObservableOnSubscribe对象。
class CreateEmitter<T> extends AtomicReference<Disposable>
implements ObservableEmitter<T>, Disposable {
}
而CreateEmitter又继承ObservableEmitter接口，又回调ObservableOnSubscribe的subscribe方法，对应着我们的：
Observable observable = Observable.create(new ObservableOnSubscribe<String>() {
@Override
public void subscribe(ObservableEmitter<String> emitter) throws Exception {
emitter.onNext("Jack1");
emitter.onNext("Jack2");
emitter.onNext("Jack3");
emitter.onComplete();
}
});
当它发送消息既调用emitter.onNext()方法时，既调用了CreateEmitter的onNext()方法：
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);
}
}
可以看到最终又回调了观察者的onNext()方法，把被观察者的数据传输给了观察者。有人会问
isDisposed()是什么意思，是判断要不要终止传递的，我们看emitter.onComplete()源码：
public void onComplete() {
if (!isDisposed()) {
try {
observer.onComplete();
} finally {
dispose();
}
}
}
public static boolean dispose(AtomicReference<Disposable> field) {
Disposable current = field.get();
Disposable d = DISPOSED;
if (current != d) {
current = field.getAndSet(d);
if (current != d) {
if (current != null) {
current.dispose();
}
return true;
}
}
return false;
}
public static boolean isDisposed(Disposable d) {
return d == DISPOSED;
}
dispose()方法是终止消息传递，也就付了个DISPOSED常量，而isDisposed()方法就是判断这个常量而已。这就是整个消息订阅发送的过程，用的是观察者模式。
线程切换
在上面模板代码的基础上，线程切换只是改变了如下代码：
observable.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(observer);
下面我们对线程切换的源码进行一下分析，分为两部分：subscribeOn()和observeOn()
subscribeOn()
首先是subscribeOn()源码如下：
public final Observable<T> subscribeOn(Scheduler scheduler) {
ObjectHelper.requireNonNull(scheduler, "scheduler is null");
return RxJavaPlugins.onAssembly(new ObservableSubscribeOn<T>(this, scheduler));
}
我们传进去了一个Scheduler类，Scheduler是一个调度类，能够延时或周期性地去执行一个任务。
Scheduler有如下类型：
| 类型 | 使用方式 | 含义 | 使用场景 |
|---|---|---|---|
| IoScheduler | Schedulers.io() | io操作线程 | 读写SD卡文件，查询数据库，访问网络等IO密集型操作 |
| NewThreadScheduler | Schedulers.newThread() | 创建新线程 | 耗时操作等 |
| SingleScheduler | Schedulers.single() | 单例线程 | 只需一个单例线程时 |
| ComputationScheduler | Schedulers.computation() | CPU计算操作线程 | 图片压缩取样、xml,json解析等CPU密集型计算 |
| TrampolineScheduler | Schedulers.trampoline() | 当前线程 | 需要在当前线程立即执行任务时 |
| HandlerScheduler | AndroidSchedulers.mainThread() | Android主线程 | 更新UI等 |
接着就没什么了，只是返回一个ObservableSubscribeOn对象而已。
observeOn()
首先看源码如下：
public final Observable<T> observeOn(Scheduler scheduler) {
return observeOn(scheduler, false, bufferSize());
}
public final Observable<T> observeOn(Scheduler scheduler, boolean delayError, int bufferSize) {
ObjectHelper.requireNonNull(scheduler, "scheduler is null");
ObjectHelper.verifyPositive(bufferSize, "bufferSize");
return RxJavaPlugins.onAssembly(new ObservableObserveOn<T>(this, scheduler, delayError, bufferSize));
}
这里也是没什么，只是最终返回一个ObservableObserveOn对象而已。
接着还是像原来那样调用subscribe()方法进行订阅，看起来好像整体变化不大，就是封装了一些对象而已，不过着恰恰是RxJava源码的精华，当他再次调用subscribeActual()方法时，已经不是之前的ObservableCreate()里subscribeActual方法了，而是最先调用ObservableObserveOn的subscribeActual()方法，对应源码如下：
protected void subscribeActual(Observer<? super T> observer) {
if (scheduler instanceof TrampolineScheduler) {
source.subscribe(observer);
} else {
Scheduler.Worker w = scheduler.createWorker();
source.subscribe(new ObserveOnObserver<T>(observer, w, delayError, bufferSize));
}
}
在这里有两点要讲，一点是ObserveOnObserver是执行观察者的线程，后面还会详解，然后就是source.subscribe，这个source.subscribe调的是ObservableSubscribeOn的subscribe方法，而subscribe方法因为继承的也是Observable，是Observable里的方法，所以和上面的ObservableCreate一样的方法，所以会调用ObservableSubscribeOn里的subscribeActual()方法，对应的代码如下：
public void subscribeActual(final Observer<? super T> s) {
final SubscribeOnObserver<T> parent = new SubscribeOnObserver<T>(s);
s.onSubscribe(parent);
parent.setDisposable(scheduler.scheduleDirect(new SubscribeTask(parent)));
}
上面代码中，首先把ObserveOnObserver返回给来的用SubscribeOnObserver“包装”起来，然后在回调Observer的onSubscribe()，就是对应模板代码的onSubscribe()方法。
接着看SubscribeTask类的源码：
final class SubscribeTask implements Runnable {
private final SubscribeOnObserver<T> parent;
SubscribeTask(SubscribeOnObserver<T> parent) {
this.parent = parent;
}
@Override
public void run() {
source.subscribe(parent);
}
}
其中的source.subscribe(parent)，就是我们执行子线程的回调方法，对应我们模板代码里的被观察者的subscribe()方法。它放在run()方法里，并且继承Runnable,说明这个类主要是线程运行。接着看scheduler.scheduleDirect()方法对应的源码如下：
public Disposable scheduleDirect(@NonNull Runnable run) {
return scheduleDirect(run, 0L, TimeUnit.NANOSECONDS);
}
public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
final Worker w = createWorker();
final Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
DisposeTask task = new DisposeTask(decoratedRun, w);
w.schedule(task, delay, unit);
return task;
}
在这里，createWorker()也是一个抽象方法，调用的是我们的调度类对应的Schedulers类里面的方法，这里是IoScheduler类，
public final class IoScheduler extends Scheduler{
final AtomicReference<CachedWorkerPool> pool;
//省略....
public Worker createWorker() {
return new EventLoopWorker(pool.get());
}
static final class EventLoopWorker extends Scheduler.Worker {
private final CompositeDisposable tasks;
private final CachedWorkerPool pool;
private final ThreadWorker threadWorker;
final AtomicBoolean once = new AtomicBoolean();
EventLoopWorker(CachedWorkerPool pool) {
this.pool = pool;
this.tasks = new CompositeDisposable();
this.threadWorker = pool.get();
}
//省略....
@NonNull
@Override
public Disposable schedule(@NonNull Runnable action, long delayTime, @NonNull TimeUnit unit) {
if (tasks.isDisposed()) {
// don't schedule, we are unsubscribed
return EmptyDisposable.INSTANCE;
}
return threadWorker.scheduleActual(action, delayTime, unit, tasks);
}
}
}
static final class CachedWorkerPool implements Runnable {
//省略....
ThreadWorker get() {
if (allWorkers.isDisposed()) {
return SHUTDOWN_THREAD_WORKER;
}
while (!expiringWorkerQueue.isEmpty()) {
ThreadWorker threadWorker = expiringWorkerQueue.poll();
if (threadWorker != null) {
return threadWorker;
}
}
ThreadWorker w = new ThreadWorker(threadFactory);
allWorkers.add(w);
return w;
}
//省略....
}
这就是IoScheduler的createWorker()的方法，其实最主要的意思就是获取线程池，以便于生成子线程，让SubscribeTask()可以运行。然后直接调用 w.schedule(task, delay, unit)方法让它在线程池里执行。上面中那ThreadWorker的源码如下：
static final class ThreadWorker extends NewThreadWorker {
private long expirationTime;
ThreadWorker(ThreadFactory threadFactory) {
super(threadFactory);
this.expirationTime = 0L;
}
//省略代码....
}
public class NewThreadWorker extends Scheduler.Worker implements Disposable {
private final ScheduledExecutorService executor;
public NewThreadWorker(ThreadFactory threadFactory) {
executor = SchedulerPoolFactory.create(threadFactory);
}
public ScheduledRunnable scheduleActual(final Runnable run, long delayTime, @NonNull TimeUnit unit, @Nullable 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) {
if (parent != null) {
parent.remove(sr);
}
RxJavaPlugins.onError(ex);
}
return sr;
}
}
可以看到，这就调了原始的javaAPI来进行线程池操作。
然后最后一环在子线程调用source.subscribe(parent)方法，然后回调刚开始创建的ObservableCreate的subscribeActual(),既：
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);
}
}
进行消息的订阅绑定。
当我们在调用 emitter.onNext(内容)时，是在io线程里的，那回调的onNext()又是什么时候切换的？那就是前面为了整个流程流畅性没讲的在observeOn()里的ObserveOnObserver是执行观察者的线程的过程。
class ObserveOnObserver<T> extends BasicIntQueueDisposable<T>
implements Observer<T>, Runnable {
//省略代码....
ObserveOnObserver(Observer<? super T> actual, Scheduler.Worker worker, boolean delayError, int bufferSize) {
this.actual = actual;
this.worker = worker;
this.delayError = delayError;
this.bufferSize = bufferSize;
}
@Override
public void onSubscribe(Disposable s) {
if (DisposableHelper.validate(this.s, s)) {
this.s = s;
if (s instanceof QueueDisposable) {
@SuppressWarnings("unchecked")
QueueDisposable<T> qd = (QueueDisposable<T>) s;
int m = qd.requestFusion(QueueDisposable.ANY | QueueDisposable.BOUNDARY);
if (m == QueueDisposable.SYNC) {
sourceMode = m;
queue = qd;
done = true;
actual.onSubscribe(this);
schedule();
return;
}
if (m == QueueDisposable.ASYNC) {
sourceMode = m;
queue = qd;
actual.onSubscribe(this);
return;
}
}
queue = new SpscLinkedArrayQueue<T>(bufferSize);
actual.onSubscribe(this);
}
}
@Override
public void onNext(T t) {
if (done) {
return;
}
if (sourceMode != QueueDisposable.ASYNC) {
queue.offer(t);
}
schedule();
}
void schedule() {
if (getAndIncrement() == 0) {
worker.schedule(this);
}
}
//省略代码....
}
当调用emitter.onNext(内容)方法，会调用上面的onNext()方法，然后在这个方法里会把数据压入一个队列，然后执行worker.schedule(this)方法，work是什么呢，还记得AndroidSchedulers.mainThread()吗，这个对应这个HandlerScheduler这个类，所以createWorker()对应着：
private static final class MainHolder {
static final Scheduler DEFAULT = new HandlerScheduler(new Handler(Looper.getMainLooper()));
}
public Worker createWorker() {
return new HandlerWorker(handler);
}
private static final class HandlerWorker extends Worker {
private final Handler handler;
private volatile boolean disposed;
HandlerWorker(Handler handler) {
this.handler = handler;
}
@Override
public Disposable schedule(Runnable run, long delay, TimeUnit unit) {
if (run == null) throw new NullPointerException("run == null");
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));
if (disposed) {
handler.removeCallbacks(scheduled);
return Disposables.disposed();
}
return scheduled;
}
}
在next()方法里，运用android自带的Handler消息机制，通过把方法包裹在Message里，同通过handler.sendMessageDelayed()发送消息，就会在ui线程里回调Next()方法，从而实现从子线程切换到android主线程的操作。我们在主线程拿到数据就可以进行各种在主线程的操作了。
总结一下：
ObservableCreate 一> ObservableSubscribeOn 一> ObservableObserveOn为初始化顺序
当调用observable.subscribe(observer)时的执行顺序
ObservableObserveOn 一> ObservableSubscribeOn 一> ObservableCreate
当发送消息的执行顺序
ObservableCreate 一> ObservableSubscribeOn 一> ObservableObserveOn
以上就是消息订阅和线程切换的源码的所有讲解了。
为了让你们理解更清楚，我仿照RxJava写了大概的消息订阅和线程切换的最基本代码和基本功能，以帮助你们理解
https://github.com/jack921/RxJava2Demo
