RxJava源码分析
2017-02-18 本文已影响26人
yangweigbh
从创建一个数组的Observable说起：
public static <T> Observable<T> from(T[] array) {
......
return create(new OnSubscribeFromArray<T>(array));
}
public static <T> Observable<T> create(OnSubscribe<T> f) {
return new Observable<T>(RxJavaHooks.onCreate(f)); //RxJavaHooks提供了改写OnSubscribe的钩子，可以认为直接返回参数
}
protected Observable(OnSubscribe<T> f) {
this.onSubscribe = f;
}
OnSubsribeFromArray:
public final class OnSubscribeFromArray<T> implements OnSubscribe<T> {
final T[] array;
public OnSubscribeFromArray(T[] array) {
this.array = array;
}
@Override
public void call(Subscriber<? super T> child) {
child.setProducer(new FromArrayProducer<T>(child, array));
}
当调用Observable的subscribe时，会创建一个ObserverSubscribe包装Observer
public final Subscription subscribe(final Observer<? super T> observer) {
if (observer instanceof Subscriber) {
return subscribe((Subscriber<? super T>)observer);
}
if (observer == null) {
throw new NullPointerException("observer is null");
}
return subscribe(new ObserverSubscriber<T>(observer));
}
public final Subscription subscribe(Subscriber<? super T> subscriber) {
return Observable.subscribe(subscriber, this);
}
static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) {
// 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
observable.onSubscribe.call(subscriber);
return 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()) {
RxJavaHooks.onError(RxJavaHooks.onObservableError(e));
} else {
// if an unhandled error occurs executing the onSubscribe we will propagate it
try {
subscriber.onError(RxJavaHooks.onObservableError(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.
RxJavaHooks.onObservableError(r);
// TODO why aren't we throwing the hook's return value.
throw r; // NOPMD
}
}
return Subscriptions.unsubscribed();
}
}
显示调用subscriber的onStart，然后用SafeSubscriber包装subscriber，SafeSubscriber是为了保证OnError和OnComplete只会执行其中之一，并且执行后不再执行onNext
接着调用OnSubscribeFromArray.call(subscriber)
@Override
public void call(Subscriber<? super T> child) {
child.setProducer(new FromArrayProducer<T>(child, array));
}
static final class FromArrayProducer<T>
extends AtomicLong
implements Producer {
/** */
private static final long serialVersionUID = 3534218984725836979L;
final Subscriber<? super T> child;
final T[] array;
int index;
public FromArrayProducer(Subscriber<? super T> child, T[] array) {
this.child = child;
this.array = array;
}
@Override
public void request(long n) {
if (n < 0) {
throw new IllegalArgumentException("n >= 0 required but it was " + n);
}
if (n == Long.MAX_VALUE) {
if (BackpressureUtils.getAndAddRequest(this, n) == 0) {
fastPath();
}
} else
if (n != 0) {
if (BackpressureUtils.getAndAddRequest(this, n) == 0) {
slowPath(n);
}
}
}
void fastPath() {
final Subscriber<? super T> child = this.child;
for (T t : array) {
if (child.isUnsubscribed()) {
return;
}
child.onNext(t);
}
if (child.isUnsubscribed()) {
return;
}
child.onCompleted();
}
void slowPath(long r) {
final Subscriber<? super T> child = this.child;
final T[] array = this.array;
final int n = array.length;
long e = 0L;
int i = index;
for (;;) {
while (r != 0L && i != n) {
if (child.isUnsubscribed()) {
return;
}
child.onNext(array[i]);
i++;
if (i == n) {
if (!child.isUnsubscribed()) {
child.onCompleted();
}
return;
}
r--;
e--;
}
r = get() + e;
if (r == 0L) {
index = i;
r = addAndGet(e);
if (r == 0L) {
return;
}
e = 0L;
}
}
}
}
Subscriber.setProducer：如果包装了其他subscriber，则转交给其他subscriber，如果没有包装subscriber，没有调用request，会调用p.request(Long.MAX_VALUE)。如果之前调用了request，则调用p.request(n)，n为累积的数量。
public void setProducer(Producer p) {
long toRequest;
boolean passToSubscriber = false;
synchronized (this) {
toRequest = requested;
producer = p;
if (subscriber != null) {
// middle operator ... we pass through unless a request has been made
if (toRequest == NOT_SET) {
// we pass through to the next producer as nothing has been requested
passToSubscriber = true;
}
}
}
// do after releasing lock
if (passToSubscriber) {
subscriber.setProducer(producer);
} else {
// we execute the request with whatever has been requested (or Long.MAX_VALUE)
if (toRequest == NOT_SET) {
producer.request(Long.MAX_VALUE);
} else {
producer.request(toRequest);
}
}
}
FromArrayProducer.request(n)
@Override
public void request(long n) {
if (n < 0) {
throw new IllegalArgumentException("n >= 0 required but it was " + n);
}
if (n == Long.MAX_VALUE) {
if (BackpressureUtils.getAndAddRequest(this, n) == 0) {
fastPath();
}
} else
if (n != 0) {
if (BackpressureUtils.getAndAddRequest(this, n) == 0) {
slowPath(n);
}
}
}
如果n 为 Long.MAX_VALUE，如果之前没有请求过，则调用fastPath
如果n不为0，则说明之前调用了request，则要支持背压，调用slowPath
void fastPath() {
final Subscriber<? super T> child = this.child;
for (T t : array) {
if (child.isUnsubscribed()) {
return;
}
child.onNext(t);
}
if (child.isUnsubscribed()) {
return;
}
child.onCompleted();
}
只要没有unsubscribe，会对数组中每个元素调用Observer的onNext，然后调用Observer.onCompleted()
Scheduler
通过subscribeOn和observeOn可以改变操作符执行的线程
subscribeOn只能调用一次，表示subscribe操作所在的线程，第一次ObserveOn之前所以操作都在subscribeOn所指定的线程
observeOn可以调用多次，调用之后所有的操作都在observeOn指定的线程上进行操作
subscribeOn，创建一个OperatorSubscribeOn将原来的Observable包装起来。
public final Observable<T> subscribeOn(Scheduler scheduler) {
if (this instanceof ScalarSynchronousObservable) {
return ((ScalarSynchronousObservable<T>)this).scalarScheduleOn(scheduler);
}
return create(new OperatorSubscribeOn<T>(this, scheduler));
}
public final class OperatorSubscribeOn<T> implements OnSubscribe<T> {
final Scheduler scheduler;
final Observable<T> source;
public OperatorSubscribeOn(Observable<T> source, Scheduler scheduler) {
this.scheduler = scheduler;
this.source = source;
}
@Override
public void call(final Subscriber<? super T> subscriber) {
final Worker inner = scheduler.createWorker();
subscriber.add(inner);
inner.schedule(new Action0() {
@Override
public void call() {
final Thread t = Thread.currentThread();
Subscriber<T> s = new Subscriber<T>(subscriber) {
@Override
public void onNext(T t) {
subscriber.onNext(t);
}
@Override
public void onError(Throwable e) {
try {
subscriber.onError(e);
} finally {
inner.unsubscribe();
}
}
@Override
public void onCompleted() {
try {
subscriber.onCompleted();
} finally {
inner.unsubscribe();
}
}
@Override
public void setProducer(final Producer p) {
subscriber.setProducer(new Producer() {
@Override
public void request(final long n) {
if (t == Thread.currentThread()) {
p.request(n);
} else {
inner.schedule(new Action0() {
@Override
public void call() {
p.request(n);
}
});
}
}
});
}
};
source.unsafeSubscribe(s);
}
});
}
}
从scheduler中create一个worker，调用worker的schedule执行包装Observable的subscribe方法。而且保证Producer的request方法在worker线程上调用。
RxJava中的Scheduler有
- NewThreadScheduler，NewThreadWorker（包装了一个ScheduledExecutorService）
- ComputationScheduler：EventLoopsScheduler，根据cpu数量确定worker的数量，EventLoopWorker包装了PoolWorker（实际是NewThreadWorker）
- IOScheduler: CachedThreadScheduler,ThreadWorker: unsubscribe的ThreadWorker可以重用，长期不用的会被清理。
Map操作符
public final <R> Observable<R> map(Func1<? super T, ? extends R> func) {
return create(new OnSubscribeMap<T, R>(this, func));
}
创建一个OnSubscribeMap包装map函数和原Observable。
public final class OnSubscribeMap<T, R> implements OnSubscribe<R> {
final Observable<T> source;
final Func1<? super T, ? extends R> transformer;
public OnSubscribeMap(Observable<T> source, Func1<? super T, ? extends R> transformer) {
this.source = source;
this.transformer = transformer;
}
@Override
public void call(final Subscriber<? super R> o) {
MapSubscriber<T, R> parent = new MapSubscriber<T, R>(o, transformer);
o.add(parent);
source.unsafeSubscribe(parent);
}
subscribe时会生成一个MapSubscriber包装原来subscriber。
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) {
RxJavaHooks.onError(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);
}
}
当MapSubscriber的onNext方法被调用时，先用map函数转换一下，然后传给包装subscriber的onNext
