Rxjava2的原理:一步一步的看得懂Rxjava源码
1 Rxjava2最简单使用方式拆解
Observable p=Observable.create(new ObservableOnSubscribe() {
@Override
public void subscribe(ObservableEmitter e) throws Exception {
e.onNext("hello world");
e.onComplete();
}
});public static <T> Observable<T> create(ObservableOnSubscribe<T> source) {
ObjectHelper.requireNonNull(source, "source is null");
return RxJavaPlugins.onAssembly(new ObservableCreate<T>(source));
}调用create方法之后实际上返回了一个ObservableCreate对象.继承了Observable,是一个被观察者对象.
p.subscribe(new Observer() {
@Override
public void onSubscribe(Disposable d) {
}
@Override
public void onNext(Object value) {
}
@Override
public void onError(Throwable e) {
}
@Override
public void onComplete() {
}
});我们看下subscribe方法.
public final void subscribe(Observer<? super T> observer) {
...
subscribeActual(observer);
...
}其他代码都删掉了,剩下最核心的 subscribeActual(observer),这个observer就是我们创建的匿名内部类对象.subscribeActual()方法是个抽象方法,我们看下ObservableCreate中是怎么实现的.
public ObservableCreate(ObservableOnSubscribe<T> source) {
this.source = source;
}
@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);
}
}CreateEmitter发射器,在这里我们调用了 observer.onSubscribe(parent)也就是我们创建的匿名observer类的onSubscribe方法.
source.subscribe(parent)最重要的方法可能没有之一,观察者和被观察者顺利会师,事件开始执行,
@Override
public void subscribe(ObservableEmitter e) throws Exception {//这里的ObservableEmitter就是parent,也就是CreateEmitter发射器对象
e.onNext("hello world");
e.onComplete();
}接下来看看CreateEmitter的onNext和onComplete方法.
@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);
}
}我们看到在发射器的onNext方法中,啥也没做,就是当了个二传手,调用了我们观察者的onNext方法.
@Override
public void onComplete() {
if (!isDisposed()) {
try {
observer.onComplete();
} finally {
dispose();
}
}
}onComplete方法中也就是调用了观察者的onComplete方法.
我们来缕缕这个过程
1 create方法传返回了一个对象是ObservableCreate,ObservableCreate的构造方法中有一个ObservableOnSubscribe对象,也就是我们使用create时候创建的匿名内部类对象.
2 p.subscribe(o)实际上调用了ObservableCreate的subscribeActual方法
3 subscribeActual中首先调用了 observer的onSubscribe方法,紧接着调用了source.subscribe(parent)也就是ObservableOnSubscribe的subscribe方法,事件开始执行
4 subscribe方法中调用CreateEmitter的onNext方法,这个方法调用了observer的onNext方法,观察者对事件进行反应.
5 subscribe方法中调用CreateEmitter的onComplete方法,这个方法调用了observer的onComplete方法,整个流程结束.
2 MAP操作符是怎么工作的
public final <R> Observable<R> map(Function<? super T, ? extends R> mapper) {
ObjectHelper.requireNonNull(mapper, "mapper is null");
return RxJavaPlugins.onAssembly(new ObservableMap<T, R>(this, mapper));
}map操作符把我们的observable对象变化成了具体的ObservableMap,参数是我们之前创建好的observable和mapper function
public ObservableMap(ObservableSource<T> source, Function<? super T, ? extends U> function) {
super(source);
this.function = function;
}
@Override
public void subscribeActual(Observer<? super U> t) {
source.subscribe(new MapObserver<T, U>(t, function));
}注意注意:这里形成了一个新的订阅关系
这里的source是我们create创建的observable,要不然会懵,创建ObservableMap时候我们传进来的this是我们生成的observable.
到这里我们会重新调用onSubscribe() subscribeActual(),这里就回到了我们最简单模式时候的调用步骤.不同的是我们真正的调用observer的方法实在MapObserver对应的方法中.
具体流程是发射器调用onNext方法-->MapObserver的onNext方法-->再到我们定义的observer的onNext方法
@Override
public void onNext(T t) {
if (done) {
return;
}
if (sourceMode != NONE) {
actual.onNext(null);
return;
}
U v;
try {
//调用mapper改变数据
**v = ObjectHelper.requireNonNull(mapper.apply(t), "The mapper function returned a null value.");**
} catch (Throwable ex) {
fail(ex);
return;
}
//actual我们定义的observer
actual.onNext(v);
}3 进阶flatMap
@SchedulerSupport(SchedulerSupport.NONE)
public final <R> Observable<R> flatMap(Function<? super T, ? extends ObservableSource<? extends R>> mapper,
boolean delayErrors, int maxConcurrency, int bufferSize) {
...
return RxJavaPlugins.onAssembly(new ObservableFlatMap<T, R>(this, mapper, delayErrors, maxConcurrency, bufferSize));看看ObservableFlatMap代码
public ObservableFlatMap(ObservableSource<T> source,
Function<? super T, ? extends ObservableSource<? extends U>> mapper,
boolean delayErrors, int maxConcurrency, int bufferSize) {
super(source);
this.mapper = mapper;
this.delayErrors = delayErrors;
this.maxConcurrency = maxConcurrency;
this.bufferSize = bufferSize;
}
@Override
public void subscribeActual(Observer<? super U> t) {
if (ObservableScalarXMap.tryScalarXMapSubscribe(source, t, mapper)) {
return;
}
source.subscribe(new MergeObserver<T, U>(t, mapper, delayErrors, maxConcurrency, bufferSize));
}是不是和MAP超级像,我们这几看MergeObserver onNext做了什么
@Override
public void onNext(T t) {
...
p = ObjectHelper.requireNonNull(mapper.apply(t), "The mapper returned a null ObservableSource");
...
subscribeInner(p);
}
@SuppressWarnings("unchecked")
void subscribeInner(ObservableSource<? extends U> p) {
for (;;) {
if (p instanceof Callable) {
} else {
InnerObserver<T, U> inner = new InnerObserver<T, U>(this, uniqueId++);
addInner(inner);
p.subscribe(inner);
break;
}
}
}省略了很多代码,我们看主要逻辑,获取到flatMap生成的observableSource,然后 p.subscribe(inner);注意这里的P不是observable
看innerObserver的onNext做了什么
//这里的onNext事件由 p.subscribe(inner)触发
@Override
public void onNext(U t) {
if (fusionMode == QueueDisposable.NONE) {
parent.tryEmit(t, this);
} else {
parent.drain();
}
}
void tryEmit(U value, InnerObserver<T, U> inner) {
if (get() == 0 && compareAndSet(0, 1)) {
actual.onNext(value);
if (decrementAndGet() == 0) {
return;
}
} else {
SimpleQueue<U> q = inner.queue;
if (q == null) {
q = new SpscLinkedArrayQueue<U>(bufferSize);
inner.queue = q;
}
q.offer(value);
if (getAndIncrement() != 0) {
return;
}
}
drainLoop();
}在这里我们终于看到我们定义的observer接收到了onNext事件
4 总结
Observable ObservableSource要分清楚,他们都有一个方法叫subscribe()
Observer Emitter分清楚,他们有共同的方法onNext() onError() onComplete()
否则话很容易晕头转向.
文章如有表述有错误,请指出,谢谢.