RXJava之链式调用（一）
阿里P7移动互联网架构师进阶视频（每日更新中）免费学习请点击：https://space.bilibili.com/474380680
Rxjava中链式调用怎么实现的？
Observable.just("a") //Observable1
.map(new Func1<String, String>() { //Observable2
@Override
public String call(String s) {
System.out.print(Thread.currentThread().getName() + ":first--" + s +"\n");
return s + s;
}
})
.subscribe(new Subscriber<String>() { //代码⑥ Subscriber
@Override
public void onCompleted() {
System.out.print(Thread.currentThread().getName()+"\n");
System.out.print("completed"+"\n");
}
@Override
public void onError(Throwable e) {
System.out.print("error");
}
@Override
public void onNext(String s) {
System.out.println(s);
}
});
}
先说说自己的理解，若把整个链条看成一个整体对象，那么just创建被观察者对象，而subscribe()里的Subscriber作为观察者；若每一步都分开看，just()和subscribe()中间的操作符即是观察者，又是被观察者。
Observable中每个操作符基本都会创建出一个新的Observable；因此可以解理成后一级的操作符去观察前一个Observable对象；以上例来说，.subscribe的Subscriber所观察的对象就是.map返回的Observable2，而.map的Subscriber所观察的对象就是 Observable.just("a")得到的对象Observable1；
下面介绍实现代码，整个链式调用真正开始的地方是.subscribe()，我们就从这里开始。省略掉一些代码，只看关键部分如下：
private static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) {
...
try {
// allow the hook to intercept and/or decorate
hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber); //代码①
return hook.onSubscribeReturn(subscriber);
}
...
}
hook.onSubscribeStart(observable, observable.onSubscribe)得到的对象就是observable.onSubscribe，而此处的observable明显就是this，也就是上例中的observable2对象，即把subscriber传入到了observable2里面以供其调用。
再跟着代码进入observable2（.map操作符）的实现。其主要实现是lift和OperatorMap。如下：
public final <R> Observable<R> map(Func1<? super T, ? extends R> func) {
return lift(new OperatorMap<T, R>(func));
}
lift和OperatorMap各自干了什么事情呢？先看OperatorMap，Func1也作为构造参数传入。关键代码：
@Override
public Subscriber<? super T> call(final Subscriber<? super R> o) { //代码②
return new Subscriber<T>(o) {
@Override
public void onCompleted() {
o.onCompleted();
}
@Override
public void onError(Throwable e) {
o.onError(e);
}
@Override
public void onNext(T t) {
try {
o.onNext(transformer.call(t));
} catch (Throwable e) {
Exceptions.throwOrReport(e, this, t);
}
}
};
}
这里new出了一个观察者对象Subscriber，它实现了什么功能通过 o.onNext(transformer.call(t));即将例子中的Func1代码执行后将结果传入到下一层。即这里运行了Func1的代码。
再看lift()操作符，看其返回值也就是我们定义的observable2对象。因此subscribe里的"代码①"的call即是此处observable2里OnSubscribe的call方法；再看call方法，“代码④”部分则是调用到了observable1对象里OnSubscribe的call方法，而“代码③”将Func1操作动作转变为Subscriber，通过call(o)完成对下一级Subscriber的引用。
public final <R> Observable<R> lift(final Operator<? extends R, ? super T> operator) {
return new Observable<R>(new OnSubscribe<R>() {
@Override
public void call(Subscriber<? super R> o) {
try {
Subscriber<? super T> st = hook.onLift(operator).call(o); //代码③
try {
// new Subscriber created and being subscribed with so 'onStart' it
st.onStart();
onSubscribe.call(st); //代码④
} 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);
}
}
});
}
到这里“代码④”执行，即到了observable1对象，也就是例子中 Observable.just("a")所得到对象的OnSubscribe的call()方法，如下：
public final static <T> Observable<T> just(final T value) {
return ScalarSynchronousObservable.create(value);
}
ScalarSynchronousObservable类代码如下：
public static final <T> ScalarSynchronousObservable<T> create(T t) {
return new ScalarSynchronousObservable<T>(t);
}
protected ScalarSynchronousObservable(final T t) {
super(new OnSubscribe<T>() {
@Override
public void call(Subscriber<? super T> s) {
/*
* We don't check isUnsubscribed as it is a significant performance impact in the fast-path use cases.
* See PerfBaseline tests and https://github.com/ReactiveX/RxJava/issues/1383 for more information.
* The assumption here is that when asking for a single item we should emit it and not concern ourselves with
* being unsubscribed already. If the Subscriber unsubscribes at 0, they shouldn't have subscribed, or it will
* filter it out (such as take(0)). This prevents us from paying the price on every subscription.
*/
s.onNext(t); //代码⑤
s.onCompleted();
}
});
this.t = t;
}
其中"代码⑤"是关键点，t即是我们just传入的"a",s则是代码④传入的st，它其实是observable2的Subscriber(观察者)，相当于observable1持有observable2的引用。通过 s.onNext(t)，完成了observable1向下一层的observable2的回调，也就是Func1对象所在的Subscriber(OperatorMap)，再通过 o.onNext(transformer.call(t));回到例子中“代码⑥”，至此，整个调用链完成。
上面的分析比较混乱，重新梳理代码执行流程 ：
1、subscribe里，hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber); //代码①
2、map里，通过lift(）将Func1操作符生成Subserber，Subscriber<? super T> st = hook.onLift(operator).call(o); //代码③
onSubscribe.call(st); //代码④
3、just里create()， s.onNext(t); //代码⑤
4、map里， OperatorMap里对象， o.onNext(transformer.call(t));
5、subscribe 的Subscriber（）;
Observable的所有链式调用，知道两个其两个关键点即可梳理清楚整个数据流传递原理；
Observable.onSubscribe对象，完成以call方法来向上一层传递；
Subserber向下一层的Subserber调用；
原文链接：https://www.jianshu.com/p/b5ca80311746
阿里P7移动互联网架构师进阶视频（每日更新中）免费学习请点击：https://space.bilibili.com/474380680
