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Rxjava中鏈式調用怎么實現的？
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>() { //代碼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);
}
});
}
先說說自己的理解，若把整個鏈條看成一個整體對象，那么just創建被觀察者對象，而subscribe()里的Subscriber作為觀察者；若每一步都分開看，just()和subscribe()中間的操作符即是觀察者，又是被觀察者。
Observable中每個操作符基本都會創建出一個新的Observable；因此可以解理成后一級的操作符去觀察前一個Observable對象；以上例來說，.subscribe的Subscriber所觀察的對象就是.map返回的Observable2，而.map的Subscriber所觀察的對象就是 Observable.just("a")得到的對象Observable1；
下面介紹實現代碼，整個鏈式調用真正開始的地方是.subscribe()，我們就從這里開始。省略掉一些代碼，只看關鍵部分如下：
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); //代碼①
return hook.onSubscribeReturn(subscriber);
}
...
}
hook.onSubscribeStart(observable, observable.onSubscribe)得到的對象就是observable.onSubscribe，而此處的observable明顯就是this，也就是上例中的observable2對象，即把subscriber傳入到了observable2里面以供其調用。
再跟着代碼進入observable2（.map操作符）的實現。其主要實現是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也作為構造參數傳入。關鍵代碼：
@Override
public Subscriber<? super T> call(final Subscriber<? super R> o) { //代碼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);
}
}
};
}
這里new出了一個觀察者對象Subscriber，它實現了什么功能通過 o.onNext(transformer.call(t));即將例子中的Func1代碼執行后將結果傳入到下一層。即這里運行了Func1的代碼。
再看lift()操作符，看其返回值也就是我們定義的observable2對象。因此subscribe里的"代碼①"的call即是此處observable2里OnSubscribe的call方法；再看call方法，“代碼④”部分則是調用到了observable1對象里OnSubscribe的call方法，而“代碼③”將Func1操作動作轉變為Subscriber，通過call(o)完成對下一級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); //代碼③
try {
// new Subscriber created and being subscribed with so 'onStart' it
st.onStart();
onSubscribe.call(st); //代碼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);
}
}
});
}
到這里“代碼④”執行，即到了observable1對象，也就是例子中 Observable.just("a")所得到對象的OnSubscribe的call()方法，如下：
public final static <T> Observable<T> just(final T value) {
return ScalarSynchronousObservable.create(value);
}
ScalarSynchronousObservable類代碼如下：
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); //代碼⑤
s.onCompleted();
}
});
this.t = t;
}
其中"代碼⑤"是關鍵點，t即是我們just傳入的"a",s則是代碼④傳入的st，它其實是observable2的Subscriber(觀察者)，相當於observable1持有observable2的引用。通過 s.onNext(t)，完成了observable1向下一層的observable2的回調，也就是Func1對象所在的Subscriber(OperatorMap)，再通過 o.onNext(transformer.call(t));回到例子中“代碼⑥”，至此，整個調用鏈完成。
上面的分析比較混亂，重新梳理代碼執行流程 ：
1、subscribe里，hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber); //代碼①
2、map里，通過lift(）將Func1操作符生成Subserber，Subscriber<? super T> st = hook.onLift(operator).call(o); //代碼③
onSubscribe.call(st); //代碼④
3、just里create()， s.onNext(t); //代碼⑤
4、map里， OperatorMap里對象， o.onNext(transformer.call(t));
5、subscribe 的Subscriber（）;
Observable的所有鏈式調用，知道兩個其兩個關鍵點即可梳理清楚整個數據流傳遞原理；
Observable.onSubscribe對象，完成以call方法來向上一層傳遞；
Subserber向下一層的Subserber調用；
原文鏈接：https://www.jianshu.com/p/b5ca80311746
阿里P7移動互聯網架構師進階視頻（每日更新中）免費學習請點擊：https://space.bilibili.com/474380680