###create()+subscribe()： 这是Rx经常写的代码
Observable<String> observable = Observable.create(new Observable.OnSubscribe<String>() { @Override public void call(Subscriber<? super String> subscriber) { subscriber.onNext("content"); subscriber.onCompleted(); } }); Subscriber<String> subscriber = new Subscriber<String>() { @Override public void onCompleted() { Log.e("onCompleted", "onCompleted"); } @Override public void onError(Throwable e) { } @Override public void onNext(String s) { Log.e("onNext", s); } }; observable.subscribe(subscriber);1.这段代码返回一个Observable对象，hook.onCreate(f)作为参数传递过去。
public static <T> Observable<T> create(OnSubscribe<T> f) { return new Observable<T>(hook.onCreate(f)); }2.看代码可知，其实返回的就是传入的参数f；
public <T> OnSubscribe<T> onCreate(OnSubscribe<T> f) { return f; }3.调用create其实就是将OnSubscribe的实现类赋值给了Observable的成员：onSubscribe
protected Observable(OnSubscribe<T> f) { this.onSubscribe = f; } public class Observable<T> { final OnSubscribe<T> onSubscribe; ... } 然后从外面拿到这个onSubscribe.4.observable在调用subscribe的时候，看onSubscribeStart（xx,xx,xx）可知，会发现返回的就是传入的第二个参数observable.onSubscribe，3中所提到的onSubscribe。
static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) { ... try { hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber); return hook.onSubscribeReturn(subscriber); } catch (Throwable e) { ... return Subscriptions.unsubscribed(); } } 这时调用onSubsribe.call(subscriber)就会回调我们在获取observble时实现的call方法，在call中调用相应的onNext...###map():
return Observable.create(new Observable.OnSubscribe<String>() { @Override public void call(Subscriber<? super String> subscriber) { subscriber.onNext(text); } }).map(new Func1<String, List<String>>() { @Override public List<String> call(String s) { List<String> list = new ArrayList<>(); list.add(s + 1); list.add(s + 2); return list; } });1.可知，可通过map获取新的Observable对象，并将“new OnSubscribeLift(onSubscribe, operator)”此对象赋值给onSubscribe。调用map之前要调用create(),所以这个对象的构造参数之一onSubscribe就是create（）中的实现类参数（相当于上一级的onSubscribe）。
public final <R> Observable<R> map(Func1<? super T, ? extends R> func) { return lift(new OperatorMap<T, R>(func)); } public final <R> Observable<R> lift(final Operator<? extends R, ? super T> operator) { return new Observable<R>(new OnSubscribeLift<T, R>(onSubscribe, operator)); } protected Observable(OnSubscribe<T> f) { this.onSubscribe = f; }2.map后调用subscribe，根据之前分析可以写成：new OnSubscribeLift(onSubscribe, operator).call(subsriber)。
hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber);3.再看看OnSubscribeLift，必然会回调次call方法，可以看到hook.onLift(operator)返回的就是operator。
@Override public void call(Subscriber<? super R> o) { try { Subscriber<? super T> st = hook.onLift(operator).call(o); try { st.onStart(); parent.call(st); } catch (Throwable e) { Exceptions.throwIfFatal(e); st.onError(e); } } catch (Throwable e) { Exceptions.throwIfFatal(e); o.onError(e); } } public <T, R> Operator<? extends R, ? super T> onLift(final Operator<? extends R, ? super T> lift) { return lift; }4.刚开始调用map的时候，new了一个OperatorMap，这里的operator就是它的实例。必然会调用这个call方法，这里的o就是我们在subscribe中传入的实现类参数，transformer就是map传入的实现call方法的实现类参数。将MapSubscriber实例返回。
public final class OperatorMap<T, R> implements Operator<R, T> { final Func1<? super T, ? extends R> transformer; public OperatorMap(Func1<? super T, ? extends R> transformer) { this.transformer = transformer; } @Override public Subscriber<? super T> call(final Subscriber<? super R> o) { MapSubscriber<T, R> parent = new MapSubscriber<T, R>(o, transformer); o.add(parent); return parent; } }5.在3中，代码走到parent.call(st);将4中返回的MapSubscriber实例传入。（特别说明一下：这个parent就是1中所说的调用create后传入的实现类，给onSubscribe第一次赋的值，也就是上一级的onSubscribe）。所以：此时回调，create方法传入的实现了call方法的实现类，走到subscriber.onNext();就会回调下面的代码
static final class MapSubscriber<T, R> extends Subscriber<T> { ... 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) { ... actual.onError(e); } @Override public void onCompleted() { ... actual.onCompleted(); } ... }6.走到result = mapper.call(t);就会回调map参数中实现call方法的实现类，进行转换，得到新的类型，再次调用actual.onNext(result);就会回调我们在subscribe方法中传入的实现类中的onNext（），将转型后的类型，发送回来。
