RxJava详解之操作符执行原理(五)
上一篇文章介绍了RxJava的执行原理。下面继续介绍一下操作符的执行原理，但是操作符太多了，这里用map来进行说明。
Observable observable = Observable.create(new Observable.OnSubscribe() {
@Override
public void call(Subscriber subscriber) {
subscriber.onNext("Hello ");
subscriber.onNext("World !");
subscriber.onCompleted();
}
});
observable.map(new Func1() {
@Override
public String call(String s) {
return "say" + s;
}
});
observable.subscribe(new Subscriber() {
@Override
public void onCompleted() {
}
@Override
public void onError(Throwable e) {
}
@Override
public void onNext(String s) {
Log.i("@@@", s);
}
});
执行结果很显然是say Hello和say World !。
我们直接进入Observable.map()方法的源码:
public final Observable map(Func1 func) {
return create(new OnSubscribeMap(this, func));
}
public static Observable create(OnSubscribe f) {
return new Observable(RxJavaHooks.onCreate(f));
}
map的内部调用了create()方法，而create()方法的源码我们再上一个版本已经介绍了，也就是说map内部会创建一个新的Observable对象，而且用一个新的OnSubscribeMap对象作为参数。
OnSubscribeMap()对象的参数分别是之前通过create()方法创建的Observable对象，以及map中传递过来的Func1类的对象。接下来就直接看OnSubscribeMap类的源码，他实现了OnSubscribe接口，并重写了call()方法:
/**
* Applies a function of your choosing to every item emitted by an {@code Observable}, and emits the results of
* this transformation as a new {@code Observable}.
*/
public final class OnSubscribeMap implements OnSubscribe {
// 最初Observable.create()创建的Observable对象
final Observable source;
// map方法传递过来的func1对象，它是一个转换功能
final Func1 transformer;
public OnSubscribeMap(Observable source, Func1 transformer) {
this.source = source;
this.transformer = transformer;
}
@Override
public void call(final Subscriber o) {
MapSubscriber parent = new MapSubscriber(o, transformer);
// 把新创建的MapSubscriber添加到Observable.subscribe(subscribe)方法传递的参数subscriber中
o.add(parent);
// unsafeSubscribe是subscribe方法的一个安全性不高的操作，可以简单理解为subscribe方法
source.unsafeSubscribe(parent);
}
static final class MapSubscriber extends Subscriber {
final Subscriber actual;
final Func1 mapper;
boolean done;
public MapSubscriber(Subscriber actual, Func1 mapper) {
this.actual = actual;
this.mapper = mapper;
}
@Override
public void onNext(T t) {
R result;
try {
// 先会执行以下转换函数的call方法，然后将结果再传递给Subscribe对象调用它的onNext方法
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);
}
}
}
因为在执行observable.subscribe(subscriber)方法会调用到call()方法，这里看一下call()方法的核心:
MapSubscriber parent = new MapSubscriber(o, transformer);
// 把新创建的MapSubscriber添加到Observable.subscribe(subscribe)方法传递的参数subscriber中
o.add(parent);
// unsafeSubscribe是subscribe方法的一个安全性不高的操作，可以简单理解为subscribe方法，注意这里传递的是parent，也就是先创建的MapSubscriber对象，而这里的source是谁呢？ 它是最初Observable.create创建的Observable对象
source.unsafeSubscribe(parent);
add()及unsasfeSubscribe()方法如下:
private final SubscriptionList subscriptions;
public final void add(Subscription s) {
subscriptions.add(s);
}
public final Subscription unsafeSubscribe(Subscriber subscriber) {
try {
// new Subscriber so onStart it
subscriber.onStart();
// allow the hook to intercept and/or decorate
RxJavaHooks.onObservableStart(this, onSubscribe).call(subscriber);
return RxJavaHooks.onObservableReturn(subscriber);
} catch (Throwable e) {
// special handling for certain Throwable/Error/Exception types
Exceptions.throwIfFatal(e);
// 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();
}
}
这一块代码就很简单了，因为和前面一篇我们分析的subscribe()方法类似，相当于直接调用了最初Observable.create()创建的Observable对象的call(subscriber)方法，而这里的subscriber又是我们创建的MapSubscriber的子类，所以这里相当于调用了MapSubscriber类中的onNext()、onComplete()和onError()方法:
static final class MapSubscriber extends Subscriber {
final Subscriber actual;
final Func1 mapper;
boolean done;
public MapSubscriber(Subscriber actual, Func1 mapper) {
this.actual = actual;
this.mapper = mapper;
}
@Override
public void onNext(T t) {
R result;
try {
// 先会执行以下转换函数的call方法，这个就是我们把Hello修改为say Hello的部分
result = mapper.call(t);
} catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
unsubscribe();
onError(OnErrorThrowable.addValueAsLastCause(ex, t));
return;
}
// 将转换函数的call方法的执行结果交给最初的Subscriber.onNext()方法的参数来执行
actual.onNext(result);
}
...
}
乱不乱？
梳理一下:
- 我们把不使用
map操作符时正常的操作创建的Observable和Subscriber分别称为1号。 - 而
map又会通过create分辨创建一个Observable2号和Subscriber2号，当我们执行map的时候，会最终执行到Subscriber2号的onNext()方法中，而该方法内部会先执行一些转换操作，然后将执行完的结果作为参数传递给并调用最初的Subscriber1号的onNext()方法。懂不？ 多看两遍，这里有点绕。
更多内容请看下一篇文章RxJava详解(六)
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