RxJava 拥有繁多的 API 和复杂的逻辑链，学习复杂的知识，一般从整体再到具体，为了学习 RxJava 的原理，参考其源码，自己动手实现一个简化的 RxJava，旨在理解调用链
阅读本文，建议先下载代码 LittleRx，毕竟在IDE里阅读代码比在网页上要清晰得多，也可以看下打印的日志
最主要的4个类：Observable、OnSubscribe、Operator、Subscriber
1、最简单的，创建一个Observable，然后订阅
Observable
.create(new OnSubscribe<Integer>() {
@Override
public void call(Subscriber<? super Integer> subscriber) {
subscriber.onNext(1);
}
})
.subscribe(new Subscriber<Integer>() {
@Override
public void onNext(Integer integer) {
System.out.println(integer);
}
});
public class Observable<T> {
private OnSubscribe<T> onSubscribe;
private Observable(OnSubscribe<T> onSubscribe) {
this.onSubscribe = onSubscribe;
}
public final void subscribe(Subscriber<? super T> subscriber) {
onSubscribe.call(subscriber);
}
public static <T> Observable<T> create(OnSubscribe<T> onSubscribe) {
return new Observable<>(onSubscribe);
}
}
这里可以看出 subscribe(subscriber)–>onSubscribe.call(subscriber)，所以没有订阅动作就不会触发 OnSubscribe.call()
2、map 和 lift
Observable
.create(new OnSubscribe<Integer>() {
@Override
public void call(Subscriber<? super Integer> subscriber) {
subscriber.onNext(1);
}
})
.map(new Func1<Integer, String>() {
@Override
public String call(Integer integer) {
return "map" + integer;
}
})
.subscribe(new Subscriber<String>() {
@Override
public void onNext(String s) {
System.out.println(s);
}
});
非链式的写法
OnSubscribe<Integer> onSubscribe = new OnSubscribe<>();
Observable<Integer> observable = Observable.create(onSubscribe);
Func1<Integer, String> func = new Func1<>();
Observable<String> observable2 = observable.map(func);
Subscriber<String> subscriber = new Subscriber<>();
observable2.subscribe(subscriber);
create() 跟之前一样，那么 map() 做了什么
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));
}
lift() 根据上一个 observable 的 onSubscribe 创建一个新的 OnSubscribeLift 返回一个新的 observable2，上面我们说过 subscribe(subscriber)–>onSubscribe.call(subscriber)，所以我们接着看 OnSubscribeLift
public class OnSubscribeLift<T, R> implements OnSubscribe<R> {
final OnSubscribe<T> parent;
final Operator<? extends R, ? super T> operator;
public OnSubscribeLift(OnSubscribe<T> parent, Operator<? extends R, ? super T> operator) {
this.parent = parent;
this.operator = operator;
}
// 先不用关心具体实现，下面讲到再看
@Override
public void call(Subscriber<? super R> r) {
Subscriber<? super T> st = operator.call(r); // 这个 operator 就是 OperatorMap
parent.call(st); // parent 就是第一个 observable 的 onSubscribe
}
}
再看下 OperatorMap
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;
}
// 先不用关心具体实现，下面讲到再看
@Override
public Subscriber<? super T> call(final Subscriber<? super R> o) {
return new MapSubscriber<T, R>(o, transformer);
}
private class MapSubscriber<T, R> extends Subscriber<T> {
private Subscriber<? super R> actual;
private Func1<? super T, ? extends R> transformer;
public MapSubscriber(Subscriber<? super R> o, Func1<? super T, ? extends R> transformer) {
this.actual = o;
this.transformer = transformer;
}
// 先不用关心具体实现，下面讲到再看
@Override
public void onNext(T t) {
R r = transformer.call(t);
actual.onNext(r);
}
}
}
我们把 map() 和 lift() 都去掉，使用最基本的类来实现
OnSubscribe<Integer> onSubscribe = new OnSubscribe<>();
Observable<Integer> observable = new Observable<>(onSubscribe);
Func1<Integer, String> func = new Func1<>();
OperatorMap<Integer, String> operatorMap = new OperatorMap<>(func);
OnSubscribeLift<Integer, String> onSubscribe2 = new OnSubscribeLift<>(onSubscribe, operatorMap);
Observable<String> observable2 = new Observable<>(onSubscribe2);
Subscriber<String> subscriber = new Subscriber<>();
observable2.subscribe(subscriber);
到这里，清楚了如何把第一个 Observable<Integer> 转成 Observable<String>，包括 OnSubscribe<Integer> onSubscribe 和 OnSubscribeLift<Integer, String> onSubscribe2 的关系
那么最终的 subscribe() 如何调用到第一个 observable.call(Subscriber<Integer>) 里面的 Subscriber<Integer>.onNext(Integer) 又如何调用到最终的订阅者 subscriber<String>().onNext(String)
**1) observable2.subscribe(subscriber) –>
- onSubscribe2.call(subscriber) 即 OnSubscribeLift.call(subscriber) –>
- Subscriber<Integer> st = operatorMap.call(subscriber) 即
- Subscriber<Integer> st = new MapSubscriber<Integer, String>(subscriber, func)
- parent.call(st) 即 onSubscribe.call(st) –>
- st.onNext(1) 即 MapSubscriber.onNext(1) –>
- String string = func.call(1)
- subscriber.onNext(string)**
至此 Observable.create().map().subscribe() 的调用链就分析完了
很多操作符本质都是 lift()，以此类推，lift() 2次
3、subscribeOn
Scheduler 内部比较繁杂，我们简化下，把 subscribeOn(Scheduler) 简化成 subscribeOnIO()
Observable
.create(new OnSubscribe<Integer>() {
@Override
public void call(Subscriber<? super Integer> subscriber) {
subscriber.onNext(1);
}
})
.subscribeOnIO()
.subscribe(new Subscriber<Integer>() {
@Override
public void onNext(Integer integer) {
System.out.println(integer);
}
});
如何实现 subscribeOnIO() 让第一个 observable 的 onSubscribe 运行在子线程
public final Observable<T> subscribeOnIO() {
return create(new OnSubscribeOnIO<T>(this));
}
public final class OnSubscribeOnIO<T> implements OnSubscribe<T> {
private static ExecutorService threadPool = Executors.newCachedThreadPool();
final Observable<T> source;
public OnSubscribeOnIO(Observable<T> source) {
this.source = source;
}
@Override
public void call(final Subscriber<? super T> subscriber) {
Runnable runnable = new Runnable() {
@Override
public void run() {
source.subscribe(subscriber); // --> onSubscribe.call(subscriber) --> subscriber.onNext(1)
}
};
threadPool.submit(runnable);
}
}
从上面看出 subscribeOnIO() 新建了一个线程并加入 CachedThreadPool，在子线程里订阅上一个 Observable，后续的调用都在这个线程里完成
再考虑下复杂点的，加入 map()
Observable
.create(new OnSubscribe<Integer>() {
@Override
public void call(Subscriber<? super Integer> subscriber) {
System.out.println(Thread.currentThread());
subscriber.onNext(1);
}
})
.map(new Func1<Integer, String>() {
@Override
public String call(Integer integer) {
System.out.println(Thread.currentThread());
return "map" + integer;
}
})
.subscribeOnIO()
.subscribe(new Subscriber<String>() {
@Override
public void onNext(String s) {
System.out.println(Thread.currentThread());
System.out.println(s);
}
});
非链式的写法
OnSubscribe<Integer> onSubscribe = new OnSubscribe<>();
Observable<Integer> observable = new Observable<>(onSubscribe);
Func1<Integer, String> func = new Func1<>();
OperatorMap<Integer, String> operatorMap = new OperatorMap<>(func);
OnSubscribeLift<Integer, String> onSubscribe2 = new OnSubscribeLift<>(onSubscribe, operatorMap);
Observable<String> observable2 = new Observable<>(onSubscribe2);
OnSubscribeOnIO<String> onSubscribe3 = new OnSubscribeOnIO(observable2);
Observable<String> observable3 = new Observable<>(onSubscribe3);
Subscriber<String> subscriber = new Subscriber<>();
observable3.subscribe(subscriber);
**1) observable3.subscribe(subscriber) –>
- onSubscribe3.call(subscriber) 即 OnSubscribeOnIO.call(subscriber) –>
- 子线程 new Runnable(){} –> observable2.subscribe(subscriber)
- onSubscribe2.call(subscriber) 即 OnSubscribeLift.call(subscriber) –>
- Subscriber<Integer> st = operatorMap.call(subscriber) 即
- Subscriber<Integer> st = new MapSubscriber<Integer, String>(subscriber, func)
- parent.call(st) 即 onSubscribe.call(st) –>
- st.onNext(1) 即 MapSubscriber.onNext(1) –>
- String string = func.call(1)
- subscriber.onNext(string)**
那要是把 map() 与 subscribeOnIO() 换下位置呢
OnSubscribe<Integer> onSubscribe = new OnSubscribe<>();
Observable<Integer> observable = new Observable<>(onSubscribe);
OnSubscribeOnIO<Integer> onSubscribe2 = new OnSubscribeOnIO(observable);
Observable<Integer> observable2 = new Observable<>(onSubscribe2);
Func1<Integer, String> func = new Func1<>();
OperatorMap<Integer, String> operatorMap = new OperatorMap<>(func);
OnSubscribeLift<Integer, String> onSubscribe3 = new OnSubscribeLift<>(onSubscrib2, operatorMap);
Observable<String> observable3 = new Observable<>(onSubscribe3);
Subscriber<String> subscriber = new Subscriber<>();
observable3.subscribe(subscriber);
**1) observable3.subscribe(subscriber) –>
- onSubscribe3.call(subscriber) 即 OnSubscribeLift.call(subscriber) –>
- Subscriber<Integer> st = operatorMap.call(subscriber) 即
- Subscriber<Integer> st = new MapSubscriber<Integer, String>(subscriber, func)
- parent.call(st) 即 onSubscribe2.call(st) 即 OnSubscribeOnIO.call(st) –>
- 子线程 new Runnable(){} –> observable.subscribe(st) –>
- onSubscribe.call(st) –>
- st.onNext(1) 即 MapSubscriber.onNext(1) –>
- String string = func.call(1)
- subscriber.onNext(string)**
看得出来，不管 subscribeOnIO() 在哪，第一个 onSubscribe.call() 总是运行在子线程
4、observeOn
先看下 demo 最终写法
Handler handler = new Handler();
Observable
.create(new OnSubscribe<Integer>() {
@Override
public void call(Subscriber<? super Integer> subscriber) {
subscriber.onNext(1);
}
})
.observeOn(handler)
.map(new Func1<Integer, String>() {
@Override
public String call(Integer integer) {
return "map" + integer;
}
})
.subscribeOnIO()
.subscribe(new Subscriber<String>() {
@Override
public void onNext(String s) {
System.out.println(s);
}
});
handler.loop(); //队列没有消息时会挂起当前线程，直到收到新的消息
同样我们也自己实现一个简单的可以切换回主线程的 observeOn(Handler)
public class Observable<T> {
...
public final Observable<T> observeOn(Handler handler) {
return lift(new OperatorObserveOn<T>(handler));
}
}
OperatorObserveOn
public final class OperatorObserveOn<T> implements Operator<T, T> {
private Handler handler;
public OperatorObserveOn(Handler handler) {
this.handler = handler;
}
@Override
public Subscriber<? super T> call(final Subscriber<? super T> subscriber) {
Subscriber<T> s = new Subscriber<T>() {
@Override
public void onNext(final T t) {
handler.post(new Runnable() {
@Override
public void run() {
subscriber.onNext(t);
}
});
}
};
return s;
}
}
自定义Handler
public class Handler {
private ArrayBlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(10);
public void loop() {
for (; ; ) {
Runnable runnable;
try {
runnable = queue.take();// 没有数据则一直阻塞，直到有数据自动唤醒
} catch (InterruptedException e) {
return;
}
if (runnable == null) {
return;
}
runnable.run();
}
}
public void post(Runnable runnable) {
try {
queue.put(runnable);// 没有空间则一直阻塞，直到有空间
} catch (InterruptedException e) {
return;
}
}
}
非链式写法
OnSubscribe<Integer> onSubscribe = new OnSubscribe<>();
Observable<Integer> observable = new Observable<>(onSubscribe);
OperatorObserveOn<Integer> operatorObserveOn = new OperatorObserveOn(handler);
OnSubscribeLift<Integer, String> onSubscribe2 = new OnSubscribeLift<>(onSubscribe, operatorObserveOn);
Func1<Integer, String> func = new Func1<>();
OperatorMap<Integer, String> operatorMap = new OperatorMap<>(func);
OnSubscribeLift<Integer, String> onSubscribe3 = new OnSubscribeLift<>(onSubscribe2, operatorMap);
Observable<String> observable2 = new Observable<>(onSubscribe3);
OnSubscribeOnIO<String> onSubscribe4 = new OnSubscribeOnIO(observable2);
Observable<String> observable3 = new Observable<>(onSubscribe4);
Subscriber<String> subscriber = new Subscriber<>();
observable3.subscribe(subscriber);
**1) observable3.subscribe(subscriber) –>
- onSubscribe4.call(subscriber) 即 OnSubscribeOnIO.call(subscriber) –>
- 子线程 new Runnable(){} –> observable2.subscribe(subscriber)
- onSubscribe3.call(subscriber) 即 OnSubscribeLift.call(subscriber) –>
- Subscriber<Integer> st = operatorMap.call(subscriber) 即
- Subscriber<Integer> st = new MapSubscriber<Integer, String>(subscriber, func)
- parent.call(st) 即 onSubscribe2.call(st) 即 OnSubscribeLift.call(st)–>
- Subscriber<Integer> st2 = operatorObserveOn.call(st) –>
- parent.call(st2) 即 onSubscribe.call(st2) –>
- st2.onNext(1) –> // onNext()里面切换到Handler所在线程
- st.onNext(1) –>
- String string = func.call(1)
- subscriber.onNext(string)**
5、其他
总的来说，调用链确实有点复杂，不过也还是可以接受的，一个调用链花点时间想想还是能清楚，只是每碰到一个调用链都要花点时间才能想清楚，还没能力能在几秒内就能想清楚，只能是多想多锻炼了。比如想想上面的，如果把 observeOn(handler) 放在 map() 后面呢
