第二章 RxJava基础知识
1. Observable
1.1 RxJava的使用三步骤
- 创建Observable
- 创建Observer
- 使用subscribe()进行订阅
Observable.just("Hello World")
.subscribe(new Consumer<String>() {
@Override
public void accept(String s) throws Exception {
System.out.println(s);
}
});
subscribe()有多个重载方法：
Disposable subscribe()
Disposable subscribe(Consumer<? super T> onNext)
Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError)
Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
Action onComplete)
Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
Action onComplete, Consumer<? super Disposable> onSubscribe)
void subscribe(Observer<? super T> observer)
Consumer是消费者。
onComplete是一个Action,它与Consumer的区别如下：
- Action:无参数
- Consumer:单一参数类型
第五种重载例子：
Observable.just("Hello World")
.subscribe(new Consumer<String>() {
@Override
public void accept(String s) throws Exception {
System.out.println(s);
}
}, new Consumer<Throwable>() {
@Override
public void accept(Throwable throwable) throws Exception {
System.out.println(throwable.getMessage());
}
}, new Action() {
@Override
public void run() throws Exception {
System.out.println("onComplete");
}
}, new Consumer<Disposable>() {
@Override
public void accept(Disposable disposable) throws Exception {
System.out.println("subscribe");
}
});
执行结果：
subscribe
Hello World
onComplete
Rxjava2中，Observable不在支持Subscriber,而是需要使用Observer作为观察者。
Observable.just("Hello World")
.subscribe(new Observer<String>() {
@Override
public void onSubscribe(Disposable d) {
System.out.println("onSubscribe");
}
@Override
public void onNext(String s) {
System.out.println(s);
}
@Override
public void onError(Throwable e) {
System.out.println(e.getMessage());
}
@Override
public void onComplete() {
System.out.println("onComplete");
}
});
执行结果：
onSubscribe
Hello World
onComplete
在Rxjava中，被观察者、观察者、subscribe()方法三者缺一不可。只有使用了subscribe()，被观察者才会开始发送数据。
1.2 RxJava2的五种观察者模式：
| Observable | 能够发射0或n个数据，并以成功或者错误事件终止 |
| Flowable | 能够发射0或n个数据，并以成功或者错误事件终止。支持背压，可以控制数据源发射的速度 |
| Single | 只发射单个数据或者错误事件 |
| Completable | 从来不发射数据，只处理onComplete和onError事件。可以看成Rx的Runnable |
| Maybe | 能够发射0或者1个数据，要么成功，要么失败。有点类似于Optional |
5种被观察者类型中只有Flowable支持背压。
1.3 do操作符
| doOnSubscribe | 一旦观察者订阅了Observable,它就会被调用 |
| doOnLifecycle | 可以在观察者订阅之后，设置是否取消订阅 |
| doOnNext | 它产生的Observable每发射一项数据就会调用它一次，它的Consumer接受发射的数据项。一般用于在subscribe之前对数据进行处理 |
| doOnEach | 它产生的Observable每发射一项数据就会调用它一次，不仅包括onNext，还包括onError和onComplete |
| doAfterNext | 在onNext之后执行，而doOnNext()是在onNext之前执行 |
| doOnComplete | 当产生的Observable在正常终止调用onComplete时会被调用 |
| doFinally | 在当它产生的Observable终止之后会被调用，无论是正常终止还是异常终止。doFinally优先于doAfterTerminate的调用 |
| doAfterTerminate | 注册一个Action，当Observable调用onComplete或onError时调用 |
public static void main(String[] args) {
Observable.just("Hello World")
.doOnNext(new Consumer<String>() {
@Override
public void accept(String s) throws Exception {
System.out.println("doOnNext:"+s);
}
}).doAfterNext(new Consumer<String>() {
@Override
public void accept(String s) throws Exception {
System.out.println("doAfterNext:"+s);
}
}).doOnComplete(new Action() {
@Override
public void run() throws Exception {
System.out.println("doOnComplete");
}
}).doOnSubscribe(new Consumer<Disposable>() {
@Override
public void accept(Disposable disposable) throws Exception {
System.out.println("doOnSubscribe:");
}
}).doAfterTerminate(new Action() {
@Override
public void run() throws Exception {
System.out.println("doAfterTerminate:");
}
}).doOnEach(new Consumer<Notification<String>>() {
@Override
public void accept(Notification<String> stringNotification) throws Exception {
System.out.println("doOnEach:"+(stringNotification.isOnNext()?"onNext":stringNotification.isOnComplete()?"onComplete":"onError")+" value:"+stringNotification.getValue());
}
}).doFinally(new Action() {
@Override
public void run() throws Exception {
System.out.println("doFinally:");
}
}).doOnLifecycle(new Consumer<Disposable>() {
@Override
public void accept(Disposable disposable) throws Exception {
System.out.println("doOnLifecycle:"+disposable.isDisposed());
}
}, new Action() {
@Override
public void run() throws Exception {
System.out.println("doOnLifecycle: run");
}
}).subscribe(new Observer<String>() {
@Override
public void onSubscribe(Disposable d) {
System.out.println("onSubscribe:");
}
@Override
public void onNext(String s) {
System.out.println("onNext:"+s);
}
@Override
public void onError(Throwable e) {
System.out.println("onError:"+e.getMessage());
}
@Override
public void onComplete() {
System.out.println("onComplete:");
}
});
执行结果：
doOnSubscribe:
doOnLifecycle:false
onSubscribe:
doOnNext:Hello World
doOnEach:onNext value:Hello World
onNext:Hello World
doAfterNext:Hello World
doOnComplete
doOnEach:onComplete value:null
onComplete:
doFinally:
doAfterTerminate:
doOnEach与doOnNext、doOnComplete()、doOnError执行顺序：谁在前面先执行谁。
doOnEach放到doOnNext上面执行：
...
.doOnEach(new Consumer<Notification<String>>() {
@Override
public void accept(Notification<String> stringNotification) throws Exception {
System.out.println("doOnEach:"+(stringNotification.isOnNext()?"onNext":stringNotification.isOnComplete()?"onComplete":"onError")+" value:"+stringNotification.getValue());
}
}).doOnNext(new Consumer<String>() {
@Override
public void accept(String s) throws Exception {
System.out.println("doOnNext:"+s);
}
})...
执行结果：
doOnSubscribe:
doOnLifecycle:false
onSubscribe:
doOnEach:onNext value:Hello World
doOnNext:Hello World
onNext:Hello World
doAfterNext:Hello World
doOnEach:onComplete value:null
doOnComplete
onComplete:
doFinally:
doAfterTerminate:
2.HotObservable和Cold Observable
2.1 Observabel分类
Observable 有 Cold 和 Hot 之分。
2.2 Cold Observable
Observable 的 just、creat、range、fromXXX 等操作符都能生成Cold Observable。
Consumer<Long> subscriber1 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println("subscriber1: "+aLong);
}
};
Consumer<Long> subscriber2 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println(" subscriber2: "+aLong);
}
};
Observable<Long> observable = Observable.create(new ObservableOnSubscribe<Long>() {
@Override
public void subscribe(@NonNull ObservableEmitter<Long> e) throws Exception {
Observable.interval(10, TimeUnit.MILLISECONDS,Schedulers.computation())
.take(Integer.MAX_VALUE)
.subscribe(e::onNext);
}
}).observeOn(Schedulers.newThread());
observable.subscribe(subscriber1);
observable.subscribe(subscriber2);
try {
Thread.sleep(100L);
} catch (InterruptedException e) {
e.printStackTrace();
}
执行结果：
subscriber1: 0
subscriber2: 0
subscriber1: 1
subscriber2: 1
subscriber1: 2
subscriber2: 2
subscriber2: 3
subscriber1: 3
subscriber1: 4
subscriber2: 4
subscriber2: 5
subscriber1: 5
subscriber1: 6
subscriber2: 6
subscriber1: 7
subscriber2: 7
subscriber1: 8
subscriber2: 8
subscriber1: 9
subscriber2: 9
可以看出，subscriber1 和 subscriber2 的结果并不一定是相同的，二者是完全独立的。
尽管 Cold Observable 很好，但是对于某些事件不确定何时发生以及不确定 Observable 发射的元素数量，那还得使用 Hot Observable。比如：UI交互的事件、网络环境的变化、地理位置的变化、服务器推送消息的到达等等。
2.3 Clod Observable转Hot Observable
1. 使用publish，生成ConnectableObservable
使用 publish 操作符，可以让 Cold Observable 转换成 Hot Observable。它将原先的 Observable 转换成 ConnectableObservable。
Consumer<Long> subscriber1 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println("subscriber1: "+aLong);
}
};
Consumer<Long> subscriber2 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println(" subscriber2: "+aLong);
}
};
Consumer<Long> subscriber3 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println(" subscriber3: "+aLong);
}
};
ConnectableObservable<Long> observable = Observable.create(new ObservableOnSubscribe<Long>() {
@Override
public void subscribe(@NonNull ObservableEmitter<Long> e) throws Exception {
Observable.interval(10, TimeUnit.MILLISECONDS,Schedulers.computation())
.take(Integer.MAX_VALUE)
.subscribe(e::onNext);
}
}).observeOn(Schedulers.newThread()).publish();
observable.connect();
observable.subscribe(subscriber1);
observable.subscribe(subscriber2);
try {
Thread.sleep(20L);
} catch (InterruptedException e) {
e.printStackTrace();
}
observable.subscribe(subscriber3);
try {
Thread.sleep(100L);
} catch (InterruptedException e) {
e.printStackTrace();
}
注意，生成的 ConnectableObservable 需要调用connect()才能真正执行。
执行结果：
subscriber1: 0
subscriber2: 0
subscriber1: 1
subscriber2: 1
subscriber1: 2
subscriber2: 2
subscriber3: 2
subscriber1: 3
subscriber2: 3
subscriber3: 3
subscriber1: 4
subscriber2: 4
subscriber3: 4
subscriber1: 5
subscriber2: 5
subscriber3: 5
subscriber1: 6
subscriber2: 6
subscriber3: 6
subscriber1: 7
subscriber2: 7
subscriber3: 7
subscriber1: 8
subscriber2: 8
subscriber3: 8
subscriber1: 9
subscriber2: 9
subscriber3: 9
subscriber1: 10
subscriber2: 10
subscriber3: 10
subscriber1: 11
subscriber2: 11
subscriber3: 11
可以看到，多个订阅的 Subscriber 共享同一事件。
在这里，ConnectableObservable 是线程安全的。
2. 使用Subject/Processor
Subject 和 Processor 的作用是相同的。Processor 是 RxJava2.x 新增的类，继承自 Flowable 支持背压控制。而 Subject 则不支持背压控制。
Consumer<Long> subscriber1 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println("subscriber1: "+aLong);
}
};
Consumer<Long> subscriber2 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println(" subscriber2: "+aLong);
}
};
Consumer<Long> subscriber3 = new Consumer<Long>() {
@Override
public void accept(@NonNull Long aLong) throws Exception {
System.out.println(" subscriber3: "+aLong);
}
};
Observable<Long> observable = Observable.create(new ObservableOnSubscribe<Long>() {
@Override
public void subscribe(@NonNull ObservableEmitter<Long> e) throws Exception {
Observable.interval(10, TimeUnit.MILLISECONDS,Schedulers.computation())
.take(Integer.MAX_VALUE)
.subscribe(e::onNext);
}
}).observeOn(Schedulers.newThread());
PublishSubject<Long> subject = PublishSubject.create();
observable.subscribe(subject);
subject.subscribe(subscriber1);
subject.subscribe(subscriber2);
try {
Thread.sleep(20L);
} catch (InterruptedException e) {
e.printStackTrace();
}
subject.subscribe(subscriber3);
try {
Thread.sleep(100L);
} catch (InterruptedException e) {
e.printStackTrace();
}
执行结果跟上面使用 publish 操作符是一样的。
Subject 既是 Observable 又是 Observer(Subscriber)。
当 Subject 作为 Subscriber 时，它可以订阅目标 Cold Observable 使对方开始发送事件。同时它又作为Observable 转发或者发送新的事件，让 Cold Observable 借助 Subject 转换为 Hot Observable。
注意，Subject 并不是线程安全的，如果想要其线程安全需要调用toSerialized()方法。(在RxJava1.x的时代还可以用 SerializedSubject 代替 Subject，但是在RxJava2.x以后SerializedSubject不再是一个public class）
然而，很多基于 EventBus 改造的 RxBus 并没有这么做，包括我以前也写过这样的 RxBus