Rx系列初探RxJava(一)

Rx出来很久了，long long ago 就学习了一下，但只是初步的了解过一些，并没怎么实战过，现在都忘的差不多了，现在公司差不多都要用RxJava RxAndroid相关开发，特此重新理解学习一下这方面的知识，写点blog做个记录和分享。

首先，Rx相关的知识最好通过github上面去获取，以免其他渠道获取的信息言辞不达意亦或者错误的理解，先下载RxJava源码到本地浏览个大概即可，脑海留着些许印象，不至于后面开发感觉代码有点抽象。

项目添加RxJava依赖

 compile 'io.reactivex:rxjava:1.0.14'

Observable和OnSubscribe是Rx最常见的两个类，我是这样理解的：Observable就是大明星，OnSubscribe是粉丝，明星blog发布，关注他的粉丝就能进行评论阅读等作出各自的反应，如果粉丝取消了关注就不会再接受到明星发布的blog的消息通知。现在先来梳理一下脉络，这两个类的内部具体实现暂且不看。

public interface Observer<T> {

    /** * Notifies the Observer that the {@link Observable} has finished sending push-based notifications. * <p> * The {@link Observable} will not call this method if it calls {@link #onError}. */
    void onCompleted();

    /** * Notifies the Observer that the {@link Observable} has experienced an error condition. * <p> * If the {@link Observable} calls this method, it will not thereafter call {@link #onNext} or * {@link #onCompleted}. * * @param e * the exception encountered by the Observable */
    void onError(Throwable e);

    /** * Provides the Observer with a new item to observe. * <p> * The {@link Observable} may call this method 0 or more times. * <p> * The {@code Observable} will not call this method again after it calls either {@link #onCompleted} or * {@link #onError}. * * @param t * the item emitted by the Observable */
    void onNext(T t);

}

onError方法和onCompleted方法不会同时存在，互斥关系，程序不管是哪里一旦出错立即onError方法回调，不会再继续执行，整个流程执行完成没出错call里手动调用onCompleted。onNext稍后再提，下面再看Subscription

public interface Subscription {

    /** * Stops the receipt of notifications on the {@link Subscriber} that was registered when this Subscription * was received. * <p> * This allows unregistering an {@link Subscriber} before it has finished receiving all events (i.e. before * onCompleted is called). */
    void unsubscribe();

    /** * Indicates whether this {@code Subscription} is currently unsubscribed. * * @return {@code true} if this {@code Subscription} is currently unsubscribed, {@code false} otherwise */
    boolean isUnsubscribed();

}

isUnsubscribed用于判断订阅者有没有取消订阅，unsubscribe就是用于取消订阅啦，这些接口定义具体实现在哪里呢，该怎么用呢？且看下面代码块来帮助我们理解

public interface Action1<T> extends Action {
    void call(T t);
}
.............

public abstract class Subscriber<T> implements Observer<T>, Subscription {

}
.......................

public class Observable<T> {
    /** * Invoked when Observable.subscribe is called. */
    public interface OnSubscribe<T> extends Action1<Subscriber<? super T>> {
        // cover for generics insanity
    }

    public final static <T> Observable<T> create(OnSubscribe<T> f) {
        return new Observable<T>(hook.onCreate(f));
    }

    public final Subscription subscribe(Subscriber<? super T> subscriber) {
        return Observable.subscribe(subscriber, this);
    }

    private static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) {

   }
}

这么直观的代码应该能理解了吧，如果还无法理解，下面通过一组实例来帮助我们加深理解吧。观察者发布事件给订阅者，订阅者在call回调方法处理事情，处理完成后会执行onCompleted回调如果这个过程中出现错误立即onError回调。如果处理事件是N个，你可以选择每次处理完成了调用onNext方法通知观察者一下，处理完了所有的在调用onCompleted（如果你在call方法里面不手动调用onCompleted后面认购订阅事件的回调不会执行的。）

public class RxSimple01 {

    private RxSimple01(){

    }

    public static RxSimple01 newInstance(){

        return new RxSimple01();
    }

    String  result = null;

    public Observable createObservable(){

        return Observable.create(new Observable.OnSubscribe<String>() {
            @Override
            public void call(Subscriber<? super String> subscriber) {
                 if(!subscriber.isUnsubscribed()){//如果还没取消订阅执行下面操作
                      for (int i=0; i<5; i++){

                          int temp = new Random().nextInt(10);
                          if (temp > 8) {
                              //if value>8, we make an error
                              subscriber.onError(new Throwable(temp+";value >8"));
                              break;
                          } else {
                              subscriber.onNext("temp"+temp+".");
                          }

                          if(i==4){
                             subscriber.onCompleted();
                          }
                      }
                 }
            }
        });
    }

    public void initTextView(final TextView textView){

        createObservable().subscribe(new Subscriber<String>(){
            @Override
            public void onCompleted() {
                textView.setText(result);
            }

            @Override
            public void onError(Throwable e) {
                textView.setText(e.getMessage());
            }

            @Override
            public void onNext(String s) {
                   result+= s;
            }
        });
    }
}

Create是最基本的创建Observable的操作符，如果你只是创建了然并卵！订阅者没有认购，这个creat只是创建了一个实例而已，也仅此而已。我们需要通过观察者调用subscribe完成订阅者的认购，这样才是一个完整的流程，call方法中的onError等相关方法回调如何关联到下面创建的Subscriber实例的呢？且看下例代码块

  private static <T> Subscription subscribe(Subscriber<? super T> subscriber, Observable<T> observable) {
     // validate and proceed 观察者发布事件被订阅者认购这件事如果订阅者没有，就会抛出异常
        if (subscriber == null) {
            throw new IllegalArgumentException("observer can not be null");
        }
        if (observable.onSubscribe == null) {
            throw new IllegalStateException("onSubscribe function can not be null.");
            /* * the subscribe function can also be overridden but generally that's not the appropriate approach * so I won't mention that in the exception */
        }

        // new Subscriber so onStart it
        subscriber.onStart();

        try {
            // allow the hook to intercept and/or decorate 有了订阅者才能在这里执行call函数的回调，所以如果没有发生调用subscribe方法，create的函数内实例的OnSubscribe对应的call还有用么，答案显而易见的
            hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber);
            return hook.onSubscribeReturn(subscriber);
        } catch (Throwable e) {
            //........................
            try {
                //出现了异常会立即终止，回调error函数
                subscriber.onError(hook.onSubscribeError(e));
            } catch (OnErrorNotImplementedException e2) {

             //...............................
             //订阅事件发生了异常会取消订阅
            return Subscriptions.unsubscribed();
        }
    }

上面注释相信非常清晰了吧，Rx我认为也就那么回事：观察者和订阅者，发布事件作为链条。好与坏且不论，我也没资格说啥，还是继续理解学习吧，下一篇操作符。

以上理解如有不对欢迎指出

强烈推荐参考资料

http://mushuichuan.com/2015/12/11/rxjava-operator-1/