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在说Observer与Subscriber的关系之前，我们下重温下相关概念。
RxJava 有四个基本概念：Observable (可观察者，即被观察者)、 Observer (观察者)、 subscribe (订阅)、事件。Observable 和 Observer 通过 subscribe() 方法实现订阅关系，从而 Observable 可以在需要的时候发出事件来通知 Observer。
与传统观察者模式不同， RxJava 的事件回调方法除了普通事件 onNext() （相当于 onClick() / onEvent()）之外，还定义了两个特殊的事件：onCompleted() 和 onError()。
- onCompleted(): 事件队列完结。RxJava 不仅把每个事件单独处理，还会把它们看做一个队列。RxJava 规定，当不会再有新的 onNext() 发出时，需要触发 onCompleted() 方法作为标志。
- onError(): 事件队列异常。在事件处理过程中出异常时，onError() 会被触发，同时队列自动终止，不允许再有事件发出。
- 在一个正确运行的事件序列中, onCompleted() 和 onError() 有且只有一个，并且是事件序列中的最后一个。需要注意的是，onCompleted() 和 onError() 二者也是互斥的，即在队列中调用了其中一个，就不应该再调用另一个。RxJava 的观察者模式大致如下图：
基于以上的概念， RxJava 的基本实现主要有三点：
1) 创建 Observer
Observer 即观察者，它决定事件触发的时候将有怎样的行为。 RxJava 中的 Observer 接口的实现方式：
Observer<Apps> observer = new Observer<Apps>() {
@Override
public void onCompleted() {
listView.onRefreshComplete();
}
@Override
public void onError(Throwable e) {
listView.onRefreshComplete();
}
@Override
public void onNext(Apps appsList) {
listView.onRefreshComplete();
appLists.addAll(appsList.apps);
adapter.notifyDataSetChanged();
}
};除了 Observer 接口之外，RxJava 还内置了一个实现了 Observer 的抽象类：Subscriber。 Subscriber 对 Observer 接口进行了一些扩展，但他们的基本使用方式是完全一样的：
Subscriber subscriber = new Subscriber<Apps>() {
@Override
public void onCompleted() {
listView.onRefreshComplete();
}
@Override
public void onError(Throwable e) {
listView.onRefreshComplete();
}
@Override
public void onNext(Apps appsList) {
listView.onRefreshComplete();
appLists.addAll(appsList.apps);
adapter.notifyDataSetChanged();
}
};不仅基本使用方式一样，实质上，在 RxJava 的 subscribe 过程中，Observer 也总是会先被转换成一个 Subscriber 再使用。所以如果你只想使用基本功能，选择 Observer 和 Subscriber 是完全一样的。
public abstract class Subscriber<T> implements Observer<T>, Subscription 而onStart()方法是Subscriber中的一个方法。它也属于回调级别的。
subscribe(Subscriber)方法中有如下代码：
// if not already wrapped 包裹一层
if (!(subscriber instanceof SafeSubscriber)) {
// assign to `observer` so we return the protected version
subscriber = new SafeSubscriber<T>(subscriber);
}他将subscriber包装起来，这个具体什么意思有待研究，继续下看。
hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber);
return hook.onSubscribeReturn(subscriber);hook是什么呢？
private static final RxJavaObservableExecutionHook hook = RxJavaPlugins.getInstance().getObservableExecutionHook();RxJavaObservableExecutionHook.java源码：
/**
* Copyright 2014 Netflix, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package rx.plugins;
import rx.Observable;
import rx.Observable.OnSubscribe;
import rx.Observable.Operator;
import rx.Subscriber;
import rx.Subscription;
import rx.functions.Func1;
/**
* Abstract ExecutionHook with invocations at different lifecycle points of {@link Observable} execution with a
* default no-op implementation.
* <p>
* See {@link RxJavaPlugins} or the RxJava GitHub Wiki for information on configuring plugins:
* <a href="https://github.com/ReactiveX/RxJava/wiki/Plugins">https://github.com/ReactiveX/RxJava/wiki/Plugins</a>.
* <p>
* <b>Note on thread-safety and performance:</b>
* <p>
* A single implementation of this class will be used globally so methods on this class will be invoked
* concurrently from multiple threads so all functionality must be thread-safe.
* <p>
* Methods are also invoked synchronously and will add to execution time of the observable so all behavior
* should be fast. If anything time-consuming is to be done it should be spawned asynchronously onto separate
* worker threads.
*
*/
public abstract class RxJavaObservableExecutionHook {
/**
* Invoked during the construction by {@link Observable#create(OnSubscribe)}
* <p>
* This can be used to decorate or replace the <code>onSubscribe</code> function or just perform extra
* logging, metrics and other such things and pass-thru the function.
*
* @param f
* original {@link OnSubscribe}<{@code T}> to be executed
* @return {@link OnSubscribe}<{@code T}> function that can be modified, decorated, replaced or just
* returned as a pass-thru
*/
public <T> OnSubscribe<T> onCreate(OnSubscribe<T> f) {
return f;
}
/**
* Invoked before {@link Observable#subscribe(rx.Subscriber)} is about to be executed.
* <p>
* This can be used to decorate or replace the <code>onSubscribe</code> function or just perform extra
* logging, metrics and other such things and pass-thru the function.
*
* @param onSubscribe
* original {@link OnSubscribe}<{@code T}> to be executed
* @return {@link OnSubscribe}<{@code T}> function that can be modified, decorated, replaced or just
* returned as a pass-thru
*/
public <T> OnSubscribe<T> onSubscribeStart(Observable<? extends T> observableInstance, final OnSubscribe<T> onSubscribe) {
// pass-thru by default
return onSubscribe;
}
/**
* Invoked after successful execution of {@link Observable#subscribe(rx.Subscriber)} with returned
* {@link Subscription}.
* <p>
* This can be used to decorate or replace the {@link Subscription} instance or just perform extra logging,
* metrics and other such things and pass-thru the subscription.
*
* @param subscription
* original {@link Subscription}
* @return {@link Subscription} subscription that can be modified, decorated, replaced or just returned as a
* pass-thru
*/
public <T> Subscription onSubscribeReturn(Subscription subscription) {
// pass-thru by default
return subscription;
}
/**
* Invoked after failed execution of {@link Observable#subscribe(Subscriber)} with thrown Throwable.
* <p>
* This is <em>not</em> errors emitted via {@link Subscriber#onError(Throwable)} but exceptions thrown when
* attempting to subscribe to a {@link Func1}<{@link Subscriber}{@code <T>}, {@link Subscription}>.
*
* @param e
* Throwable thrown by {@link Observable#subscribe(Subscriber)}
* @return Throwable that can be decorated, replaced or just returned as a pass-thru
*/
public <T> Throwable onSubscribeError(Throwable e) {
// pass-thru by default
return e;
}
/**
* Invoked just as the operator functions is called to bind two operations together into a new
* {@link Observable} and the return value is used as the lifted function
* <p>
* This can be used to decorate or replace the {@link Operator} instance or just perform extra
* logging, metrics and other such things and pass-thru the onSubscribe.
*
* @param lift
* original {@link Operator}{@code <R, T>}
* @return {@link Operator}{@code <R, T>} function that can be modified, decorated, replaced or just
* returned as a pass-thru
*/
public <T, R> Operator<? extends R, ? super T> onLift(final Operator<? extends R, ? super T> lift) {
return lift;
}
}RxJavaObservableExecutionHook类的作用很特殊，似乎没有什么太大的作用，传进去什么（类型）参数，返回什么（类型）参数。
如下代码所示：
public <T> OnSubscribe<T> onCreate(OnSubscribe<T> f) {
return f;
}
public <T> OnSubscribe<T> onSubscribeStart(Observable<? extends T> observableInstance, final OnSubscribe<T> onSubscribe) {
// pass-thru by default
return onSubscribe;
}至于最后关键的返回结果：
public <T> Subscription onSubscribeReturn(Subscription subscription) {
// pass-thru by default
return subscription;
}说白了，就是返回订阅的Observer对象。
它们的区别对于使用者来说主要有两点：
- onStart(): 这是 Subscriber 增加的方法。它会在 subscribe 刚开始，而事件还未发送之前被调用，可以用于做一些准备工作，例如数据的清零或重置。这是一个可选方法，默认情况下它的实现为空。需要注意的是，如果对准备工作的线程有要求（例如弹出一个显示进度的对话框，这必须在主线程执行）， onStart() 就不适用了，因为它总是在 subscribe 所发生的线程被调用，而不能指定线程。要在指定的线程来做准备工作，可以使用 doOnSubscribe() 方法，具体可以在后面的文中看到。
- unsubscribe(): 这是 Subscriber 所实现的另一个接口 Subscription 的方法，用于取消订阅。在这个方法被调用后，Subscriber 将不再接收事件。一般在这个方法调用前，可以使用 isUnsubscribed() 先判断一下状态。 unsubscribe() 这个方法很重要，因为在 subscribe() 之后， Observable 会持有 Subscriber 的引用，这个引用如果不能及时被释放，将有内存泄露的风险。所以最好保持一个原则：要在不再使用的时候尽快在合适的地方（例如 onPause() onStop() 等方法中）调用 unsubscribe() 来解除引用关系，以避免内存泄露的发生。
2) 创建 Observable
Observable 即被观察者，它决定什么时候触发事件以及触发怎样的事件。 RxJava 使用 create() 方法来创建一个 Observable ，并为它定义事件触发规则：
Observable observable = Observable.create(new Observable.OnSubscribe<String>() {
@Override
public void call(Subscriber<? super String> subscriber) {
subscriber.onNext("Hello");
subscriber.onNext("John");
subscriber.onCompleted();
}
});可以看到，这里传入了一个 OnSubscribe 对象作为参数。OnSubscribe 会被存储在返回的 Observable 对象中，它的作用相当于一个计划表，当 Observable 被订阅的时候，OnSubscribe 的 call() 方法会自动被调用，事件序列就会依照设定依次触发（对于上面的代码，就是观察者Subscriber 将会被调用两次 onNext() 和一次 onCompleted()）。这样，由被观察者调用了观察者的回调方法，就实现了由被观察者向观察者的事件传递，即观察者模式。
create() 是 RxJava 最基本的创造事件序列的操作符。基于这个操作符， RxJava 还提供了一些方法用来快捷创建事件队列，详见RxJava操作符系列文章：http://blog.csdn.net/jdsjlzx/article/details/51485861
3) Subscribe (订阅)
创建了 Observable 和 Observer 之后，再用 subscribe() 方法将它们联结起来，整条链子就可以工作了。代码形式很简单：
observable.subscribe(observer);
// 或者：
observable.subscribe(subscriber);Observable.subscribe(Subscriber) 的内部实现是这样的（仅核心代码）：
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) {
// validate and proceed
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();
/*
* See https://github.com/ReactiveX/RxJava/issues/216 for discussion on "Guideline 6.4: Protect calls
* to user code from within an Observer"
*/
// if not already wrapped
if (!(subscriber instanceof SafeSubscriber)) {
// assign to `observer` so we return the protected version
subscriber = new SafeSubscriber<T>(subscriber);
}
// The code below is exactly the same an unsafeSubscribe but not used because it would
// add a significant depth to already huge call stacks.
try {
// allow the hook to intercept and/or decorate
hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber);
return hook.onSubscribeReturn(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(hook.onSubscribeError(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 RuntimeException("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.
hook.onSubscribeError(r);
// TODO why aren‘t we throwing the hook‘s return value.
throw r;
}
return Subscriptions.unsubscribed();
}
}可以看到，subscriber() 做了3件事：
整个过程中对象间的关系如下图：
除了 subscribe(Observer) 和 subscribe(Subscriber) ，subscribe() 还支持不完整定义的回调，RxJava 会自动根据定义创建出 Subscriber 。形式如下：
Action1<String> onNextAction = new Action1<String>() {
// onNext()
@Override
public void call(String s) {
Log.d(tag, s);
}
};
Action1<Throwable> onErrorAction = new Action1<Throwable>() {
// onError()
@Override
public void call(Throwable throwable) {
// Error handling
}
};
Action0 onCompletedAction = new Action0() {
// onCompleted()
@Override
public void call() {
Log.d(tag, "completed");
}
};
// 自动创建 Subscriber ，并使用 onNextAction 来定义 onNext()
observable.subscribe(onNextAction);
// 自动创建 Subscriber ，并使用 onNextAction 和 onErrorAction 来定义 onNext() 和 onError()
observable.subscribe(onNextAction, onErrorAction);
// 自动创建 Subscriber ，并使用 onNextAction、 onErrorAction 和 onCompletedAction 来定义 onNext()、 onError() 和 onCompleted()
observable.subscribe(onNextAction, onErrorAction, onCompletedAction);注：正如前面所提到的，Observer 和 Subscriber 具有相同的角色，而且 Observer 在 subscribe() 过程中最终会被转换成 Subscriber 对象，因此，从某种程度上来说用 Subscriber 来代替 Observer ，这样会更加严谨。
根据目前的经验来看，Observer与Subscriber的主要区别在于onNext方法执行完毕后是否取消了订阅。
首先，我们分别定义mSubscriber 和 mObserver 。
如下代码：
protected Subscriber<D> mSubscriber = new Subscriber<D>() {
@Override
public void onCompleted() {
executeOnLoadFinish();
}
@Override
public void onError(Throwable e) {
TLog.error("onError " + e.toString());
executeOnLoadDataError(null);
}
@Override
public void onNext(D d) {
TLog.log("onNext " );
List<T> list = d;
TLog.log("entity " + list.size());
executeOnLoadDataSuccess(list);
TLog.log("onSuccess totalPage " + totalPage);
}
};
protected Observer<D> mObserver = new Observer<D>() {
@Override
public void onCompleted() {
executeOnLoadFinish();
}
@Override
public void onError(Throwable e) {
TLog.error("onError " + e.toString());
executeOnLoadDataError(null);
}
@Override
public void onNext(D d) {
TLog.log("onNext " );
List<T> list = d;
TLog.log("entity " + list.size());
executeOnLoadDataSuccess(list);
TLog.log("onSuccess totalPage " + totalPage);
}
};observable.subscribeOn(Schedulers.io())
.map(new Func1<Response<D>,D>() {
@Override
public D call(Response<D> response) {
if(response == null){
throw new ApiException(100);
}
totalPage = response.total;
return response.result;
}
})
.observeOn(AndroidSchedulers.mainThread())
//.subscribe(mObserver);
.subscribe(mSubscriber);subscribe(mObserver)和subscribe(mSubscriber)执行结果就会有区别：
提醒：个人以为subscribe(mObserver)这个方式更适合分页加载。
请注意，如果你每次都使用subscribe(new Subscriber< T>() {｝）方式实现订阅，就不会出现上面的问题。
如下代码：
private void toSubscribe(Observable<Response<D>> observable) {
observable.subscribeOn(Schedulers.io())
.map(new Func1<Response<D>,D>() {
@Override
public D call(Response<D> response) {
if(response == null){
throw new ApiException(100);
}
totalPage = response.total;
return response.result;
}
})
.observeOn(AndroidSchedulers.mainThread())
.subscribe(new Subscriber<D>() {
@Override
public void onCompleted() {
executeOnLoadFinish();
}
@Override
public void onError(Throwable e) {
TLog.error("onError " + e.toString());
executeOnLoadDataError(null);
}
@Override
public void onNext(D d) {
TLog.log("onNext " );
List<T> list = d;
TLog.log("entity " + list.size());
executeOnLoadDataSuccess(list);
TLog.log("onSuccess totalPage " + totalPage);
}
});
}当然，这个方式实现分页加载也是可以的。至于哪个更好，还需要再验证。
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原文地址：http://blog.csdn.net/jdsjlzx/article/details/51534504
