RxJava Observer與Subscriber的關係
RxJava系列教程:
在說Observer與Subscriber的關係之前,我們下重溫下相關概念。
RxJava 的觀察者模式
RxJava 有四個基本概念:Observable (可觀察者,即被觀察者)、 Observer (觀察者)、 subscribe (訂閱)、事件。Observable 和 Observer 通過 subscribe() 方法實現訂閱關係,從而 Observable 可以在需要的時候發出事件來通知 Observer。
與傳統觀察者模式不同, RxJava 的事件回撥方法除了普通事件 onNext() (相當於 onClick() / onEvent())之外,還定義了兩個特殊的事件:onCompleted() 和 onError()。
- onCompleted(): 事件佇列完結。RxJava 不僅把每個事件單獨處理,還會把它們看做一個佇列。RxJava 規定,當不會再有新的 onNext() 發出時,需要觸發 onCompleted() 方法作為標誌。
- onError(): 事件佇列異常。在事件處理過程中出異常時,onError() 會被觸發,同時佇列自動終止,不允許再有事件發出。
- 在一個正確執行的事件序列中, onCompleted() 和 onError() 有且只有一個,並且是事件序列中的最後一個。需要注意的是,onCompleted() 和 onError() 二者也是互斥的,即在佇列中呼叫了其中一個,就不應該再呼叫另一個。
RxJava 的觀察者模式大致如下圖:
RxJava的實現
基於以上的概念, RxJava 的基本實現主要有三點:
1) 建立 Observer
Observer 即觀察者,它決定事件觸發的時候將有怎樣的行為。 RxJava 中的 Observer 介面的實現方式:
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 介面之外,RxJava 還內建了一個實現了 Observer 的抽象類:Subscriber。 Subscriber 對 Observer 介面進行了一些擴充套件,但他們的基本使用方式是完全一樣的:
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();
}
};
不僅基本使用方式一樣,實質上,在 RxJava 的 subscribe 過程中,Observer 也總是會先被轉換成一個 Subscriber 再使用。所以如果你只想使用基本功能,選擇 Observer 和 Subscriber 是完全一樣的。
Subscriber是Observer的實現類
public abstract class Subscriber<T> implements Observer<T>, Subscription
而onStart()方法是Subscriber中的一個方法。它也屬於回撥級別的。
subscribe(Subscriber)方法中有如下程式碼:
// if not already wrapped 包裹一層
if (!(subscriber instanceof SafeSubscriber)) {
// assign to `observer` so we return the protected version
subscriber = new SafeSubscriber<T>(subscriber);
}
他將subscriber包裝起來,這個具體什麼意思有待研究,繼續下看。
hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber);
return hook.onSubscribeReturn(subscriber);
hook是什麼呢?
private static final RxJavaObservableExecutionHook hook = RxJavaPlugins.getInstance().getObservableExecutionHook();
RxJavaObservableExecutionHook.java原始碼:
/**
* 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類的作用很特殊,似乎沒有什麼太大的作用,傳進去什麼(型別)引數,返回什麼(型別)引數。
如下程式碼所示:
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;
}
至於最後關鍵的返回結果:
public <T> Subscription onSubscribeReturn(Subscription subscription) {
// pass-thru by default
return subscription;
}
說白了,就是返回訂閱的Observer物件。
Observer與Subscriber的區別
它們的區別對於使用者來說主要有兩點:
- onStart(): 這是 Subscriber 增加的方法。它會在 subscribe 剛開始,而事件還未傳送之前被呼叫,可以用於做一些準備工作,例如資料的清零或重置。這是一個可選方法,預設情況下它的實現為空。需要注意的是,如果對準備工作的執行緒有要求(例如彈出一個顯示進度的對話方塊,這必須在主執行緒執行), onStart() 就不適用了,因為它總是在 subscribe 所發生的執行緒被呼叫,而不能指定執行緒。要在指定的執行緒來做準備工作,可以使用 doOnSubscribe() 方法,具體可以在後面的文中看到。
- unsubscribe(): 這是 Subscriber 所實現的另一個介面 Subscription 的方法,用於取消訂閱。在這個方法被呼叫後,Subscriber 將不再接收事件。一般在這個方法呼叫前,可以使用 isUnsubscribed() 先判斷一下狀態。 unsubscribe() 這個方法很重要,因為在 subscribe() 之後, Observable 會持有 Subscriber 的引用,這個引用如果不能及時被釋放,將有記憶體洩露的風險。所以最好保持一個原則:要在不再使用的時候儘快在合適的地方(例如 onPause() onStop() 等方法中)呼叫 unsubscribe() 來解除引用關係,以避免記憶體洩露的發生。
2) 建立 Observable
Observable 即被觀察者,它決定什麼時候觸發事件以及觸發怎樣的事件。 RxJava 使用 create() 方法來建立一個 Observable ,併為它定義事件觸發規則:
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,這裡傳入了一個 OnSubscribe 物件作為引數。OnSubscribe 會被儲存在返回的 Observable 物件中,它的作用相當於一個計劃表,當 Observable 被訂閱的時候,OnSubscribe 的 call() 方法會自動被呼叫,事件序列就會依照設定依次觸發(對於上面的程式碼,就是觀察者Subscriber 將會被呼叫兩次 onNext() 和一次 onCompleted())。這樣,由被觀察者呼叫了觀察者的回撥方法,就實現了由被觀察者向觀察者的事件傳遞,即觀察者模式。
3) Subscribe (訂閱)
建立了 Observable 和 Observer 之後,再用 subscribe() 方法將它們聯結起來,整條鏈子就可以工作了。程式碼形式很簡單:
observable.subscribe(observer);
// 或者:
observable.subscribe(subscriber);
Observable.subscribe(Subscriber) 的內部實現是這樣的(僅核心程式碼):
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:
- 呼叫 Subscriber.onStart() 。這個方法在前面已經介紹過,是一個可選的準備方法。
- 呼叫 Observable 中的 OnSubscribe.call(Subscriber) 。在這裡,事件傳送的邏輯開始執行。從這也可以看出,在 RxJava 中, Observable 並不是在建立的時候就立即開始傳送事件,而是在它被訂閱的時候,即當 subscribe() 方法執行的時候。
- 將傳入的 Subscriber 作為 Subscription 返回。這是為了方便 unsubscribe().
整個過程中物件間的關係如下圖:
除了 subscribe(Observer) 和 subscribe(Subscriber) ,subscribe() 還支援不完整定義的回撥,RxJava 會自動根據定義創建出 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");
}
};
// 自動建立 Subscriber ,並使用 onNextAction 來定義 onNext()
observable.subscribe(onNextAction);
// 自動建立 Subscriber ,並使用 onNextAction 和 onErrorAction 來定義 onNext() 和 onError()
observable.subscribe(onNextAction, onErrorAction);
// 自動建立 Subscriber ,並使用 onNextAction、 onErrorAction 和 onCompletedAction 來定義 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() 過程中最終會被轉換成 Subscriber 物件,因此,從某種程度上來說用 Subscriber 來代替 Observer ,這樣會更加嚴謹。
根據目前的經驗來看,Observer與Subscriber的主要區別在於onCompleted()方法執行完畢後是否取消了訂閱。
首先,我們分別定義mSubscriber 和 mObserver 。
如下程式碼:
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)執行結果就會有區別:
- mObserver可以重複使用,也就是subscribe(mObserver)可以重複訂閱;
- 當使用Observable.create方式建立Observable時,mSubscriber也能重複使用,也可以實現subscribe(mSubscriber)重複訂閱,但當使用Observable.just、Observable.from方式建立Observable時,mSubscriber不能重複使用,也就不能重複訂閱了(詳見文章:http://blog.csdn.net/jdsjlzx/article/details/54959052)。
- RxJava給出的建議是每次使用都new一個新的Subscriber 或者是使用Observer,也就是不要重複使用mSubscriber。
提醒:個人以為subscribe(mObserver)這個方式更適合分頁載入。
請注意,如果你每次都使用subscribe(new Subscriber< T>() {})方式實現訂閱,就不會出現上面的問題。
如下程式碼:
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);
}
});
}
當然,這個方式實現分頁載入也是可以的。至於哪個更好,還需要再驗證。
引用:
Subscriber是Observer介面的一個抽象實現;推薦使用Subscriber,實際上RxJava在subscibe過程中,會先將Observer轉換為一個Subscriber。