RxJava的Single、Completable以及Maybe

時間 2019-11-16

標籤 rxjava single completable 以及欄目 Java 简体版

原文原文鏈接

一般狀況下，若是咱們想要使用 RxJava 首先會想到的是使用Observable，若是要考慮到Backpressure的狀況，在 RxJava2.x 時代咱們會使用Flowable。除了Observable和Flowable以外，在 RxJava2.x 中還有三種類型的Observables：Single、Completable、Maybe。java

類型	描述
Observable	可以發射0或n個數據，並以成功或錯誤事件終止。
Flowable	可以發射0或n個數據，並以成功或錯誤事件終止。支持Backpressure，能夠控制數據源發射的速度。
Single	只發射單個數據或錯誤事件。
Completable	它歷來不發射數據，只處理 onComplete 和 onError 事件。能夠當作是Rx的Runnable。
Maybe	可以發射0或者1個數據，要麼成功，要麼失敗。有點相似於Optional

從上面的表格能夠看出，這五種被觀察者類型中只有Flowable能支持Backpressure，若是有須要Backpressure的狀況，仍是必需要使用Flowable。react

Single

從SingleEmitter的源碼能夠看出，Single 只有 onSuccess 和 onError 事件。express

/** * Copyright (c) 2016-present, RxJava Contributors. * * 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 io.reactivex;

import io.reactivex.annotations.*;
import io.reactivex.disposables.Disposable;
import io.reactivex.functions.Cancellable;

/** * Abstraction over an RxJava {@link SingleObserver} that allows associating * a resource with it. * <p> * All methods are safe to call from multiple threads. * <p> * Calling onSuccess or onError multiple times has no effect. * * @param <T> the value type to emit */
public interface SingleEmitter<T> {

    /** * Signal a success value. * @param t the value, not null */
    void onSuccess(@NonNull T t);

    /** * Signal an exception. * @param t the exception, not null */
    void onError(@NonNull Throwable t);

    /** * Sets a Disposable on this emitter; any previous Disposable * or Cancellation will be unsubscribed/cancelled. * @param s the disposable, null is allowed */
    void setDisposable(@Nullable Disposable s);

    /** * Sets a Cancellable on this emitter; any previous Disposable * or Cancellation will be unsubscribed/cancelled. * @param c the cancellable resource, null is allowed */
    void setCancellable(@Nullable Cancellable c);

    /** * Returns true if the downstream cancelled the sequence. * @return true if the downstream cancelled the sequence */
    boolean isDisposed();
}複製代碼

其中，onSuccess()用於發射數據(在Observable/Flowable中使用onNext()來發射數據)。並且只能發射一個數據，後面即便再發射數據也不會作任何處理。apache

Single的SingleObserver中只有onSuccess、onError，並無onComplete。這是 Single 跟其餘四種被觀察者最大的區別。api

Single.create(new SingleOnSubscribe<String>() {

            @Override
            public void subscribe(@NonNull SingleEmitter<String> e) throws Exception {

                e.onSuccess("test");
            }
        }).subscribe(new Consumer<String>() {
            @Override
            public void accept(@NonNull String s) throws Exception {
                System.out.println(s);
            }
        }, new Consumer<Throwable>() {
            @Override
            public void accept(@NonNull Throwable throwable) throws Exception {
                throwable.printStackTrace();
            }
        });複製代碼

上面的代碼，因爲Observer中有兩個Consumer，還能夠進一步簡化成網絡

Single.create(new SingleOnSubscribe<String>() {

            @Override
            public void subscribe(@NonNull SingleEmitter<String> e) throws Exception {

                e.onSuccess("test");
            }
        }).subscribe(new BiConsumer<String, Throwable>() {
            @Override
            public void accept(String s, Throwable throwable) throws Exception {
                System.out.println(s);
            }
        });複製代碼

Single 能夠經過toXXX方法轉換成Observable、Flowable、Completable以及Maybe。架構

Completable

Completable在建立後，不會發射任何數據。從CompletableEmitter的源碼能夠看到app

/** * Copyright (c) 2016-present, RxJava Contributors. * * 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 io.reactivex;

import io.reactivex.annotations.*;
import io.reactivex.disposables.Disposable;
import io.reactivex.functions.Cancellable;

/** * Abstraction over an RxJava {@link CompletableObserver} that allows associating * a resource with it. * <p> * All methods are safe to call from multiple threads. * <p> * Calling onComplete or onError multiple times has no effect. */
public interface CompletableEmitter {

    /** * Signal the completion. */
    void onComplete();

    /** * Signal an exception. * @param t the exception, not null */
    void onError(@NonNull Throwable t);

    /** * Sets a Disposable on this emitter; any previous Disposable * or Cancellation will be disposed/cancelled. * @param d the disposable, null is allowed */
    void setDisposable(@Nullable Disposable d);

    /** * Sets a Cancellable on this emitter; any previous Disposable * or Cancellation will be disposed/cancelled. * @param c the cancellable resource, null is allowed */
    void setCancellable(@Nullable Cancellable c);

    /** * Returns true if the downstream disposed the sequence. * @return true if the downstream disposed the sequence */
    boolean isDisposed();
}複製代碼

Completable 只有 onComplete 和 onError 事件，同時 Completable 並無map、flatMap等操做符，它的操做符比起 Observable/Flowable 要少得多。less

咱們能夠經過fromXXX操做符來建立一個Completable。這是一個Completable版本的Hello World。ide

Completable.fromAction(new Action() {
            @Override
            public void run() throws Exception {

                System.out.println("Hello World");
            }
        }).subscribe();複製代碼

Completable 常常會結合andThen操做符

Completable.create(new CompletableOnSubscribe() {
            @Override
            public void subscribe(@NonNull CompletableEmitter emitter) throws Exception {

                try {
                    TimeUnit.SECONDS.sleep(1);
                    emitter.onComplete();
                } catch (InterruptedException e) {
                    emitter.onError(e);
                }
            }
        }).andThen(Observable.range(1, 10))
        .subscribe(new Consumer<Integer>() {
            @Override
            public void accept(@NonNull Integer integer) throws Exception {
                System.out.println(integer);
            }
        });複製代碼

在這裏emitter.onComplete()執行完以後，代表Completable已經徹底執行完畢，接下來是執行andThen裏的操做。

打印結果以下：

在Completable中，andThen有多個重載的方法，正好對應了五種被觀察者的類型。

Completable andThen(CompletableSource next) <T> Maybe<T> andThen(MaybeSource<T> next) <T> Observable<T> andThen(ObservableSource<T> next) <T> Flowable<T> andThen(Publisher<T> next) <T> Single<T> andThen(SingleSource<T> next)複製代碼

Completable 也能夠經過toXXX方法轉換成Observable、Flowable、Single以及Maybe。

在網絡操做中，若是遇到更新的狀況，也就是Restful架構中的PUT操做，通常要麼返回原先的對象要麼只提示更新成功。下面兩個接口使用了Retrofit，分別是用於獲取短信驗證碼和更新用戶信息，其中更新用戶信息若是用PUT會更符合Restful的API。

/** * 獲取短信驗證碼 * @param param * @return */
    @POST("v1/user-auth")
    Completable getVerificationCode(@Body VerificationCodeParam param);

    /** * 用戶信息更新接口 * @param param * @return */
    @POST("v1/user-update")
    Completable update(@Body UpdateParam param);複製代碼

在model類中大體會這樣寫。

/** * Created by Tony Shen on 2017/7/24. */

public class VerificationCodeModel extends HttpResponse {

    /** * 獲取驗證碼 * @param activity * @param param * @return */
    public Completable getVerificationCode(AppCompatActivity activity, VerificationCodeParam param) {

        return apiService
                .getVerificationCode(param)
                .compose(RxJavaUtils.<VerificationCodeModel>completableToMain())
                .compose(RxLifecycle.bind(activity).<VerificationCodeModel>toLifecycleTransformer());
    }
}複製代碼

特別要注意的是getVerificationCode返回的是Completable而不是Completable 。

獲取驗證碼成功則給出相應地toast提示，失敗能夠作出相應地處理。

VerificationCodeModel model = new VerificationCodeModel();
model.getVerificationCode(RegisterActivity.this,param)
           .subscribe(new Action() {
                      @Override
                      public void run() throws Exception {
                              showShort(RegisterActivity.this,"發送驗證碼成功");
                      }
            },new RxException<Throwable>(){
                      @Override
                     public void accept(@NonNull Throwable throwable) throws Exception {
                              throwable.printStackTrace();
                              ......
                     }
            });複製代碼

Maybe

Maybe 是 RxJava2.x 以後纔有的新類型，能夠當作是Single和Completable的結合。

Maybe建立以後，MaybeEmitter 和 SingleEmitter 同樣並無onNext()方法，一樣須要經過onSuccess()方法來發射數據。

Maybe.create(new MaybeOnSubscribe<String>() {

            @Override
            public void subscribe(@NonNull MaybeEmitter<String> e) throws Exception {
                e.onSuccess("testA");
            }
        }).subscribe(new Consumer<String>() {

            @Override
            public void accept(@NonNull String s) throws Exception {

                System.out.println("s="+s);
            }
        });複製代碼

打印出來的結果是

s=testA複製代碼

Maybe也只能發射0或者1個數據，即便發射多個數據，後面發射的數據也不會處理。

Maybe.create(new MaybeOnSubscribe<String>() {

            @Override
            public void subscribe(@NonNull MaybeEmitter<String> e) throws Exception {
                e.onSuccess("testA");
                e.onSuccess("testB");
            }
        }).subscribe(new Consumer<String>() {

            @Override
            public void accept(@NonNull String s) throws Exception {

                System.out.println("s="+s);
            }
        });複製代碼

打印出來的結果仍然是

s=testA複製代碼

跟第一次執行的結果是一致的。

若是MaybeEmitter先調用了onComplete()，即便後面再調用了onSuccess()也不會發射任何數據。

Maybe.create(new MaybeOnSubscribe<String>() {

            @Override
            public void subscribe(@NonNull MaybeEmitter<String> e) throws Exception {
                e.onComplete();
                e.onSuccess("testA");
            }
        }).subscribe(new Consumer<String>() {

            @Override
            public void accept(@NonNull String s) throws Exception {

                System.out.println("s="+s);
            }
        });複製代碼

此次就沒有打印任何數據了。

咱們對上面的代碼再作一下修改，在subscribe()中也加入onComplete()，看看打印出來的結果會是這樣的？由於SingleObserver中是沒有onComplete()方法。

Maybe.create(new MaybeOnSubscribe<String>() {

            @Override
            public void subscribe(@NonNull MaybeEmitter<String> e) throws Exception {
                e.onComplete();
                e.onSuccess("testA");
            }
        }).subscribe(new Consumer<String>() {

            @Override
            public void accept(@NonNull String s) throws Exception {

                System.out.println("s=" + s);
            }
        }, new Consumer<Throwable>() {
            @Override
            public void accept(@NonNull Throwable throwable) throws Exception {

            }
        }, new Action() {
            @Override
            public void run() throws Exception {
                System.out.println("Maybe onComplete");
            }
        });複製代碼

此次打印的結果是

Maybe onComplete複製代碼

經過查看Maybe相關的源碼

@CheckReturnValue
    @SchedulerSupport(SchedulerSupport.NONE)
    public final Disposable subscribe(Consumer<? super T> onSuccess, Consumer<? super Throwable> onError, Action onComplete) {
        ObjectHelper.requireNonNull(onSuccess, "onSuccess is null");
        ObjectHelper.requireNonNull(onError, "onError is null");
        ObjectHelper.requireNonNull(onComplete, "onComplete is null");
        return subscribeWith(new MaybeCallbackObserver<T>(onSuccess, onError, onComplete));
    }複製代碼

咱們能夠獲得，Maybe在沒有數據發射時候subscribe會調用MaybeObserver的onComplete()。若是Maybe有數據發射或者調用了onError()，是不會再執行MaybeObserver的onComplete()。

咱們也能夠將 Maybe 轉換成Observable、Flowable、Single，只需相應地調用toObservable()、toFlowable()、toSingle()。

接下來咱們再來看看 Maybe 跟 Retrofit 是怎樣結合使用的？
下面的網絡請求，最初返回的類型是Flowable，可是這個網絡請求並非一個連續事件流，咱們只會發起一次 Post 請求返回數據而且只收到一個事件。所以，能夠考慮將 onComplete() 能夠跟 onNext() 合併。在這裏，嘗試咱們將Flowable改爲Maybe。

@POST("v1/contents")
    Maybe<ContentModel> loadContent(@Body ContentParam param);複製代碼

在model類中，咱們大體會這樣寫。

public class ContentModel extends HttpResponse {

    public List<ContentItem> data;

    /** * 獲取內容 * @param fragment * @param param * @param cacheKey * @return */
    public Maybe<ContentModel> getContent(Fragment fragment,ContentParam param,String cacheKey) {

        return apiService.loadContent(param)
                .compose(RxLifecycle.bind(fragment).<ContentModel>toLifecycleTransformer())
                .compose(RxJavaUtils.<ContentModel>maybeToMain())
                .compose(RxUtils.<ContentModel>toCacheTransformer(cacheKey,App.getInstance().cache));
    }

    ......
}複製代碼

其中，maybeToMain()方法是用Kotlin編寫的工具方法，這些工具方法由Kotlin來編寫會顯得比較簡單和清晰，特別是lambda表達式更加直觀。

@JvmStatic
    fun <T> maybeToMain(): MaybeTransformer<T, T> {

        return MaybeTransformer{
            upstream ->
            upstream.subscribeOn(Schedulers.io())
                    .observeOn(AndroidSchedulers.mainThread())
        }
    }複製代碼

最後是真正地使用model類，若是網絡請求成功則將數據展現到recyclerview上，若是失敗也會作出相應地處理。

model.getContent(this,param,cacheKey)
                .subscribe(new Consumer<ContentModel>() {
                    @Override
                    public void accept(@io.reactivex.annotations.NonNull ContentModel model) throws Exception {

                        adapter = new NewsAdapter(mContext, model);
                        recyclerview.setAdapter(adapter);
                        spinKitView.setVisibility(View.GONE);
                    }
                }, new RxException<Throwable>() {
                    @Override
                    public void accept(@NonNull Throwable throwable) throws Exception {
                        throwable.printStackTrace();
                        spinKitView.setVisibility(View.GONE);

                        ......
                    }
                });複製代碼