RxJava 介紹與 Android 中的 RxJava ⿈黃千碩 (Kros) oSolve Ltd. / Wish8 Co,. Ltd. Mobile App Developer 

Outline • RxJava Introduction • Observable • Operators • Subject • Scheduler • Android Lifecycle • Testing • Performance 

What is RxJava? • RxJava 是 Reactive X (Reactive Extensions) 對 Java VM 的實作 • 是⼀一個 Library，利⽤用資料流 (observable sequences) 來處理 「asynchronous 與 event- base 」類型的程式。 • 主要⾓角⾊色為：observable 與 observer 。 

What is Reactive? • 翻譯：「響應式」「反應式」開發 • 把資料 (data) 或是事件 (event) 變成「可觀察」 (observer pattern) 的「資料流」。 • 並加上運算元 (operators) 來操作這些資料。 

What is FRP ? • FRP - Functional Reactive Programming. • Reactive 是⺫⽬目的 • 為了能讓開發者不落⼊入如何處理（事件）資料的 繁雜程式邏輯中，利⽤用 函數式 (Functional) 的⽅方 法來處理資料流 • filter(), map(), flatMap(), …etc. 

Why FRP? • Concurrency • thread 的控管複雜 • Asynchronous Programming • 為追求 60fps，許多事情我們會丟到背景處理 • Callback Hell • 當 Callback 太多時，眼睛都花了。 

– 林信良 “若開發者是以務實且不斷提升作為⾃自我期許，更 ⾼高階的抽象化作法將會是必修的課題” 

Observable • Observable 為發射資料的⼈人 

Create Observable • Observable.just() • Observable.from() • …etc. 

Observable.just() • 把「資料」轉變成 Observable。 Observable.just("Hello World!") 

Observer • Observer 為對這些資料有興趣的⼈人 • 透過 subscribe method 連結 observer 與 observable. • Observer 透過 subscribe 來監聽⼀一個 Observable. 

Subscribe • 連結 observable 與 observer • 通常必須實作 subscribe 的 interface. • onNext, onError, onComplete public final Subscription subscribe(final Action1 onNext, final Action1 onError, final Action0 onComplete) { /* ... */ } 

>>>>>>>>>>>>>>>>>>> s:Hello World! Observable.just("Hello World!").subscribe(new Action1() {
 @Override
 public void call(String s) {
 System.out.println(">>>>>>>>>>>>>>>>>>> s:" + s);
 }
 }, new Action1() {
 @Override
 public void call(Throwable throwable) {
 }
 }, new Action0() {
 @Override
 public void call() {
 }
 }); 

>>>>>>>>>>>>>>>>>>> s:Hello World! Observable.just("Hello World!").subscribe(new Action1() {
 @Override
 public void call(String s) {
 System.out.println(">>>>>>>>>>>>>>>>>>> s:" + s);
 }
 }, new Action1() {
 @Override
 public void call(Throwable throwable) {
 }
 }, new Action0() {
 @Override
 public void call() {
 }
 }); Observable.just("Hello World!").subscribe(new Action1() {
 @Override
 public void call(String s) {
 System.out.println(">>>>>>>>>>>>>>>>>>> s:" + s);
 }
 }); 可以只實作感興趣的 callback 

>>>>>>>>>>>>>>>>>>> s:Hello World! Observable.just("Hello World!").subscribe(new Action1() {
 @Override
 public void call(String s) {
 System.out.println(">>>>>>>>>>>>>>>>>>> s:" + s);
 }
 }, new Action1() {
 @Override
 public void call(Throwable throwable) {
 }
 }, new Action0() {
 @Override
 public void call() {
 }
 }); Observable.just("Hello World!").subscribe(new Action1() {
 @Override
 public void call(String s) {
 System.out.println(">>>>>>>>>>>>>>>>>>> s:" + s);
 }
 }); Observable.just("Hello World!").subscribe(s -> {
 System.out.println(">>>>>>>>>>>>>>>>>>> s:" + s);
 }); 套⽤用 retrolambda，採⽤用 java8 lambda，讓程式碼更簡潔 

Observable.from • 把「⼀一包資料」轉變成 Observable。⽽而這個 Observable 每次只發射資料中的單⼀一資料 Observable.from(listOfIntegers) 

Observable.from >>>>>>>>>>>>>>>>>>> integer:1 >>>>>>>>>>>>>>>>>>> integer:2 >>>>>>>>>>>>>>>>>>> integer:3 >>>>>>>>>>>>>>>>>>> integer:4 >>>>>>>>>>>>>>>>>>> integer:5 >>>>>>>>>>>>>>>>>>> integer:6 >>>>>>>>>>>>>>>>>>> integer:7 List integers = new ArrayList<>();
 integers.add(1);
 // ...
 integers.add(7); 
 Observable.from(integers).subscribe(integer -> {
 System.out.println(">>>>>>>>>>>>>>>>>>> integer:" + integer);
 }); 

“Hot” and “Cold” Observable • Observable 什麼時候會發射資料呢？ • Hot observable • 當它⼀一建⽴立時就會發射資料 • Cold observable • 當有 observer subscribe 時，才會發射資料 

Operators • Creating Observables (ex: create, from, just, …) • Transforming Observables (ex: map, flatMap, …) • Filtering Observables • Combining Observables • Error Handling Operators • Observable Utility Operators • ……etc. 

Observable.just("Hello World!").map(s -> s + " Android Taipei")
 .subscribe(s -> {
 System.out.println(">>>>>>>>>>>>>>>>>>> s:" + s);
 }); Observable.from(integers)
 .map(integer -> integer + 10)
 .subscribe(integer -> {
 System.out.println(">>>>>>>>>>>>>>>>>>> integer:" + integer);
 }); >>>>>>>>>>>>>>>>>>> s:Hello World! Android Taipei >>>>>>>>>>>>>>>>>>> integer:11 >>>>>>>>>>>>>>>>>>> integer:12 >>>>>>>>>>>>>>>>>>> integer:13 >>>>>>>>>>>>>>>>>>> integer:14 >>>>>>>>>>>>>>>>>>> integer:15 >>>>>>>>>>>>>>>>>>> integer:16 >>>>>>>>>>>>>>>>>>> integer:17 對 "Hello World!" 加⼯工 對 list 中每個 element 加⼯工 

台北市 公廁查詢系統 利⽤用台北市政府 Open Data http://data.taipei/ ⺫⽬目的：找尋離我最近的公廁 

http://data.taipei/ 

Callback 版本 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 void listToiletCallback(@Query("rid") String rid, 
 @Query("scope") String scope,
 @Query("limit") int limit,
 @Query("offset") int offset,
 Callback callback); 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 void listToiletCallback(@Query("rid") String rid, 
 @Query("scope") String scope,
 @Query("limit") int limit,
 @Query("offset") int offset,
 Callback callback); @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE); fetchNearestToilet(); } 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 void listToiletCallback(@Query("rid") String rid, 
 @Query("scope") String scope,
 @Query("limit") int limit,
 @Query("offset") int offset,
 Callback callback); @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE); fetchNearestToilet(); } ⺫⽬目的：找出距離我 5 km 以內的公廁， 並按照遠近排序 

private void fetchNearestToilet() {
 apiService.listToiletCallback(RID, SCOPE, 500, 0, new Callback() {
 @Override
 public void success(ApiResponse apiResponse, Response response) {
 List filtered = new ArrayList<>();
 for (Toilet toilet : apiResponse.getResult().getToilets()) {
 if (lessThan5Km(toilet)) {
 filtered.add(toilet);
 }
 }
 Collections.sort(filtered, new Comparator() {
 @Override
 public int compare(Toilet lhs, Toilet rhs) {
 return compareDistance(lhs, rhs);
 }
 });
 adapter.reset(filtered);
 progressBar.setVisibility(View.GONE);
 }
 
 @Override
 public void failure(RetrofitError error) {
 progressBar.setVisibility(View.GONE);
 ViewHelper.showError(getActivity(), error);
 }
 });
 } 

RxJava 版本 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 Observable listToilet(@Query("rid") String rid, 
 @Query("scope") String scope, 
 @Query("limit") int limit, 
 @Query("offset") int offset); 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 Observable listToilet(@Query("rid") String rid, 
 @Query("scope") String scope, 
 @Query("limit") int limit, 
 @Query("offset") int offset); @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe((toilets) -> {
 adapter.reset(toilets);
 },
 throwable -> ViewHelper.showError(getActivity(), throwable)); } } 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 Observable listToilet(@Query("rid") String rid, 
 @Query("scope") String scope, 
 @Query("limit") int limit, 
 @Query("offset") int offset); @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe((toilets) -> {
 adapter.reset(toilets);
 },
 throwable -> ViewHelper.showError(getActivity(), throwable)); } } 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 Observable listToilet(@Query("rid") String rid, 
 @Query("scope") String scope, 
 @Query("limit") int limit, 
 @Query("offset") int offset); @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable)); } Java 8 的 method reference 

// - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 Observable listToilet(@Query("rid") String rid, 
 @Query("scope") String scope, 
 @Query("limit") int limit, 
 @Query("offset") int offset); @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable)); } 

private Observable> fetchNearestToilet() {
 return apiService.listToilet(RID, SCOPE, 500, 0)
 .flatMap(response -> Observable.from(response.getResult().getToilets()))
 .filter(this::lessThan5Km)
 .toSortedList(this::compareDistance);
 } // - 跟 Server 抓取公廁資料 @GET("/apiAccess")
 Observable listToilet(@Query("rid") String rid, 
 @Query("scope") String scope, 
 @Query("limit") int limit, 
 @Query("offset") int offset); @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable));
 } } 

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Subject • 翻譯：主題 • A Subject is a sort of bridge or proxy that is available in some implementations of ReactiveX that acts both as an observer and as an Observable. • Subject 可以是發送 event 的⼈人 (observable)， 也可以是註冊 event 的⼈人 (observer)。 • ⽤用途：Event Bus 

Subject • Subject 有很多種： • AsyncSubject • BehaviorSubject • PublishSubject • ReplaySubject 

Publish Subject • 會發送給每個 observers • 只會接收到 subscribe 之後的 event 

Subject 

Activity 1 Subject 

Activity 1 Subject subject.subscribe(); 

Activity 1 Subject startActivity(); subject.subscribe(); 

Activity 1 Activity 2 Subject startActivity(); subject.subscribe(); 

Activity 1 Activity 2 Subject startActivity(); // Do something… subject.onNext(Event); finish(); subject.subscribe(); 

Activity 1 Activity 2 Subject startActivity(); subject.subscribe(); // Do something… subject.onNext(Event); finish(); 

Activity 1 Activity 2 Subject startActivity(); subject.subscribe(); // Do something… subject.onNext(Event); finish(); 

Activity 1 Activity 2 Subject startActivity(); subject.subscribe(); // Do something… subject.onNext(Event); finish(); 

Activity 1 Activity 2 Subject startActivity(); subject.subscribe(); // Do something… subject.onNext(Event); finish(); 

Activity 2 Subject startActivity(); subject.subscribe(); // Do something… subject.onNext(Event); finish(); Activity 1 (with Event) 

Activity 1 (with Event) Activity 2 Subject startActivity(); subject.subscribe(); // Do something… subject.onNext(Event); finish(); 

Subject subject.subscribe(); Activity 1 (with Event) 

Subject subject.subscribe(); Activity 1 (with Event) Activity 2 (with Event) Activity 3 (with Event) subject.subscribe(); subject.subscribe(); 可以很多⼈人註冊 

Scheduler • If you want to introduce multithreading into your cascade of Observable operators, you can do so by instructing those operators (or particular Observables) to operate on particular Schedulers. • 可以利⽤用 Scheduler 來實作 thread 的切換。 

Scheduler @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable));
 } } 

Android Lifecycle • Activity 與 Fragment 都有各⾃自的 lifecycle. • Activity, onCreate(), onResume(), onPause(), onDestory(), ..etc. • Fragment, onCreate(), onCreateView(), onResume(), onPause(), onDestory(), ..etc • 如果 Activity/Fragment 被 destroy 時，你的 async task 還沒做完怎麼辦？ 

Android Lifecycle • 會導致 Memory leak 或是 NPE. • Activity 與 Fragment 都有各⾃自的 lifecycle. • Activity, onCreate(), onResume(), onPause(), onDestory(), ..etc. • Fragment, onCreate(), onCreateView(), onResume(), onPause(), onDestory(), ..etc • 如果 Activity/Fragment 被 destroy 時，你的 async task 還沒做完怎麼辦？ 

Android Lifecycle • Import RxJava Android 版
 compile 'io.reactivex:rxandroid:0.25.0' • 使⽤用 Android 相關的 observable 與 event.
 rx.android.lifecycle.LifecycleObservable
 rx.android.lifecycle.LifecycleEvent 

@Override
 protected void onStart() {
 super.onStart();
 lifecycleSubject.onNext(LifecycleEvent.START);
 }
 
 @Override
 protected void onResume() {
 super.onResume();
 lifecycleSubject.onNext(LifecycleEvent.RESUME);
 }
 
 @Override
 protected void onPause() {
 lifecycleSubject.onNext(LifecycleEvent.PAUSE);
 super.onPause();
 }
 
 @Override
 protected void onStop() {
 lifecycleSubject.onNext(LifecycleEvent.STOP);
 super.onStop();
 }
 
 @Override
 protected void onDestroy() {
 lifecycleSubject.onNext(LifecycleEvent.DESTROY);
 super.onDestroy();
 } private final BehaviorSubject lifecycleSubject = BehaviorSubject.create(); public class BaseActivity extends AppCompatActivity { } 

public class BaseActivity extends AppCompatActivity { /* reset code */ public Observable lifecycle() {
 return lifecycleSubject.asObservable();
 } protected Observable bind(Observable observable) {
 return LifecycleObservable.bindActivityLifecycle(lifecycle(),
 observable.observeOn(AndroidSchedulers.mainThread()));
 } /* reset code */ } 

@Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 bind(fetchNearestToilet())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable));
 } } @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable));
 } } bind() 的功能：當 fragment 被 destroyed 時，會⾃自動 unsubscribe 此 observable. 

@Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 fetchNearestToilet()
 .observeOn(AndroidSchedulers.mainThread())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable));
 } } @Override
 public void onViewCreated(View view, Bundle savedInstanceState) {
 super.onViewCreated(view, savedInstanceState);
 progressBar.setVisibility(View.VISIBLE);
 bind(fetchNearestToilet())
 .finallyDo(() -> progressBar.setVisibility(View.GONE))
 .subscribe(adapter::reset,
 throwable -> ViewHelper.showError(getActivity(), throwable));
 } } bind() 的功能：當 fragment 被 destroyed 時，會⾃自動 unsubscribe 此 observable. 

Testing • 測試容易 • 簡易的⽅方法, toBlocking() • 正規的⽅方法, TestSubscriber() 

public class AccountDaemon { public Observable login(final Account account) {
 return Observable.just(account).map(account1 -> {
 checkAccount(account);
 return accountService.login(account);
 });
 } private void checkAccount(Account account) throws IllegalArgumentException {
 if (TextUtils.isEmpty(account.getEmail())
 || TextUtils.isEmpty(account.getPassword())) {
 throw new IllegalArgumentException("Email or password can not be empty.");
 }
 } } public class Account {
 private final String email;
 private final String password;
 public static Account createLoginAccount(final String email, 
 final String password) {
 return new Account(email, password);
 }
 // rest implementation… } 

public void testLogin_empty_email() throws Exception {
 Account account = Account.createLoginAccount(null, "password");
 try {
 accountDaemon.login(account).toBlocking().single();
 fail("method should throw exception");
 } catch (Throwable ex) {
 assertEquals("Email or password can not be empty.", ex.getLocalizedMessage());
 }
 } // Official way public void testLogin_using_test_subscriber() {
 TestSubscriber testSubscriber = new TestSubscriber<>();
 Account account = Account.createLoginAccount("email", "password");
 accountDaemon.login(account).subscribe(testSubscriber);
 
 Account expect = Account.createLoginAccount("email", "password");
 testSubscriber.assertNoErrors();
 testSubscriber.assertValue(expect);
 } // Blocking way public void testLogin() throws Exception {
 Account account = Account.createLoginAccount("email", "password");
 Account result = accountDaemon.login(account).toBlocking().single();
 assertEquals("email", result.getEmail());
 assertEquals("password", result.getPassword());
 } 

Performance 

public void testPerformance_rx() {
 List data = new ArrayList<>();
 for (int i = 0; i < 100000; ++i) {
 data.add(i);
 }
 List result = Observable.from(data)
 .filter(integer -> integer % 2 == 0).toList().toBlocking().first();
 assertEquals(100000 / 2, result.size());
 } public void testPerformance_for_loop() {
 List data = new ArrayList<>();
 for (int i = 0; i < 100000; ++i) {
 data.add(i);
 }
 List result = new ArrayList<>();
 for (int i = 0, size = data.size(); i < size; i++) {
 if (i % 2 == 0) {
 result.add(i);
 }
 }
 assertEquals(100000 / 2, result.size());
 } 

不是每個語⾔言的 Rx 版本效 能都很好，導⼊入時需注意 例如:ReactiveCocoa 

RAC-ReactiveCocoa - (void)testRACPerformance {
 NSArray *array = [self getTestArray];
 [self measureBlock:^{
 RACSequence *sequence = [array.rac_sequence filter:^BOOL(NSNumber *number) {
 return number.intValue % 2 == 0;
 }];
 NSArray *results = sequence.array;
 XCTAssertEqualObjects(@(100000 / 2), @(results.count));
 }];
 } - (void)testNativePerformance {
 NSArray *array = [self getTestArray];
 [self measureBlock:^{
 NSMutableArray *results = [NSMutableArray array];
 for (int i = 0; i < array.count; ++i) {
 NSNumber *number = array[i];
 if (number.intValue % 2 == 0) {
 [results addObject:number];
 }
 }
 XCTAssertEqualObjects(@(100000 / 2), @(results.count));
 }];
 } 

優缺點 • 優點 • 程式碼清楚，簡潔 • 容易進⾏行 Asynchronous Programming • 缺點 • 學習成本⾼高(map????, flatMap?????, amb???) • ⼊入侵式的，所有 API 被迫改成 Observable 

public void fetchUserProfile() {
 // code
 }
 
 public void fetchFriends() {
 // code
 }
 
 public void fetchShippingInfo() {
 // code
 } public Observable fetchUserProfile() {
 // code
 }
 
 public Observable> fetchFriends() {
 // code
 }
 
 public Observable fetchShippingInfo() {
 // code
 } 

Reference • ReactiveX
 http://reactivex.io/ • FRP與函數式-林信良
 http://www.ithome.com.tw/voice/91328 • RxJava Android Patterns
 http://stablekernel.com/blog/replace-asynctask-asynctaskloader-rx- observable-rxjava-android-patterns/ • Architecting Android…The evolution
 http://fernandocejas.com/2015/07/18/architecting-android-the-evolution/ • Unit Testing RxJava Observables
 https://medium.com/ribot-labs/unit-testing-rxjava-6e9540d4a329 • Demo Project
 https://github.com/ch8908/rxjava-demo