创建Observables

https://github.com/ReactiveX/RxJava/wiki/Creating-Observables#create

1：Observable.fromAction使用效果：

Observable<Integer> observable = Observable.fromAction(
                () -> {
                    System.out.println("我去，被执行了");
                }
        );
        observable.subscribe(new Observer<Integer>() {
            @Override
            public void onSubscribe(@NonNull Disposable d) {
                System.out.println("onSubscribe1");
            }

            @Override
            public void onNext(@NonNull Integer integer) {
                System.out.println("onNext1" + integer);
            }

            @Override
            public void one rror(@NonNull Throwable e) {
                System.out.println("onError1");
            }

            @Override
            public void onComplete() {
                System.out.println("onComplete1");
            }
        });
        observable.subscribe(new Observer<Integer>() {
            @Override
            public void onSubscribe(@NonNull Disposable d) {
                System.out.println("onSubscribe2");
            }

            @Override
            public void onNext(@NonNull Integer integer) {
                System.out.println("onNext2" + integer);
            }

            @Override
            public void one rror(@NonNull Throwable e) {
                System.out.println("onError2");
            }

            @Override
            public void onComplete() {
                System.out.println("onComplete2");
            }
        });

onSubscribe1
我去，被执行了
onComplete1
onSubscribe2
我去，被执行了
onComplete2

2：Observable.just(有限个item)

内部使用fromArray()

3：Observable.create()

Observable<String> observable1 = Observable.create((emitter -> {
            emitter.onNext("a");
            emitter.onNext("b");
            emitter.onNext("c");
            emitter.onNext("d");
            emitter.onComplete();这边调用onNext等方法
        }));

        observable1.subscribe(new Observer<String>() {
            @Override
            public void onSubscribe(@NonNull Disposable d) {
                System.out.println("onSubscribe");
            }

            @Override
            public void onNext(@NonNull String s) {
                System.out.println("onNext");
            }

            @Override
            public void one rror(@NonNull Throwable e) {
                System.out.println("onError");
            }

            @Override
            public void onComplete() {
                System.out.println("onComplete");
            }
        });

官方例子：

ScheduledExecutorService executor = Executors.newSingleThreadScheduledExecutor();

        ObservableOnSubscribe<String> handler = emitter -> {

            Future<Object> future = executor.schedule(() -> {
                emitter.onNext("Hello");
                emitter.onNext("World");
                return "result";
            }, 3, TimeUnit.SECONDS);

            String result = (String) future.get();//阻塞
            emitter.onNext(result);
            emitter.onComplete();//onComplete之后调用onNext无用
            emitter.setCancellable(() -> future.cancel(false));
        };

        Observable<String> observable = Observable.create(handler);

        observable.subscribe(item -> System.out.println(item), error -> error.printStackTrace(),
                () -> System.out.println("Done"));
        
        executor.shutdown();

 注：需要注意回调的调用时机

 Observable.create(ObservableOnSubscribe<Int> { emitter ->
                    log("create ${Thread.currentThread().name}")
                    emitter.onNext(1)
                    emitter.onNext(2)
                    emitter.onNext(3)
                    emitter.onComplete()
                }).compose(RxUtil.ioAndMainObservable())
                    .subscribe(object : Observer<Int> {
                        override fun onSubscribe(d: Disposable) {
                          log("onSubscribe ${Thread.currentThread().name}")
                        }

                        override fun onNext(t: Int) {
                            log("onNext ${Thread.currentThread().name}")
                        }

                        override fun one rror(e: Throwable) {
                            log("onError ${Thread.currentThread().name}")
                        }

                        override fun onComplete() {
                            log("onComplete ${Thread.currentThread().name}")
                        }
                    })

onSubscribe main
create RxCachedThreadScheduler-1
onNext main
onNext main
onComplete main

就是说，onScubcribe在emitter的那段代码之前被调用，并且subscribeOn切换的线程只是emitter中的，不影响onSubscribe，onSubscribe的线程和onNext一样。

4：处理异常

官方例子：https://github.com/ReactiveX/RxJava/wiki/How-To-Use-RxJava#error-handling

Hot & Cold Observable

When does an Observable begin emitting its sequence of items? It depends on the Observable. A “hot” Observable may begin emitting items as soon as it is created, and so any observer who later subscribes to that Observable may start observing the sequence somewhere in the middle. A “cold” Observable, on the other hand, waits until an observer subscribes to it before it begins to emit items, and so such an observer is guaranteed to see the whole sequence from the beginning.

In some implementations of ReactiveX, there is also something called a “Connectable” Observable. Such an Observable does not begin emitting items until its Connect method is called, whether or not any observers have subscribed to it.

Scheduler

https://reactivex.io/documentation/scheduler.html

巨人的肩膀：

一些很重要的概念：https://reactivex.io/documentation/observable.html