# RxJava概念

主要的想法是觀察者模式(Observer Pattern)，由觀察者(Observer)及被觀察者(Observable)組成

***觀察者***透過註冊/訂閱(Subscribe)***被觀察者***，監控被觀察者。***被觀察者***只要偵測到變化就會通知***觀察者***

***快速判斷***：要判斷觀察者和被觀察者的角色分別，就看***哪一個是會先改變的***，會先改變的那個就是***被觀察者***，接收被觀察者通知才改變的就是***觀察者***。

在Java的寫法和觀念顛倒：

```
observable.subscribe(observer)
```

## Observer

針對Observable的通知行為應該要有些什麼反應

```java
Observer<String> observe = new Observer<String> {
    @Override
    public void onNext(String s) {
        Log.d(TAG, "Item: " + s);
    }
    @Override
    public void onCompleted() {
        Log.d(TAG, "Completed!");
    }
    @Override
    public void onError(Throwable e) {
        Log.d(TAG, e.getMessage());
    }
}
```

onNext() -> 接到通知以後會逐條執行的行為

onCompleted() -> 全部onNext()執行完會去執行的方法

onError() -> 執行途中發生錯誤會去執行的方法

## Observable

被觀察者如何通知觀察者

```java
Observable observable = Observable.create(new Observable.onSubscribe<String>(){
    @Override
    public void call(Subscriber<? super String> subscriber) {
        subscriber.onNext("Hello!");
        subscriber.onNext("World!");
        subscriber.onCompleted();
    }
})
```

一旦被訂閱就會觸發call()

## Subscribe

```java
observable.subscribe(observer)
```


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