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<li><a href="#前言">前言</a></li>
<li><a href="#rxjava是什么">RxJava是什么</a>
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<li><a href="#reactive-x">Reactive X</a></li>
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<li><a href="#hot-or-cold-observables">“Hot” or “Cold” Observables</a></li>
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<li><a href="#rxjava入门">RxJava入门</a>
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<li><a href="#hello-world">Hello World</a></li>
<li><a href="#操作符-operators">操作符 Operators</a>
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<li><a href="#interval">Interval</a>
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<li><a href="#参数">参数</a></li>
</ul></li>
</ul></li>
<li><a href="#转换操作符">转换操作符</a>
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<li><a href="#buffer">Buffer</a></li>
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<li><a href="#背压操作符">背压操作符</a>
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<li><a href="#背压支持">背压支持</a></li>
<li><a href="#onbackpressurebuffer-onbackpressuredrop">onBackpressurebuffer/onBackpressureDrop</a></li>
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href="/hugo-theme-pure/2019/08/rxjava-guide/"
>RxJava入门</a
>
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<div class="article-meta">
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<a class="article-tag-link" href="/hugo-theme-pure/tags/rxjava/"> rxjava </a>
<a class="article-tag-link" href="/hugo-theme-pure/tags/%E5%93%8D%E5%BA%94%E5%BC%8F%E7%BC%96%E7%A8%8B/"> 响应式编程 </a>
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<div class="article-entry marked-body" itemprop="articleBody">
<blockquote>
<p>本文基于 rxjava 1.x 版本</p>
</blockquote>
<h2 id="前言">前言</h2>
<p>写这篇文章是因为之前在看Hystrix时,觉得响应式编程很有意思,之前也了解到Spring5主打特性就是响应式,就想来试试水,入个门.本文主要介绍RxJava的特点,入门操作</p>
<h2 id="rxjava是什么">RxJava是什么</h2>
<h3 id="reactive-x">Reactive X</h3>
<p><code>ReactiveX</code>是使用<code>Observable</code>序列来组合异步操作且基于事件驱动的一个库.其继承自<a href="https://zh.wikipedia.org/wiki/观察者模式">观察者模式</a>来支持数据流或者事件流通过添加操作符(operators)的方式来声明式的操作,并抽象出对低级别线程(low-level thread),同步,线程安全,并发数据结构,非阻塞IO问题的关注.</p>
<p>ReactiveX 在不同语言中都有实现,RxJava 只是在JVM上实现的一套罢了.</p>
<h3 id="概念">概念</h3>
<blockquote>
<p>观察者模式是该框架的灵魂~</p>
</blockquote>
<p><img src="https://i.loli.net/2019/08/21/PT56HZ2obO8vU3V.png" alt="WX20190821-171445.png" /></p>
<p>上图可以表述为: <strong>观察者</strong>(Observer) <strong>订阅</strong>(subscribe)<strong>被观察者</strong>(Observable),当Observable产生事件或数据时,会调用Observer的方法进行回调.</p>
<p>听起来有点别扭,这里举一个形象点的例子.</p>
<p><strong>显示器开关</strong></p>
<p>显示器开关即为 Observable, 显示器为 Observer,这两个组件就会形成联系.当开关按下时,显示器就会通电点亮,这里即可抽象成Observable发出一个事件,Observer对事件做了处理.做什么样的处理其实在Subscribe时就已经决定了.</p>
<p><strong>回调方法</strong></p>
<p>在subscribe时会要求实现对应的回调方法,标准方法有以下三个:</p>
<ul>
<li><strong>onNext</strong></li>
</ul>
<p>Observable调用这个方法发射数据，方法的参数就是Observable发射的数据，这个方法可能会被调用多次，取决于你的实现。</p>
<ul>
<li><strong>onError</strong></li>
</ul>
<p>当Observable遇到错误或者无法返回期望的数据时会调用这个方法，这个调用会终止Observable，后续不会再调用onNext和onCompleted，onError方法的参数是抛出的异常。</p>
<ul>
<li><strong>onCompleted</strong></li>
</ul>
<p>正常终止，如果没有遇到错误，Observable在最后一次调用onNext之后调用此方法。</p>
<h3 id="hot-or-cold-observables">“Hot” or “Cold” Observables</h3>
<p>Observable何时开始发送数据呢?基于此问题,可以将Observable分为两类: <code>Hot</code> & <code>Cold</code> . 可以理解为主动型和被动型.</p>
<p><strong>Hot Observable</strong>: Observable一经创建,就会开始发送数据. 所以后面订阅的Observer可能消费不到Observable完整的数据.</p>
<p><strong>Cold Observable</strong>: Observable会等到有Observer订阅时才开始发送数据,此时Observer会消费到完整的数据</p>
<h2 id="rxjava入门">RxJava入门</h2>
<h3 id="hello-world">Hello World</h3>
<pre><code class="language-java">Observable.create(new Observable.OnSubscribe<String>() {
@Override
public void call(Subscriber<? super String> subscriber) {
subscriber.onNext("Hello World");
subscriber.onCompleted();
//subscriber.onError(new RuntimeException("error"));
}
}).subscribe(new Subscriber<String>() {
@Override
public void onCompleted() {
System.out.println("观察结束啦~~~");
}
@Override
public void onError(Throwable e) {
System.out.println("观察出错啦~~~");
}
@Override
public void onNext(String s) {
System.out.println("onNext:" + s);
}
});
}
</code></pre>
<pre><code class="language-java">// onNext:Hello World
// 观察结束啦~~~
// 注释掉上一行 打开下一行注释 就会输出
// onNext:Hello World
// 观察出错啦~~~
</code></pre>
<p>上述即为一个标准的创建观察者被观察者并订阅,实现订阅逻辑.</p>
<p><strong>疑问</strong></p>
<ol>
<li>为什么<code>subscribe</code>方法的参数是<code>Subscriber</code>呢?</li>
</ol>
<p>在rxjava中Observer是接口,Subscriber实现了Observer并提供了拓展.所以普遍用这个.</p>
<ol>
<li>为什么是Observable.subscribe(Observer)?用上面的显示器开关的例子来说就相当于显示器开关订阅显示器.</li>
</ol>
<p>为了保证流式风格~rxjava提供了一系列的操作符来对Observable发出的数据做处理,流式风格可以使操作符使用起来更友好.所以就当做Observable订阅了Observer吧🤦♂</p>
<h3 id="操作符-operators">操作符 Operators</h3>
<p>单纯的使用上面的<code>Hello World</code>撸码只能说是观察者模式的运用罢了,操作符才是<code>ReactiveX</code>最强大的地方.我们可以通过功能不同的操作符对Observable发出的数据做过滤(filter),转换(map)来满足业务的需求.其实就可以当作是Java8的<code>lambda</code>特性.</p>
<blockquote>
<p>Observable在经过操作符处理后还是一个Observable,对应上述的<strong>流式风格</strong></p>
</blockquote>
<p>案例: 假设我们需要监听鼠标在一个直角坐标系中的点击,取得所有在第一象限点击的坐标.</p>
<p><img src="https://i.loli.net/2019/08/22/eQnOjomU7IufBpk.png" alt="marble.png" /></p>
<p>从该流程图可以看出,鼠标点击后会发出很多数据,一次点击一个点,我们对数据进行filter,得到了下方时间轴上的数据源.这就是我们想要的.下面来看下常用的操作符有哪些?</p>
<h4 id="创建型操作符">创建型操作符</h4>
<blockquote>
<p>用于创建Observable对象的操作符</p>
</blockquote>
<h5 id="create">Create</h5>
<p>创建一个Observable,需要传递一个<code>Function</code>来完成调用Observer的逻辑.</p>
<p>一个标准的Observable必须只能调用一次(<strong>Exactly Once</strong>)<code>onCompleted</code>或者<code>onError</code>,并且在调用后不能再调用Observer的其他方法(eg: onNext).</p>
<p><strong>sample code</strong></p>
<pre><code class="language-java">Observable.create(new Observable.OnSubscribe<Integer>() {
@Override
public void call(Subscriber<? super Integer> observer) {
try {
if (!observer.isUnsubscribed()) {
for (int i = 1; i < 5; i++) {
observer.onNext(i);
}
observer.onCompleted();
}
} catch (Exception e) {
observer.onError(e);
}
}
} ).subscribe(new Subscriber<Integer>() {
@Override
public void onNext(Integer item) {
System.out.println("Next: " + item);
}
@Override
public void onError(Throwable error) {
System.err.println("Error: " + error.getMessage());
}
@Override
public void onCompleted() {
System.out.println("Sequence complete.");
}
});
</code></pre>
<pre><code class="language-bash">Next: 1
Next: 2
Next: 3
Next: 4
Sequence complete.
</code></pre>
<h5 id="defer">Defer</h5>
<p>直到有Observer订阅时才会创建,并且会为每一个Observer创建新的Observable,这样可以保证所有Observer可以看到相同的数据,并且从头开始消费.</p>
<p><strong>sample code</strong></p>
<pre><code class="language-java">Observable<String> defer = Observable.defer(new Func0<Observable<String>>() {
@Override
public Observable<String> call() {
return Observable.just("Hello", "World");
}
});
defer.subscribe(new Subscriber<String>() {
@Override
public void onCompleted() {
System.out.println("第一个订阅完成啦~");
}
@Override
public void onError(Throwable e) {
System.out.println("第一个订阅报错啦~");
}
@Override
public void onNext(String s) {
System.out.println("第一个订阅收到:" + s);
}
});
defer.subscribe(new Subscriber<String>() {
//与上一个订阅逻辑相同
});
</code></pre>
<pre><code class="language-bash">第一个订阅收到:Hello
第一个订阅收到:World
第一个订阅完成啦~
第二个订阅收到:Hello
第二个订阅收到:World
第二个订阅完成啦~
</code></pre>
<p><strong>Note:</strong></p>
<p>Defer在RxJava中的实现其实有点像指派,可以看到构建时,传参为<code>Func0<Observable<T>></code>,Observer真正订阅的是传参中的Observable.</p>
<h5 id="just">Just</h5>
<p>在上文<code>Defer</code>中代码中就用了<code>Just</code>,指的是可以发送特定的数据.代码一致就不作展示了.</p>
<h5 id="interval">Interval</h5>
<p>可以按照指定时间间隔从0开始发送无限递增序列.</p>
<h6 id="参数">参数</h6>
<ul>
<li>initalDelay 延迟多长时间开始第一次发送</li>
<li>period 指定时间间隔</li>
<li>unit 时间单位</li>
</ul>
<p>如下例子:延迟0秒后开始发送,每1秒发送一次. 因为sleep 100秒,会发送0-99终止</p>
<p><strong>sample code</strong></p>
<pre><code class="language-java">Observable.interval(0,1,TimeUnit.SECONDS).subscribe(new Action1<Long>() {
// 这里只实现了OnNext方法,onError和onCompleted可以有默认实现.一种偷懒写法
@Override
public void call(Long aLong) {
System.out.println(aLong);
}
});
try {
//阻塞当前线程使程序一直跑
TimeUnit.SECONDS.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
</code></pre>
<h4 id="转换操作符">转换操作符</h4>
<blockquote>
<p>将Observable发出的数据进行各类转换的操作符</p>
</blockquote>
<h5 id="buffer">Buffer</h5>
<p><img src="https://i.loli.net/2019/08/22/IC3vbhSDGgUYku5.png" alt="buffer.png" /></p>
<p>如上图所示,buffer定期将数据收集到集合中,并将集合打包发送.</p>
<p><strong>sample code</strong></p>
<pre><code class="language-java">Observable.just(2,3,5,6)
.buffer(3)
.subscribe(new Action1<List<Integer>>() {
@Override
public void call(List<Integer> integers) {
System.out.println(integers);
}
});
</code></pre>
<pre><code class="language-bash">[2, 3, 5]
[6]
</code></pre>
<p><strong>Window</strong></p>
<p>window和buffer是非常像的两个操作符,区别在于buffer会将存起来的item打包再发出去,而window则只是单纯的将item堆起来,达到阈值再发出去,不对原数据结构做修改.</p>
<p><img src="https://i.loli.net/2019/08/22/kqIb5j6xfPDdOZn.png" alt="window.png" /></p>
<p><strong>sample code</strong></p>
<pre><code class="language-java">Observable.just(2,3,5,6)
.window(3)
.subscribe(new Action1<Observable<Integer>>() {
@Override
public void call(Observable<Integer> integerObservable) {
integerObservable.subscribe(new Action1<Integer>() {
@Override
public void call(Integer integer) {
// do anything
}
});
}
});
</code></pre>
<h4 id="合并操作符">合并操作符</h4>
<blockquote>
<p>将多个Observable合并为一个的操作符</p>
</blockquote>
<h5 id="zip">Zip</h5>
<p>使用一个函数组合多个Observable发射的数据集合，然后再发射这个结果。如果多个Observable发射的数据量不一样，则以最少的Observable为标准进行组合.</p>
<p><img src="https://i.loli.net/2019/08/22/Nu4TCMEHKGA3Y1V.png" alt="zip.png" /></p>
<p><strong>sample code</strong></p>
<pre><code class="language-java">Observable<Integer> observable1=Observable.just(1,2,3,4);
Observable<Integer> observable2=Observable.just(4,5,6);
Observable.zip(observable1, observable2, new Func2<Integer, Integer, String>() {
@Override
public String call(Integer item1, Integer item2) {
return item1+"and"+item2;
}
}).subscribe(new Action1<String>() {
@Override
public void call(String s) {
System.out.println(s);
}
});
</code></pre>
<pre><code class="language-bash">1and4
2and5
3and6
</code></pre>
<h4 id="背压操作符">背压操作符</h4>
<blockquote>
<p>用于平衡Observer消费速度,Observable生产速度的操作符</p>
</blockquote>
<p>背压是指在异步场景中,被观察者发送事件速度远快于观察者的处理速度的情况下，一种告诉上游的被观察者降低发送速度的策略.下图可以很好阐释背压机制是如何运行的.</p>
<p><img src="https://i.loli.net/2019/09/02/4TyQFgBkIAO1R2c.png" alt="backpressure.png" /></p>
<p>宗旨就是<strong>下游告诉上游我能处理多少你就给我发多少.</strong></p>
<pre><code class="language-java">//被观察者将产生100000个事件
Observable observable=Observable.range(1,100000);
observable.observeOn(Schedulers.newThread())
.subscribe(new Subscriber() {
@Override
public void onCompleted() {
}
@Override
public void onError(Throwable e) {
}
@Override
public void onNext(Object o) {
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("on Next Request...");
request(1);
}
});
</code></pre>
<h5 id="背压支持">背压支持</h5>
<p>上述样例代码中创建Observable使用的是<code>range</code>操作符,这是因为他是支持背压的,如果用<code>interval</code>,request的方法将不起作用.因为<code>interval</code>不支持背压.那什么样的Observable支持背压呢?</p>
<p>在前面介绍概念时,有提到过<code>Hot</code>&<code>Cold</code>的区别,<code>Hot</code>类型的Observable,即一经创建就开始发送,<strong>不支持</strong>背压,<code>Cold</code>类型的Observable也只是<strong>部分</strong>支持.</p>
<h5 id="onbackpressurebuffer-onbackpressuredrop">onBackpressurebuffer/onBackpressureDrop</h5>
<p>不支持背压的操作符我们可以如何实现背压呢?就通过<code>onBackpressurebuffer/onBackpressureDrop</code>来实现.顾名思义一个是缓存,一个是丢弃.</p>
<p>这里以<code>drop</code>方式来展示.</p>
<pre><code class="language-java">Observable.interval(1, TimeUnit.MILLISECONDS)
.onBackpressureDrop()
//指定observer调度io线程上,并将缓存size置为1,这个缓存会提前将数据存好在消费,
//默认在PC上是128,设置小一点可以快速的看到drop的效果
.observeOn(Schedulers.io(), 1)
.subscribe(new Subscriber<Long>() {
@Override
public void onCompleted() {
}
@Override
public void onError(Throwable e) {
System.out.println("Error:" + e.getMessage());
}
@Override
public void onNext(Long aLong) {
System.out.println("订阅 " + aLong);
try {
TimeUnit.MILLISECONDS.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
})
</code></pre>
<pre><code class="language-bash">订阅 0
订阅 103
订阅 207
订阅 300
订阅 417
订阅 519
订阅 624
订阅 726
订阅 827
订阅 931
订阅 1035
订阅 1138
订阅 1244
订阅 1349
</code></pre>
<p>可以很明显的看出很多数据被丢掉了,这就是背压的效果.</p>
<h2 id="总结">总结</h2>
<p>写了这么多后,想来说说自己的感受.</p>
<ol>
<li>转变思想: 响应式编程的思想跟我们现在后端开发思路是有区别的.可能刚开始会不适应.</li>
<li>不易调试: 流式风格写着爽,调着难</li>
</ol>
<h2 id="参考">参考</h2>
<p><a href="http://reactivex.io/documentation/">ReactiveX官网</a></p>
<p><a href="https://zhuanlan.zhihu.com/p/24473022">关于RxJava最友好的文章——背压（Backpressure)</a></p>
<p><a href="http://zhangtielei.com/posts/blog-rxjava-backpressure.html">如何形象地描述RxJava中的背压和流控机制？</a></p>
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