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<h1>浅谈Rxjava</h1>
2015-12-14
by <a class="url fn" href="http://www.codemixwine.com/author/slock.html">Slock</a>
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<div><p><strong>RxJava</strong>正在Android开发者中变的越来越流行。唯一的问题就是上手不容易，尤其是大部分人之前都是使用命令式编程语言。但是一旦你弄明白了，你就会发现RxJava真是太棒了。
这里仅仅是帮助你了解RxJava，整个系列共有四篇文章，希望你看完这四篇文章之后能够了解RxJava背后的思想，并且喜欢上RxJava。</p>
<h2>基础</h2>
<p>RxJava最核心的两个东西是Observables（被观察者，事件源）和Subscribers（观察者）。Observables发出一系列事件，Subscribers处理这些事件。这里的事件可以是任何你感兴趣的东西（触摸事件，web接口调用返回的数据。。。）</p>
<p>一个Observable可以发出零个或者多个事件，知道结束或者出错。每发出一个事件，就会调用它的Subscriber的onNext方法，最后调用Subscriber.onNext()或者Subscriber.onError()结束。</p>
<p>Rxjava的看起来很想设计模式中的观察者模式，但是有一点明显不同，那就是如果一个Observerble没有任何的的Subscriber，那么这个Observable是不会发出任何事件的。</p>
<h2>Hello World</h2>
<p>创建一个Observable对象很简单，直接调用Observable.create即可</p>
<div class="highlight"><pre><span class="nx">Observable</span><span class="o"><</span><span class="nb">String</span><span class="o">></span> <span class="nx">myObservable</span> <span class="o">=</span> <span class="nx">Observable</span><span class="p">.</span><span class="nx">create</span><span class="p">(</span>
<span class="k">new</span> <span class="nx">Observable</span><span class="p">.</span><span class="nx">OnSubscribe</span><span class="o"><</span><span class="nb">String</span><span class="o">></span><span class="p">()</span> <span class="p">{</span>
<span class="err">@</span><span class="nx">Override</span>
<span class="kr">public</span> <span class="k">void</span> <span class="nx">call</span><span class="p">(</span><span class="nx">Subscriber</span><span class="cp"><?</span> <span class="nx">super</span> <span class="nx">String</span><span class="o">></span> <span class="nx">sub</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">sub</span><span class="o">.</span><span class="nx">onNext</span><span class="p">(</span><span class="s2">"Hello, world!"</span><span class="p">);</span>
<span class="nx">sub</span><span class="o">.</span><span class="nx">onCompleted</span><span class="p">();</span>
<span class="p">}</span>
<span class="p">});</span>
</pre></div>
<p>这里定义的Observable对象仅仅发出一个Hello World字符串，然后就结束了。接着我们创建一个Subscriber来处理Observable对象发出的字符串。</p>
<div class="highlight"><pre><span class="nx">Subscriber</span><span class="o"><</span><span class="kt">String</span><span class="o">></span> <span class="n">mySubscriber</span> <span class="o">=</span> <span class="nb">new</span> <span class="nx">Subscriber</span><span class="o"><</span><span class="kt">String</span><span class="o">></span><span class="p">()</span> <span class="p">{</span>
<span class="p">@</span><span class="nx">Override</span>
<span class="k">public</span> <span class="nf">void</span> <span class="nx">onNext</span><span class="p">(</span><span class="kt">String</span> <span class="nb">s</span><span class="p">)</span> <span class="p">{</span> <span class="nx">System.out.println</span><span class="p">(</span><span class="nb">s</span><span class="p">);</span> <span class="p">}</span>
<span class="p">@</span><span class="nx">Override</span>
<span class="k">public</span> <span class="nf">void</span> <span class="nx">onCompleted</span><span class="p">()</span> <span class="p">{</span> <span class="p">}</span>
<span class="p">@</span><span class="nx">Override</span>
<span class="k">public</span> <span class="nf">void</span> <span class="nx">onError</span><span class="p">(</span><span class="nx">Throwable</span> <span class="nx">e</span><span class="p">)</span> <span class="p">{</span> <span class="p">}</span>
<span class="p">};</span>
</pre></div>
<p>这里subscriber仅仅就是打印observable发出的字符串。通过subscribe函数就可以将我们定义的myObservable对象和mySubscriber对象关联起来，这样就完成了subscriber对observable的订阅。</p>
<div class="highlight"><pre><span class="n">myObservable</span><span class="p">.</span><span class="n">subscribe</span><span class="p">(</span><span class="n">mySubscriber</span><span class="p">);</span>
</pre></div>
<p>一旦mySubscriber订阅了myObservable，myObservable就是调用mySubscriber对象的onNext和onComplete方法，mySubscriber就会打印出Hello World！</p>
<h2>更简洁的代码</h2>
<p>是不是觉得仅仅为了打印一个hello world要写这么多代码太啰嗦？我这里主要是为了展示RxJava背后的原理而采用了这种比较啰嗦的写法，RxJava其实提供了很多便捷的函数来帮助我们减少代码。</p>
<p>首先来看看如何简化Observable对象的创建过程。RxJava内置了很多简化创建Observable对象的函数，比如Observable.just就是用来创建只发出一个事件就结束的Observable对象，上面创建Observable对象的代码可以简化为一行</p>
<div class="highlight"><pre><span class="nx">Observable</span><span class="o"><</span><span class="kt">String</span><span class="o">></span> <span class="n">myObservable</span> <span class="o">=</span> <span class="nx">Observable.just</span><span class="p">(</span><span class="s2">"Hello, world!"</span><span class="p">);</span>
</pre></div>
<p>接下来看看如何简化Subscriber，上面的例子中，我们其实并不关心OnComplete和OnError，我们只需要在onNext的时候做一些处理，这时候就可以使用Action1类。</p>
<div class="highlight"><pre><span class="nx">Action1</span><span class="o"><</span><span class="kt">String</span><span class="o">></span> <span class="n">onNextAction</span> <span class="o">=</span> <span class="nb">new</span> <span class="nx">Action1</span><span class="o"><</span><span class="kt">String</span><span class="o">></span><span class="p">()</span> <span class="p">{</span>
<span class="p">@</span><span class="nx">Override</span>
<span class="k">public</span> <span class="nf">void</span> <span class="nb">call</span><span class="p">(</span><span class="kt">String</span> <span class="nb">s</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">System.out.println</span><span class="p">(</span><span class="nb">s</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">};</span>
</pre></div>
<p>subscribe方法有一个重载版本，接受三个Action1类型的参数，分别对应OnNext，OnComplete， OnError函数。</p>
<div class="highlight"><pre><span class="n">myObservable</span><span class="p">.</span><span class="n">subscribe</span><span class="p">(</span><span class="n">onNextAction</span><span class="p">,</span> <span class="n">onErrorAction</span><span class="p">,</span> <span class="n">onCompleteAction</span><span class="p">);</span>
</pre></div>
<p>这里我们并不关心onError和onComplete，所以只需要第一个参数就可以</p>
<div class="highlight"><pre><span class="n">myObservable</span><span class="p">.</span><span class="n">subscribe</span><span class="p">(</span><span class="n">onNextAction</span><span class="p">);</span>
<span class="c1">//Outputs "Hello, world!"</span>
</pre></div>
<p>上面的代码最终可以写成这样</p>
<div class="highlight"><pre><span class="nx">Observable.just</span><span class="p">(</span><span class="s2">"Hello, world!"</span><span class="p">)</span>
<span class="bp">.</span><span class="nx">subscribe</span><span class="p">(</span><span class="nb">new</span> <span class="nx">Action1</span><span class="o"><</span><span class="kt">String</span><span class="o">></span><span class="p">()</span> <span class="p">{</span>
<span class="p">@</span><span class="nx">Override</span>
<span class="k">public</span> <span class="nf">void</span> <span class="nb">call</span><span class="p">(</span><span class="kt">String</span> <span class="nb">s</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">System.out.println</span><span class="p">(</span><span class="nb">s</span><span class="p">);</span>
<span class="p">}</span>
<span class="p">});</span>
</pre></div>
<p>使用java8的lambda可以使代码更简洁</p>
<div class="highlight"><pre><span class="n">Observable</span><span class="p">.</span><span class="n">just</span><span class="p">(</span><span class="s">"Hello, world!"</span><span class="p">)</span>
<span class="p">.</span><span class="n">subscribe</span><span class="p">(</span><span class="n">s</span> <span class="o">-></span> <span class="n">System</span><span class="p">.</span><span class="n">out</span><span class="p">.</span><span class="n">println</span><span class="p">(</span><span class="n">s</span><span class="p">));</span>
</pre></div>
<p>Android开发中，强烈推荐使用retrolambda这个gradle插件，这样你就可以在你的代码中使用lambda了。</p></div>
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