RxJava基本流程和lift源码分析

最新推荐文章于 2022-03-17 14:52:54 发布

转载最新推荐文章于 2022-03-17 14:52:54 发布 · 307 阅读

本文深入浅出地介绍了RxJava的基本工作原理，包括观察者模式的实现、数据转换操作符的运用，以及如何通过lift方法自定义操作符。

基本结构

我们先来看一段最基本的代码，分析这段代码在RxJava中是如何实现的。

<code class="language-Java hljs java has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">Observable.OnSubscribe<String> onSubscriber1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Observable.OnSubscribe<String>() {
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">call</span>(Subscriber<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> String> subscriber) {
        subscriber.onNext(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"1"</span>);
        subscriber.onCompleted();
    }
};
Subscriber<String> subscriber1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Subscriber<String>() {
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onCompleted</span>() {

    }

    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onError</span>(Throwable e) {

    }

    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onNext</span>(String s) {

    }
};

Observable.create(onSubscriber1)
        .subscribe(subscriber1);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li></ul>

首先我们来看一下Observable.create的代码

<code class="language-Java hljs java has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> <T> Observable<T> <span class="hljs-title" style="box-sizing: border-box;">create</span>(OnSubscribe<T> f) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Observable<T>(hook.onCreate(f));
}

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">protected</span> <span class="hljs-title" style="box-sizing: border-box;">Observable</span>(OnSubscribe<T> f) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.onSubscribe = f;
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

直接就是调用了Observable的构造函数来创建一个新的Observable对象，这个对象我们暂时标记为observable1，以便后面追溯。
同时，会将我们传入的OnSubscribe对象onSubscribe1保存在observable1的onSubscribe属性中，这个属性在后面的上下文中很重要，大家留心一下。

接下来我们来看看subscribe方法。

<code class="language-Java hljs r has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">public final Subscription subscribe(Subscriber<? super <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>> subscriber) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> Observable.subscribe(subscriber, this);
}

private static <<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>> Subscription subscribe(Subscriber<? super <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>> subscriber, Observable<<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">T</span>> observable) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">...</span>
    subscriber.onStart();
    hook.onSubscribeStart(observable, observable.onSubscribe).call(subscriber);
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> hook.onSubscribeReturn(subscriber);
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

可以看到，subscribe之后，就直接调用了observable1.onSubscribe.call方法，也就是我们代码中的onSubscribe1对象的call方法
，传入的参数就是我们代码中定义的subscriber1对象。call方法中所做的事情就是调用传入的subscriber1对象的onNext和onComplete方法。
这样就实现了观察者和被观察者之间的通讯，是不是很简单？

<code class="language-Java hljs java has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">call</span>(Subscriber<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> String> subscriber) {
    subscriber.onNext(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"1"</span>);
    subscriber.onCompleted();
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

subscribe基本流程

lift

讲之前先上一个简单的lift流程图吧
lift简化流程图

lift方法是RxJava中实现自定义operator的关键，这里我们以最简单的map为例，来分析一下lift方法的工作原理，我们对上面的demo代码稍作修改

<code class="language-Java hljs java has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;">Observable.OnSubscribe<String> onSubscriber1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Observable.OnSubscribe<String>() {
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">call</span>(Subscriber<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> String> subscriber) {
        subscriber.onNext(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"1"</span>);
        subscriber.onCompleted();
    }
};
Subscriber<Integer> subscriber1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Subscriber<Integer>() {
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onCompleted</span>() {

    }

    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onError</span>(Throwable e) {

    }

    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onNext</span>(Integer i) {

    }
};
Func1<String, Integer> transformer1 = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Func1<String, Integer>() {
    <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Integer <span class="hljs-title" style="box-sizing: border-box;">call</span>(String s) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> Integer.parseInt(s);
    }
};

Observable.create(onSubscriber1)
        .map(transformer1)
        .subscribe(subscriber1);</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li><li style="box-sizing: border-box; padding: 0px 5px;">28</li><li style="box-sizing: border-box; padding: 0px 5px;">29</li><li style="box-sizing: border-box; padding: 0px 5px;">30</li><li style="box-sizing: border-box; padding: 0px 5px;">31</li><li style="box-sizing: border-box; padding: 0px 5px;">32</li><li style="box-sizing: border-box; padding: 0px 5px;">33</li></ul>

和刚才不同的是我们在create之后调用了map方法，然后才调用subscribe方法。
map方法的代码如下:

<code class="hljs java has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <R> Observable<R> <span class="hljs-title" style="box-sizing: border-box;">map</span>(Func1<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> T, ? extends R> func) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> lift(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> OperatorMap<T, R>(func));
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

一堆泛型参数是不是略晕啊，别急，我们慢慢来看。

首先来介绍一下Func这个接口。RxJava中有一系列Action+数字，Func+数字的接口，这些接口中都只有一个call方法，其中Action接口的call方法都没有返回值，
Func接口的call方法都有返回值，后面的那个数字表示call方法接受几个泛型类型的参数。

其实主要是因为Java中函数不是一等公民，所以只能用接口这么啰嗦的格式，还好我们可以使用lambda简化我们的代码。（羡慕函数式语言）

这里map方法接收的参数类型为Func1<? super T, ? extends R> func，表示func的call方法接收一个T类型的参数，返回一个R类型的返回值。

OperatorMap又是什么鬼呢？

<code class="language-Java hljs php has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">OperatorMap</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">T</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">R</span>> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Operator</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">R</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">T</span>>

<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">public</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">interface</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Operator</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">R</span>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">T</span>> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Func1</span><<span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Subscriber</span><? <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">super</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">R</span>>, <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Subscriber</span><? <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">super</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">T</span>>></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li></ul>

这里可以看到OperatorMap继承自Operator, 而Operator又继承自Func1接口，也就是说Operator接口的call方法会接收一个Subscriber类型的参数，
并且返回另外一个Subscriber类型的对象。Operator.call方法返回一个Subscriber对象，其实我们可以这么理解，每一个operator也是一个订阅者，
它返回的Subscriber对象正好用来订阅Observable发出来的消息。

有一点需要注意的是OperatorMap和Operator的泛型参数顺序刚好是相反的，为什么要这么做呢？其实很简单，因为Operator本身是对Observable发出的数据
进行转换的，所以经常会出现operator转换之后返回的数据类型变了，而OperatorMap这里刚好颠倒了一下顺序，就可以保证call方法返回的Subscriber类型
可以订阅Observable发出的数据。

OperatorMap的代码我们先不看，先来看一下lift方法中都做了些啥吧。

<code class="language-Java hljs java has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> <R> Observable<R> <span class="hljs-title" style="box-sizing: border-box;">lift</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Operator<? extends R, ? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> T> operator) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Observable<R>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> OnSubscribe<R>() {
        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">call</span>(Subscriber<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> R> o) {
            Subscriber<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> T> st = hook.onLift(operator).call(o);
            st.onStart();
            onSubscribe.call(st);
        }
    });
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li></ul>

lift方法会返回一个新创建的Observable对象，这里我们给这个Observable一个标识observable2。observable2的onSubscribe属性就是lift中new出来的这个
OnSubscribe对象。

对照demo中的代码，我们调用map之后，就调用了subscribe方法，也就是调用了这里的observable2的subscribe方法。
根据上面的介绍，调用subscribe之后，就会调用observable2.onSubscribe.call方法，call中首先做的事情就是调用OperatorMap的call方法

<code class="language-Java hljs java has-numbering" style="display: block; padding: 0px; background-color: transparent; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-top-left-radius: 0px; border-top-right-radius: 0px; border-bottom-right-radius: 0px; border-bottom-left-radius: 0px; word-wrap: normal; background-position: initial initial; background-repeat: initial initial;"><span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> Subscriber<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> T> <span class="hljs-title" style="box-sizing: border-box;">call</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">final</span> Subscriber<? <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span> R> o) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Subscriber<T>(o) {
        <span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onNext</span>(T t) {
            o.onNext(transformer.call(t));
        }
    };
}</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; background-color: rgb(238, 238, 238); top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right;"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

OperatorMap的call方法返回了一个Subscriber对象，这里我们标记为subscriber$map，改对象有一个transformer属性，就是我们在demo中定义的transformer1对象，
他是一个Func1类型，用来对每一个数据项进行变换。这里call方法中接收的Subscriber其实就是demo中的subscriber1对象。

我们回到lift中创建的observable2的call方法中继续，拿到OperatorMap返回的Subscriber对象之后，接着调用了onSubscribe.call方法，并且将返回的Subscriber
对象传递进去。这里需要注意的一点就是onSubscribe对象就是我们在demo中定义的onSubscribe1变量，所以就是调用了onSubscribe1.call(subscriber$map)方法。

现在我们就可以从onSubscribe1.call的调用来分析一下数据的转换过程，首先调用了subscriber map.onNext("1"),subscriber map.onNext中会首先调用
transformer1.call(“1”),然后使用返回值1，来调用onSubscribe1.onNext(1)方法，最终demo中的onSubscribe1就收到了1这个值。