node-red教程 3.4 switch控件介绍

迈不过友情╰ 2022-05-22 07:56 182阅读 0赞

Switch本意是开关、转换，用在node-red里有点像一个岔路口，node-red里边有数据流，数据流可能有不同的分支，在“岔路口”管理数据流向的，就可以是switch控件。控件根据数据的一些属性来判断它的流向。就比如你到了公共卫生间，你是男生，就去左边的男卫生间；女生去右边的女卫生间。Switch控件可以根据数据的topic或是payload，或者其它属性来判断数据该发给哪一个出口。  
　　Switch控件属于“function”类型的控件，function一般翻译为功能或者函数。说到函数可能就会让人想到数学上那些抽象的公式，让人望而却步了。好在编程中用到的函数跟数学上的不太一样，一般是有一定功能的程序。对，就是写程序。但是不要怕，程序里的逻辑一般都是很简单的，简单到可以用朴素的语言描述。比如你在回家的路上，你妈妈告诉你：“要是看到卖包子的，买6个包子。没有卖包子的话，买4个馒头”。在程序里，可能这么表示这个逻辑关系

if (卖包子 == true)
    buy(6,包子);
    else
    buy(4,馒头)；

![这里写图片描述][70]

## 3.4.1 switch的说明信息 ##

在附带这些说明的时候，我其实是忐忑不安的。因为我发现，这个说明信息可能把简单的问题复杂化了。我在没有看这些说明的时候就摸索出了这个节点的基本用法，看了说明以后反而有点不知所云。这是因为，switch节点的高级用法我们可能暂时没有用到，因此如果读一下信息让你感到疑惑，你大可以跳过这段，毕竟我们以应用为主。  
以后别的控件的说明信息也可以先跳过，掌握用法以后再来看，可能会好一些。

根据其属性值或序列位置路由消息。（路由在这里可以理解为数据转发）  
细节  
当消息到达时，节点将评估每一个已定义的规则，并将消息转发到任何匹配规则的相应输出。  
可以选择配置为，一旦找到匹配的规则，节点就可以停止评估规则。  
可以根据单个消息属性、流或全局上下文属性或JSONata表达式的结果来评估规则。  
规则  
有四种类型的规则：  
数值：规则是根据配置的属性进行评估的  
序列：规则可用于消息序列，如分割节点生成的序列规则  
表达式：可以提供一个JSONata表达式，它将对整个消息进行评估，并且如果表达式返回true值，则会匹配。  
其它规则：如果没有匹配的规则，则可以使用另一种规则来匹配。

处理消息序列  
默认情况下，节点不会修改消息的msg.parts属性，这是序列的一部分  
重新创建的消息序列选项可以为匹配的每条规则生成新的消息序列。在这种模式下，节点将在发送新序列之前缓冲整个传入序列。运行时设置nodeMessageBufferMaxLength 可以用来限制有多少消息节点将被缓冲。

## 3.4.2 switch判断数值 ##

在程序设计中，最重要的不是写程序，而是设计。就像建筑、机械等行业的要画设计图、施工图，程序设计的思路也有必要用图的形式画出来。画图的过程就是思考的过程，由于其直观性，画图的过程本身又促进了思考。在软件工程中，已经发展出了很多种实用的图，为软件产品设计的质量提供保证。接下来我们就来学习程序设计中最简单的一种图——“程序流程图”。  
　　在程序设计之前，我们先来梳理一下任务：可能会有一些数据需要我们判断大小，然后把数据发到不同的输出分支。有点像你提着三个篮子去摘苹果，大苹果放在篮子1，中苹果放在篮子2，小苹果放在篮子3。我们就以“10”作为判断的标准，把1,10,100作为测试数据。分析任务，可以得到以下流程图。  
![这里写图片描述][70 1]  
　　测试数据的输入使用inject节点，三个输出使用debug节点，相信大家早已轻车熟路。然后拖入一个switch节点。按图示修改  
![这里写图片描述][70 2]  
![这里写图片描述][70 3]  
　　改完以后发现switch节点的输出变为3个了，从上到下分别是输出1，输出2与输出3。连接并部署。  
![这里写图片描述][70 4]  
　　分别点击3个inject节点的输入按钮。可以在调试窗口观察到以下现象。可以看出，100是OUT1节点收到的，也就是第一个判断条件（>10）的输出,100>10，没有问题。其它的数也根据判断条件正确输出，说明我们的任务完成了。  
![这里写图片描述][70 5]

[{"id":"d7d8d28a.66a29","type":"switch","z":"a1f259d6.8791a8","name":"判断数字","property":"payload","propertyType":"msg","rules":[{"t":"gt","v":"10","vt":"num"},{"t":"eq","v":"10","vt":"str"},{"t":"lt","v":"10","vt":"str"}],"checkall":"true","repair":false,"outputs":3,"x":380,"y":140,"wires":[["a9c17a9c.6080d8"],["460c1daf.59a374"],["e05c0491.0a53d8"]]},{"id":"5bb99142.a869a","type":"inject","z":"a1f259d6.8791a8","name":"","topic":"","payload":"100","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":80,"wires":[["d7d8d28a.66a29"]]},{"id":"fa6bef86.f1046","type":"inject","z":"a1f259d6.8791a8","name":"","topic":"","payload":"10","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":140,"wires":[["d7d8d28a.66a29"]]},{"id":"883913c2.9c3ab","type":"inject","z":"a1f259d6.8791a8","name":"","topic":"","payload":"1","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":200,"wires":[["d7d8d28a.66a29"]]},{"id":"a9c17a9c.6080d8","type":"debug","z":"a1f259d6.8791a8","name":"OUT1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":590,"y":80,"wires":[]},{"id":"460c1daf.59a374","type":"debug","z":"a1f259d6.8791a8","name":"OUT2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":590,"y":140,"wires":[]},{"id":"e05c0491.0a53d8","type":"debug","z":"a1f259d6.8791a8","name":"OUT3","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":590,"y":200,"wires":[]}]

## 3.4.3 switch判断数组 ##

刚刚的例子只能判断单个数字，实际使用中，我们常常需要判断数组的某一位。接下来，在刚刚的例子上进行一点点的修改，来实现可以根据数组的第二位的情况来分发数据。  
　　注入内容改为数组。  
![这里写图片描述][70 6]  
　　Switch节点需要注意属性，是msg.payload\[1\]！因为数组下标从0开始，所以数组的第一位是msg.payload\[0\]，第二位才是msg.payload\[1\]。  
　　除了刚刚使用的大于，等于和小于，还有很多判断条件，感兴趣的可以尝试下。我这里随便用了几个。  
　　顺带提一下，在switch的说明信息里，有很多关于sequence rules和JSONata exp的描述，这些可能是很有用的判断条件，不过我们暂时没有用到，因此看不懂说明信息。感兴趣的可以查一下。  
![这里写图片描述][70 7]  
![这里写图片描述][70 8]

[{"id":"fc2f904.f999f7","type":"switch","z":"35ef3f16.088cc","name":"判断数字","property":"payload[1]","propertyType":"msg","rules":[{"t":"eq","v":"2","vt":"num"},{"t":"btwn","v":"5","vt":"num","v2":"6","v2t":"num"},{"t":"cont","v":"7","vt":"num"}],"checkall":"true","repair":false,"outputs":3,"x":440,"y":220,"wires":[["b425dfbd.2d50c"],["ee083586.6939a8"],["96d980b9.81bc7"]]},{"id":"48f7a616.f295d8","type":"inject","z":"35ef3f16.088cc","name":"","topic":"","payload":"[1,2,3]","payloadType":"bin","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":230,"y":160,"wires":[["fc2f904.f999f7"]]},{"id":"ab2bdee9.29b41","type":"inject","z":"35ef3f16.088cc","name":"","topic":"","payload":"[4,5,6]","payloadType":"bin","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":230,"y":220,"wires":[["fc2f904.f999f7"]]},{"id":"518c940.856a46c","type":"inject","z":"35ef3f16.088cc","name":"","topic":"","payload":"[7,78,9]","payloadType":"bin","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":230,"y":280,"wires":[["fc2f904.f999f7"]]},{"id":"b425dfbd.2d50c","type":"debug","z":"35ef3f16.088cc","name":"OUT1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":650,"y":160,"wires":[]},{"id":"ee083586.6939a8","type":"debug","z":"35ef3f16.088cc","name":"OUT2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":650,"y":220,"wires":[]},{"id":"96d980b9.81bc7","type":"debug","z":"35ef3f16.088cc","name":"OUT3","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":650,"y":280,"wires":[]}]

## 3.4.4 switch判断topic ##

前边两个例子都是根据msg.payload属性来判断的，msg里边的其它属性也可以作为判断条件，在3.1.5小节中提到过，接下来尝试操作一下。  
　　拖入3个inject节点，topic要填写，我填写的topic是“apple”，“banana”和“orange”。Switch节点进行如下修改。  
![这里写图片描述][70 9]  
　　连线并部署。点击inject按钮观察现象。  
![这里写图片描述][70 10]  
　　我想，除了点击一下apple的输入按钮，OUT1与OUT3都收到数据以外，其它的现象都和你预料的一致。Apple为什么会输出两次？  
　　这是因为条件3是包含“e”，apple也是包含“e”的，所以apple是满足条件3的。  
![这里写图片描述][70 11]  
　　如果想让apple满足条件1以后就不再判断条件3，可以选择“接受第一条匹配消息后停止”。  
![这里写图片描述][70 12]

代码如下：

[{"id":"6b822d6c.b64d34","type":"switch","z":"2cabb877.dd50b8","name":"判断主题","property":"topic","propertyType":"msg","rules":[{"t":"eq","v":"apple","vt":"str"},{"t":"eq","v":"banana","vt":"str"},{"t":"cont","v":"e","vt":"str"}],"checkall":"false","repair":false,"outputs":3,"x":360,"y":160,"wires":[["5723c235.995c1c"],["7a3d0ed7.1fd12"],["24f93dff.b007a2"]]},{"id":"3eeb3b18.884db4","type":"inject","z":"2cabb877.dd50b8","name":"","topic":"apple","payload":"apple","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":170,"y":100,"wires":[["6b822d6c.b64d34"]]},{"id":"9ee97777.be6de8","type":"inject","z":"2cabb877.dd50b8","name":"","topic":"banana","payload":"banana","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":180,"y":160,"wires":[["6b822d6c.b64d34"]]},{"id":"1215b92e.3a45f7","type":"inject","z":"2cabb877.dd50b8","name":"","topic":"orange","payload":"orange","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":180,"y":220,"wires":[["6b822d6c.b64d34"]]},{"id":"5723c235.995c1c","type":"debug","z":"2cabb877.dd50b8","name":"OUT1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":570,"y":100,"wires":[]},{"id":"7a3d0ed7.1fd12","type":"debug","z":"2cabb877.dd50b8","name":"OUT2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":570,"y":160,"wires":[]},{"id":"24f93dff.b007a2","type":"debug","z":"2cabb877.dd50b8","name":"OUT3","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":570,"y":220,"wires":[]}]

[70]: /images/20220522/0157aebf7fb141d6a0b2ad034d81d75b.png
[70 1]: /images/20220522/1cddbac500c24a3ebaa16efeab1e3a4a.png
[70 2]: /images/20220522/2d171a09e4f34e00a7bc4a48b40ccc94.png
[70 3]: /images/20220522/74d314377fc04861ad3ddcbc0aa45d87.png
[70 4]: /images/20220522/2296f504cffe43d888c03272243c609c.png
[70 5]: /images/20220522/88aeae982b0943459c0730b40230f263.png
[70 6]: /images/20220522/06a6bb38fde74e48afec149264b0e978.png
[70 7]: /images/20220522/28f80c76329242af816f6287e31ff020.png
[70 8]: /images/20220522/ca1fc79ce0534c50b38c782ba4ce3bb8.png
[70 9]: /images/20220522/af2121cbcf9940a0b606f9961ad21bf1.png
[70 10]: /images/20220522/e6a94ff977e4441f84ba4899e664965f.png
[70 11]: /images/20220522/50f7c75ece3f4191ad407dd40232a5d6.png
[70 12]: /images/20220522/9c00d8c127344dc7949504a2d15b8b6c.png