5.4 context与global的应用
5.4.1 使用context实现计数器
在之前的流中，我们总是在执行函数节点时新建一个变量。函数节点执行完毕以后，这个变量的值就会丢失，没有办法保存。如果需要保存一个变量的值该如何操作？
可以借助context对象。它可以理解为上下文，或者语境，用于保存内存中的数据，这个数据可以持续保存到下一个消息到来。Context可用于保存索引，计数或者消息中的数据。
拖入inject节点与debug节点。拖入一个函数节点，进行如下设置：
完成以后连线并部署。多次点击inject节点的输入按钮，并在调试窗口观察现象。
发现调试窗口中的数据在递增
接下来结合现象分析代码：
var count = context.get('count')||0;
count += 1;
context.set('count',count);
msg.payload = count;
return msg;
我们可以查到Context的一些API（应用程序接口）如下
context.get(..) : 获取一个节点范围内的上下文属性
context.set(..) : 设置一个节点范围内的上下文属性
context.keys(..) : 返回所有节点范围上下文属性键的列表
context.flow : 同 flow
context.global : 同lobal
每次的输入都不变，count每次都+1,最后输出递增的数，说明count记录了上一次输入的值。执行Context.get以后，就可以获取到之前保存的值。
执行context.set以后，就能把处理过后的值保存下来。保存到哪里？’count’,带单引号的count里。接下来用流程图来表示逻辑。注意，带单引号的’count’与变量count不一样
流程保存在这里
[{"id":"1e2326a1.bc3da9","type":"inject","z":"dfef6fbd.55491","name":"","topic":"","payload":"1","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":690,"y":160,"wires":[["ecb5fce2.ce1dc"]]},{"id":"7e2fd11b.d4b4","type":"debug","z":"dfef6fbd.55491","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":1100,"y":160,"wires":[]},{"id":"ecb5fce2.ce1dc","type":"function","z":"dfef6fbd.55491","name":"计数器","func":"var count = context.get('count')||0;\n//如果count不存在就初始化为0，已存在则获取count的值\ncount += 1;\ncontext.set('count',count);\n//执行完+1操作以后保存count的值\nmsg.payload = count;\nreturn msg;","outputs":1,"noerr":0,"x":880,"y":160,"wires":[["7e2fd11b.d4b4"]]}]
5.4.2 使用flow在不同节点之间传递参数
在5.4.1小节中，函数节点计数器实现了用’count’来保存数据的方法，那么这个’count’里边的内容，能否被别的节点使用？我们来尝试一下。
在上一章节的基础上进行修改，添加计数器2，并做如下修改：
如果计数器2可以使用’count’的内容，那么payload里的内容应该是递增的数据。然而，部署以后，观察实验现象，可以发现OUT2的内容并没有变化。这说明计数器2没有办法获取’count’里的数据。
细心的读者可以发现，在context的API里，有这么一句解释：
context.get(..) : 获取一个节点范围内的上下文属性
也就是使用context属性只能影响一个节点范围内。然而在实际的应用中，不同的节点之间可能会用到同一个变量，也就是说，有一个“全局变量”，该如何传递？
我们查阅了函数的API，找到了flow的context用法，地址是
https://nodered.org/docs/writing-functions
flow.get(..) : 获得流作用域上下文属性
flow.set(..) : 设置流作用域上下文属性
flow.keys(..) : 返回所有流作用域上下文属性键的列表
修改为“计数器”节点
修改“计数器2”节点
部署并点击inject节点的输入按钮，看到现象如下
这说明计数器2获取到了计数器中的count值，也就是通过flow.set与flow.get实现了不同节点的参数传递。
流程分析如下：
流的信息保存在这里：
[{"id":"d5052172.91827","type":"inject","z":"a1f259d6.8791a8","name":"","topic":"","payload":"1","payloadType":"num","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":140,"y":180,"wires":[["9ddd2b2f.536d28"]]},{"id":"811f3cdb.bf65b","type":"debug","z":"a1f259d6.8791a8","name":"OUT1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":530,"y":180,"wires":[]},{"id":"9ddd2b2f.536d28","type":"function","z":"a1f259d6.8791a8","name":"计数器","func":"var count = context.get('count')||0;\n//如果count不存在就初始化为0，已存在则获取count的值\ncount += 1;\ncontext.set('count',count);\n//执行完+1操作以后保存count的值\nflow.set('flowSave',count);\n//设置一个flowSave，来保存变量count的值\nmsg.payload = count;\nreturn msg;","outputs":1,"noerr":0,"x":330,"y":180,"wires":[["811f3cdb.bf65b","dbecc13a.c9642"]]},{"id":"dbecc13a.c9642","type":"function","z":"a1f259d6.8791a8","name":"计数器2","func":"var count = flow.get('flowSave')||0;\n//如果count不存在就初始化为0，已存在则获取count的值\nmsg.payload = count;\nreturn msg;","outputs":1,"noerr":0,"x":450,"y":260,"wires":[["71a9791a.cd5f28"]]},{"id":"71a9791a.cd5f28","type":"debug","z":"a1f259d6.8791a8","name":"OUT2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":610,"y":260,"wires":[]}]
5.4.3 使用flow实现计时器
使用flow的属性在不同的节点之间传递参数，这个功能十分有用。接下来尝试使用flow实现一个节点执行时间的计数器。
在JS中，可以使用Data对象的getTime方法获取从1970年1月1日至今的毫秒数。跟时间戳得到的数据类似。在开始节点和结束节点分别获取时间，两个值的差就是这两个节点之间所用的时间了。
另外，需要一个延时节点来放在两个计时节点之间。在功能控件里恰好有一个名为“delay”的控件满足要求。它可以控制消息通过自己的时间或者限制它的传递速度。我们会使用它的随机延时的功能。
这次我们先来画一个流程图来分析思路。所谓“谋定而后动”，在编程里也是一个好习惯。
这是开始时间的设置
这是结束时间的设置
这是delay节点的设置
整个流连线如下。完成以后部署并点击inject节点的输入按钮
可以看到，delay节点出现了一个蓝色的标记，以及表示时间的数字。这就是节点的状态status。
调试窗口信息如下
这是代码，懒得自己输入的可以直接导入看结果。
[{"id":"710e935.672cf6c","type":"inject","z":"b0555ded.82baa","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":150,"y":200,"wires":[["c3b7267d.1578c8"]]},{"id":"8bc81259.86e39","type":"debug","z":"b0555ded.82baa","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":560,"y":200,"wires":[]},{"id":"c3b7267d.1578c8","type":"function","z":"b0555ded.82baa","name":"开始时间","func":"flow.set('stratTime',new Date().getTime());\n//使用data对象的getTime方法获取开始时间，\n//并存到'startTime'里\nreturn msg;","outputs":1,"noerr":0,"x":260,"y":280,"wires":[["28cea295.2be12e"]]},{"id":"9152870f.6bf4e8","type":"function","z":"b0555ded.82baa","name":"结束时间","func":"var currentTime = new Date().getTime();\n//获取当前时间\nvar stratTime = flow.get('stratTime',stratTime);\n//用flow.get获取之间节点储存的开始时间\nvar timeElapsed = (currentTime - stratTime)/1000;\n//计算时间差并从毫秒转换为秒\nmsg.payload = \"逝去的时间是: \"+timeElapsed+\"s\";\nreturn msg;","outputs":1,"noerr":0,"x":460,"y":280,"wires":[["8bc81259.86e39"]]},{"id":"28cea295.2be12e","type":"delay","z":"b0555ded.82baa","name":"","pauseType":"random","timeout":"5","timeoutUnits":"seconds","rate":"1","nbRateUnits":"1","rateUnits":"second","randomFirst":"1","randomLast":"5","randomUnits":"seconds","drop":false,"x":350,"y":200,"wires":[["9152870f.6bf4e8"]]}]
5.4.4 使用全局变量
全局变量名为global，它与flow功能类似，但是作用域比flow大，并且，即便重新部署，global储存的值也不会丢失。以下是global的API。
global.get(..) ：获取全局范围的上下文属性
global.set(..) ：设置全局范围的上下文属性
global.keys(..) ：返回所有全局作用域上下文属性键的列表
在5.4.3中见到的例子使用global也完全可以实现，只需要把里边所有的“flow”换成“global”就可以，由于太简单，就不演示了。Flow的作用于只局限于同一个流，此处做一点flow做不到的事情——流之间的数据传递。程序设计思路如下：
整个流程如下设计
开始时间与当前时间分别设计如下
完成以后部署，并先点击开始时间的输入按钮，再点击当前时间的输入按钮。调试窗口现象如下：
从现象中可以看出来虽然两个流没有直接连接，但是第二个流仍然获取了在第一个流中设置的变量。说明实验成功。
接下来，观察重新部署时，global变量能否保存。为节点做一些无关紧要的修改，例如out改为OUT，以便能够点击“部署”按钮。
部署完成以后，不要点击“开始时间”的输入按钮，直接点击“当前时间”的输入按钮，可以观察到如下的现象。
逝去的时间并没有被重置，说明重新部署不影响global的内容。
程序的代码保存在这里
[{"id":"eaf64e67.5277","type":"inject","z":"3769aa0d.6513d6","name":"开始时间","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":140,"y":220,"wires":[["8a1f7741.6eb3b8"]]},{"id":"d6468738.ed2948","type":"debug","z":"3769aa0d.6513d6","name":"startTime","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":460,"y":220,"wires":[]},{"id":"8a1f7741.6eb3b8","type":"function","z":"3769aa0d.6513d6","name":"开始时间","func":"global.set('stratTime',new Date().getTime());\n//使用data对象的getTime方法获取开始时间，\n//并存到'startTime'里\nreturn msg;","outputs":1,"noerr":0,"x":280,"y":220,"wires":[["d6468738.ed2948"]]},{"id":"4c55cdab.648834","type":"function","z":"3769aa0d.6513d6","name":"当前时间","func":"var currentTime = new Date().getTime();\n//获取当前时间\nvar stratTime = global.get('stratTime',stratTime);\n//用global.get获取之间节点储存的开始时间\nvar timeElapsed = (currentTime - stratTime)/1000;\n//计算时间差并从毫秒转换为秒\nmsg.payload = \"逝去的时间是: \"+timeElapsed+\"s\";\nreturn msg;","outputs":1,"noerr":0,"x":280,"y":300,"wires":[["c5123924.d3f928"]]},{"id":"9f83e717.eaf888","type":"inject","z":"3769aa0d.6513d6","name":"当前时间","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":120,"y":300,"wires":[["4c55cdab.648834","357d85e7.a522ba"]]},{"id":"c5123924.d3f928","type":"debug","z":"3769aa0d.6513d6","name":"OUT","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":440,"y":300,"wires":[]},{"id":"357d85e7.a522ba","type":"debug","z":"3769aa0d.6513d6","name":"crrentTime","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":280,"y":400,"wires":[]}]
接下来，可以试一试不同的页面内，global的数值能否传递。新建一个页面。点击这个加号：
把第一个流放在“开始的时间的流”标签下。第二个流放在“当前时间的流”标签下。
部署，并先后点击开始时间与当前时间的输入按钮，可以在调试窗口看到如下现象，说明，即便是跨标签页，global的数据仍可以传递。
另外，我在官方的博客见到了没有使用set与get方法的另外一种写法，经过实验也可以使用，把它写在这里。
context.global.startTime = new Date().getTime();
//global.set('stratTime',new Date().getTime());
var timeElapsed = (currentTime - context.global.startTime)/1000;
//var stratTime = global.get('stratTime',stratTime);
//var timeElapsed = (currentTime - stratTime)/1000;
5.5 函数控件的其它功能
函数控件的功能强大，甚至可以用函数控件实现其它所有功能控件的功能。在函数空间中，除了可以调用JS自带的各种方法以外，含有一些node-red特有的API可以使用，我把它写在这里
node.id :函数节点的ID，在0.19版本添加
node.name :函数节点的名字，在0.19版本增加
node.log(..) : 记录一条消息
node.warn(..) : 记录一条警告消息
node.error(..) : 记录一条错误消息
node.debug(..) :记录一条调试消息
node.trace(..) : 记录跟踪消息
node.on(..) : 注册一个事件处理函数
node.status(..) : 更新节点的状态
node.send(..) : 发送消息
这其中从log到trace都是用来向命令换输出信息的，由于用法十分简单，就不讲解了。
5.5.1 使用send方法发送数据
在5.2.3小节时，我们遇到一个问题：如果“apple”既满足条件1，又满足条件3，但是条件1执行return 的操作以后，条件3就不再执行了，有没有既可以发送数据，又不会结束函数的方法？当然有。我们查询了节点可以调用的API，发现有send函数，满足我们的要求。
查一下之前的代码，把“判断主题”节点做如下修改
然后部署并观察现象，可以发现，当点击apple的输入按钮时，OUT1与OUT3都可以收到消息，说明实验成功。
代码如下
[{"id":"aaa307ba.874e58","type":"function","z":"b0555ded.82baa","name":"判断主题","func":"if (msg.topic === \"apple\") {\n node.send([ msg, null, null]);\n} \nif(msg.topic === \"banana\"){\n node.send( [ null, msg, null]);\n}\nif(msg.topic.indexOf(\"e\") != -1){\n node.send([null,null, msg ]);\n}\n","outputs":3,"noerr":0,"x":360,"y":300,"wires":[["e8ac6685.9b2a48"],["503899c3.750508"],["1053e26d.f8c7ae"]]},{"id":"8e22df59.c7795","type":"inject","z":"b0555ded.82baa","name":"","topic":"apple","payload":"apple","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":150,"y":240,"wires":[["aaa307ba.874e58"]]},{"id":"1692dfa1.47a2a","type":"inject","z":"b0555ded.82baa","name":"","topic":"banana","payload":"banana","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":160,"y":300,"wires":[["aaa307ba.874e58"]]},{"id":"bb012d65.92486","type":"inject","z":"b0555ded.82baa","name":"","topic":"orange","payload":"orange","payloadType":"str","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":160,"y":360,"wires":[["aaa307ba.874e58"]]},{"id":"e8ac6685.9b2a48","type":"debug","z":"b0555ded.82baa","name":"OUT1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":550,"y":240,"wires":[]},{"id":"503899c3.750508","type":"debug","z":"b0555ded.82baa","name":"OUT2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":550,"y":300,"wires":[]},{"id":"1053e26d.f8c7ae","type":"debug","z":"b0555ded.82baa","name":"OUT3","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":550,"y":360,"wires":[]}]
5.5.2 流程之间传递函数
Context元素除了可以传递变量以外，也可以传递函数。我们在流1中新建一个名为hello的函数，在流2中调用它。
流1的函数节点
流2的函数节点
先点击IN1的输入按钮，再点击IN2的输入按钮，可以看到OUT2中打印出了hello world，其中的“hello”是通过调用流1 的hello()函数打印出来的，说明实验成功了。
代码保存在这里
[{"id":"eaf64e67.5277","type":"inject","z":"3769aa0d.6513d6","name":"IN1","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":210,"y":160,"wires":[["1012b4b4.a746fb"]]},{"id":"d6468738.ed2948","type":"debug","z":"3769aa0d.6513d6","name":"OUT1","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":530,"y":160,"wires":[]},{"id":"1012b4b4.a746fb","type":"function","z":"3769aa0d.6513d6","name":"hello","func":"context.global.hello = function(){return \"hello\"};\nmsg.payload = context.global.hello();\nreturn msg;","outputs":1,"noerr":0,"x":370,"y":160,"wires":[["d6468738.ed2948"]]},{"id":"8331681d.a15558","type":"function","z":"3769aa0d.6513d6","name":"+world","func":"msg.payload = context.global.hello() + \" World\";\nreturn msg;","outputs":1,"noerr":0,"x":350,"y":260,"wires":[["8214b226.35f87"]]},{"id":"eff72cea.339ae","type":"inject","z":"3769aa0d.6513d6","name":"IN2","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":190,"y":260,"wires":[["8331681d.a15558"]]},{"id":"8214b226.35f87","type":"debug","z":"3769aa0d.6513d6","name":"OUT2","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","x":520,"y":260,"wires":[]}]
5.5.3 使用状态标志
在5.4.3小节时，我们第一次使用delay节点，用于产生一个随机数。此节点在在运行的时候显示出了一个表示时间的字符，显示这个随机的时间是多少。我觉得这个功能很有用，很直观的可以显示出我们感兴趣的一些数据或状态，可以省去到处用debug节点打印的烦恼。翻看函数节点的API，正好有一条可以用来显示状态：
node.status(..) : 更新节点的状态
我们来研究一下如何使用。
首先要搭建一个可变的输入信息，并且要持续一段时间，避免状态变得太快看不到。相信详细学过函数控件用法以后，对于聪明的你来说，这一定不是难事。——用函数节点+delay节点可以实现。先拖拽节点，形成数据流。
第一个函数节点用于发送5个数字：
Delay节点用于限制数据包的速度，每秒一条。
如此就能实现，第二个函数节点每秒接收到一个数字。
第二个函数节点的代码如下：
node.status({fill:"green",shape:"dot",text:"Received "+msg.payload});
return msg;
也很好理解：填充绿色的点，文本显示“Received + 载荷”，只是格式死板一点，照着填写，不要遗漏标点或拼写错误。
接下来点击inject的输入按钮，可以在调试窗口看到每隔一秒收到一个数据。这都是意料中的现象。
同时，第二个函数节点也能显示出一些信息。绿色的点，以及显示内容都是我们代码中自己编写的，所以，任务完成。
代码在这里
[{"id":"5e91200b.6cd0c","type":"inject","z":"3769aa0d.6513d6","name":"","topic":"","payload":"","payloadType":"date","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":140,"y":120,"wires":[["27878fd7.889b1"]]},{"id":"27878fd7.889b1","type":"function","z":"3769aa0d.6513d6","name":"send1-5","func":"for (var i=0; i<5 ; i++){\n node.send({payload:i});\n}\nreturn null;","outputs":1,"noerr":0,"x":240,"y":200,"wires":[["b99e8ad0.ce0d28"]]},{"id":"4705bd05.3042f4","type":"debug","z":"3769aa0d.6513d6","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"false","x":630,"y":120,"wires":[]},{"id":"b99e8ad0.ce0d28","type":"delay","z":"3769aa0d.6513d6","name":"","pauseType":"rate","timeout":"5","timeoutUnits":"seconds","rate":"1","nbRateUnits":"1","rateUnits":"second","randomFirst":"1","randomLast":"5","randomUnits":"seconds","drop":false,"x":370,"y":120,"wires":[["1d46ef67.107e81"]]},{"id":"1d46ef67.107e81","type":"function","z":"3769aa0d.6513d6","name":"received1-5","func":"node.status({fill:\"green\",shape:\"dot\",text:\"Received \"+msg.payload});\nreturn msg;","outputs":1,"noerr":0,"x":510,"y":200,"wires":[["4705bd05.3042f4"]]}]
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