feature: add signalk-subscribe to subscribe to specific messages

Author: sbender9

package.json

@@ -18,7 +18,8 @@
       "signalk-send-put": "signalk-send-put.js",
       "signalk-send-notification": "signalk-send-notification.js",
       "signalk-send-nmea2000": "signalk-send-nmea2000.js",
-      "signalk-send-nmea0183": "signalk-send-nmea0183.js"
+      "signalk-send-nmea0183": "signalk-send-nmea0183.js",
+      "signalk-subscribe": "signalk-subscribe.js"
     }
   }
 }

signalk-subscribe.html

@@ -0,0 +1,41 @@
+<script type="text/javascript">
+    RED.nodes.registerType('signalk-subscribe',{
+        category: 'input',
+        color: '#ffcc01',
+        defaults: {
+          name: {value:""},
+          context: {value:"vessels.self"},
+          path: {value:""},
+          period: {value: 1000}
+        },
+        inputs:0,
+        outputs:1,
+        icon: "bridge.png",
+        label: function() {
+            return this.name||"signalk-subscribe";
+        }
+    });
+</script>
+
+<script type="text/x-red" data-template-name="signalk-subscribe">
+    <div class="form-row">
+        <label for="node-input-name"><i class="icon-tag"></i> Name</label>
+        <input type="text" id="node-input-name" placeholder="Name">
+  </div>
+  <div class="form-row">
+        <label for="node-input-context"><i class="icon-tag"></i> Context</label>
+        <input type="text" id="node-input-context" placeholder="Context">
+  </div>
+  <div class="form-row">
+        <label for="node-input-path"><i class="icon-tag"></i> Path</label>
+        <input type="text" id="node-input-path" placeholder="Path">
+  </div>
+  <div class="form-row">
+  <label for="node-input-period"><i class="icon-tag"></i> Period (ms)</label>
+        <input type="text" id="node-input-period" placeholder="Period">
+  </div>
+</script>
+
+<script type="text/x-red" data-help-name="signalk-subscribe">
+    <p>Input that sends messages for every delta from the given path</p>
+</script>

signalk-subscribe.js

@@ -0,0 +1,60 @@
+
+module.exports = function(RED) {
+  function input(config) {
+    RED.nodes.createNode(this,config);
+    var node = this;
+
+    var signalk = node.context().global.get('signalk')
+    var app = node.context().global.get('app')
+    var smanager = node.context().global.get('subscriptionmanager')
+    var onStop = []
+
+    if ( !config.path || config.path.length == 0 ) {
+      node.error('no path specified')
+      return
+    }
+
+    let subscription = {
+      "context": config.context,
+      subscribe: [{
+        path: config.path,
+        period: config.period
+      }]
+    }
+
+
+    function on_delta(delta) {
+      if ( delta.updates ) {
+        var copy = JSON.parse(JSON.stringify(delta))
+        copy.updates = []
+        delta.updates.forEach(update => {
+          if ( update.values &&
+               (!update.$source || !update.$source.startsWith('signalk-node-red') )) {
+            copy.updates.push(update)
+          }
+        })
+
+        if ( copy.updates.length > 0 ) {
+          if ( copy.context == app.selfContext ) {
+            copy.context = 'vessels.self'
+          }
+          node.send({ payload: copy })
+        }
+      }
+    }
+
+    smanager.subscribe(subscription,
+                       onStop,
+                       (err) => {
+                         node.error('subscription error', err)
+                       },
+                       on_delta);
+
+    
+    node.on('close', function() {
+      onStop.forEach(f => f());
+      onStop = []
+    })
+  }
+  RED.nodes.registerType("signalk-subscribe", input);
+}