Merge pull request #989 from flowforge/zj-core-nodes-doc-inject

core nodes: Use case for inject

src/_includes/core-nodes/inject-use-case.md

@@ -0,0 +1,35 @@
+## What's the Inject node in Node-RED used for?
+
+The Inject node is the beginning of many flows that are triggered manually. The box to the left of the node sends a message to connected nodes. For that reason it's often used for debugging too, to inject values at a point of choosing. The message item can be empty too. The message to send defaults to the timestamp as payload and an empty topic.
+
+Message properties can be set to flow, or global variable values, and many other types, including JSONata expressions.
+
+Flows can also be triggered once right after Node-RED starts the flows or with a delay. This is useful to set an initial state from a flow on boot.
+
+An inject node can also start a flow based on a schedule. The schedules have capabilities mimicing cron. In the bottom section of the properties pane the repeat section one can select "at a specific time".
+
+Repeating the measure on an interval is done by selecting "interval" in the "repeat" section. On a schedule requires an input higher than 1 and below 2^31. When the repeat value is 0 or below Node-RED will not display an error.
+
+## Examples
+
+### Inject on Node-RED start
+
+To setup state when starting Node-RED, the inject node can be set to a zero delay to trigger a flow. When an Inject node is set to run only once, a small '1' is displayed after the label inside the node.
+
+![Inject once on start](./images/inject-node-once.png)
+
+```json
+[{"id":"73cc510bee68600f","type":"inject","z":"80987f27785245a7","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"","once":true,"onceDelay":"0.1","topic":"","payload":"","payloadType":"date","x":190,"y":200,"wires":[["7f83bf24bdf7bc68"]]},{"id":"7f83bf24bdf7bc68","type":"debug","z":"80987f27785245a7","name":"Output once","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":370,"y":200,"wires":[]}]
+```
+
+### Run a flow daily at midnight
+
+By selecting "at a specific" time in the Repeat section the inject node can generate a message at set times. Useful for data processing at set times.
+
+Do set the correct timezone in the [editor settings](/docs/user/instance-settings/#editor).
+
+![Inject nightly](./images/inject-node-nightly.png)
+
+```json
+[{"id":"998e844a7e50e275","type":"inject","z":"80987f27785245a7","name":"","props":[{"p":"payload"}],"repeat":"","crontab":"59 23 * * *","once":false,"onceDelay":"0","topic":"","payload":"","payloadType":"date","x":190,"y":320,"wires":[["1e80f5229516e910"]]},{"id":"1e80f5229516e910","type":"debug","z":"80987f27785245a7","name":"Output daily at night","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"payload","targetType":"msg","statusVal":"","statusType":"auto","x":400,"y":320,"wires":[]}]
+```