--- title: Patch Nodes --- Patch Nodes =========== What you see on the tab in the main workspace is called a *patch*. A XOD patch corresponds to a single source file or module in other programming languages. Until now, you’ve created single-patch programs and the patch was automatically named `main`. Creating a whole project in a single patch would become messy once the number of nodes and links passed some limit. It is often a good idea to split complex patches into several simpler patches that are easier to understand and edit. Furthermore, you can reuse patches several times in your project with slightly different parameters, thus avoiding node duplication. When you use one patch as a node in other patches it is called a *patch node*. In this chapter, we’re going to build a simple watering station for two plants. The idea is to water a plant if its soil became too dry and constantly show the current soil measurements on an LCD screen. Single plant station on a single patch -------------------------------------- To start, let's build a device that will work with a single plant. We’ll use a single patch called `main`, as in previous chapters. Create a new project called `water-station` and wire up your circuit: ![Single plant circuit](./single-plant.fz.png) Now make the following patch to control the device: ![Single plant patch](./single-plant.patch.png) Be sure to set port values properly for all hardware nodes. You may have noticed it is very similar to the previously implemented smart light project. We've only changed the sensor and the actuator. The moisture sensor replaced the light sensor, and the pump replaced the LED. Upload the program to your board and test the device. Put the sensor into a glass of water and take it out. See how the pump reacts. Observe the text shown on the LCD. Let’s improve the program a bit by adding pretty formatting to the messages so that instead of `"0.42"`, the LCD would show something like `"Cactus: 42%"`. We achieve this by adding two nodes. First, a `to-percent` node will convert a number from the sensor to a string like `"42%"`. Second, the `concat` node will concatenate the constant prefix `"Cactus: "` with the percent string: ![Single plant patch with percents](./single-plant-percent.patch.png)

Hint You can click the help button in Inspector or Project Browser next to a node name to open node’s reference and learn what the node does.

Extracting plant logic to a separate patch ------------------------------------------ So far, so good. Now consider that we want to extend the device to handle two plants at once. We have another sensor, yet another relay, and a pump. What we want to share is the LCD. Each plant’s message should be shown on its own line. The very staightforward way to do it would be to duplicate most of nodes related to reading and comparing data, and formatting the result. But it would quickly become unmanagable and error prone. Changes in one place would always require manually mirroring those changes in other places. Patch nodes to the rescue. What do the different plants have in common? And what is different? The logic is common, but the name of the plant and its watering threshold value differ. The ports used to connect the sensor and the relay are also different. So these things need to be provided as parameters to our patch. What might the patch output to the outside world? It could be a status message string. Create a new patch with File → New Patch, and name it `plant`. Look at Project Browser. You’ll see that a `plant` patch has appeared next to our `main`. First of all, we’re going to define its inputs and outputs. Expand `xod/patch-nodes` in Project Browser and notice nodes with names like `input-xxx` and `output-xxx`. They are called *terminals*, and define the patch's input and output pins. Place a few inputs and outputs according to what we’ve planned to parametrize: ![Plant patch terminals](./plant-terminals.patch.png) Now give the terminal nodes informative labels so we can remember which one means what: ![Plant patch terminals with labels](./plant-terminals-labeled.patch.png) Switch back to the `main` patch and try to add two nodes of our newly created type `plant`: ![Patch nodes](./single-plant-with-patch-nodes.patch.png) We’re going to use these two nodes to manage plants and move the existing logic to the shared `plant` patch. Let’s do it.

XOD T0D0 Currently there is no cut/copy/paste in XOD. Yes, that’s a pain. We’ll implement it in future versions. If you would like to give the feature more priority, we welcome you to share your opinion on our forum.

Here is the final `main` patch: ![Main patch for two plants](./two-plants-main.patch.png) And the `plant` patch: ![Plant patch](./plant.patch.png) Wire up the circuit: ![Single plant circuit](./two-plants.fz.png) Set parameters for your plants with Inspector. Then upload the program and see how both plants are served simultaneously with a single patch. What’s next ----------- Our quick tutorial is almost complete. The last thing to learn is what you can do to build arbitrary projects with arbitrary hardware. See the [Complex Projects](../complex-projects/) chapter to learn more.