SDKs and languages

The repository has two optional helpers in sdk/, one file each with no dependencies. Copy the file next to your program.

Python

sdk/python/fenn.py works with Python 3.8 and later.

python

import fenn

app = fenn.Plugin()
tasks = app.load("tasks", [])              # JSON in FENN_DATA_DIR

def draw():
    app.tile(fenn.row([fenn.ring(0.3, label=str(len(tasks))), fenn.text("Tasks")], align="center"))
    app.page(fenn.column([
        fenn.text_input("new", hint="Add a task"),
        fenn.list_view([fenn.item(str(i), t) for i, t in enumerate(tasks)]),
    ], scroll=True))

@app.on("new")
def add(e):
    if e["kind"] == "submit":
        tasks.append(e["value"]); app.save("tasks", tasks); draw()

@app.every(60)
def tick(): draw()

draw()
app.run()

It has a builder for every component (column, row, text, ring, button, canvas, …), canvas shapes (rect, circle, arc, label, glyph), badge_ring, badge_glyph, badge_pulse, show, copy, open, load and save, plus poll_event() for poll mode.

Node.js

sdk/node/fenn.js works with Node 18 and later.

const fenn = require("./fenn");
const app = new fenn.Plugin();
let on = false;
const draw = () => app.setTile(fenn.row([
  fenn.icon("lightbulb", { fill: on, color: on ? "yellow" : "dim" }),
  fenn.spacer(),
  fenn.toggle("light", on),
], { align: "center" }));
app.on("light", (e) => { on = e.value; draw(); /* call your smart-home API here */ });
draw();

Any other language

There's nothing else to it. Read lines from stdin, write lines to stdout, and flush after each line.

Flush after every line. In C, Go, Rust or C#, flush stdout yourself.
Write UTF-8, one JSON object per line, with no newlines inside the object.
stderr goes to Fenn's log, and its last lines appear on the tile if your plugin stops. Use it for debug prints.
A compiled plugin can include its binary: run = ["bin/my-plugin.exe"].

out := bufio.NewWriter(os.Stdout)
send := func(v any) { b, _ := json.Marshal(v); out.Write(append(b, '\n')); out.Flush() }
send(map[string]any{"type": "tile", "view": map[string]any{"type": "text", "text": "Hi from Go"}})
sc := bufio.NewScanner(os.Stdin)
for sc.Scan() { /* json.Unmarshal(sc.Bytes(), &msg) … */ }