Services: a Python helper for your app

Some things QML can't do, such as opening a raw TCP connection to a game, reading a device, or running a library. An app can bring a service for them: a Python 3 script that runs while the app's window is open. The app talks to it over HTTP on the device itself.

Pip-Boy, the Fallout 4 Pip-Boy, is a complete example. Its service talks to the game, and its QML draws the Pip-Boy.

Only when you must. An app with a service runs a program on the user's device with all of their access, so users have to trust it more than a QML app. The Barry Launcher plugin asks them first. If QML (with XMLHttpRequest or QtWebSockets) can do the job, leave the service out.

Adding one

Name the script in barry-app.json:

{
  "format": 1,
  "id": "io.github.you.counter",
  "name": "Counter",
  "version": "1.0.0",
  "main": "main.qml",
  "service": "service.py"
}

Barry Launcher then does the following:

1. Before the window opens, it starts python3 -s service.py in the app's folder. The environment has these variables:

   BARRY_SERVICE_PORT	The port to serve on, at 127.0.0.1
   BARRY_SERVICE_TOKEN	A random token. Answer only requests that carry it.
   BARRY_APP_ID, BARRY_APP_DIR, BARRY_APP_DATA	Your app's id, its folder, and its data folder
   XDG_CONFIG_HOME, XDG_DATA_HOME	Inside the data folder, as for the window

2. It hands the window barry.serviceUrl, barry.serviceToken and barry.request().

3. When the window closes, or the app is closed, updated or removed, it stops the service: SIGTERM first, then SIGKILL after 3 seconds.

The service's output goes to the app's log, together with the window's.

The rules

• Python 3 and its standard library only. It is the system's Python (3.12 on SteamOS), without the user's site-packages. You can bring pure Python modules in your app folder (your script's folder is on the path). Pip packages with compiled parts won't work.
• Serve on 127.0.0.1 only, on BARRY_SERVICE_PORT.
• Check the token on every request: an X-Barry-Token header (what barry.request sends), or a token query parameter for an Image's source. This stops web pages open on the device from using your service. It doesn't stop other programs on the device, because the token is in the window's command line, so don't make a service that is dangerous in anyone's hands.
• Start fast and quit on SIGTERM. The window may already be asking for the service while it starts, so barry.request reports status 0 until it answers. Retry.
• Save what matters as it changes. A service that doesn't stop within 3 seconds is killed.

A small example

service.py counts and answers with the count:

import json, os
from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer

PORT = int(os.environ["BARRY_SERVICE_PORT"])
TOKEN = os.environ["BARRY_SERVICE_TOKEN"]
count = 0

class Handler(BaseHTTPRequestHandler):
    def do_POST(self):
        global count
        if self.headers.get("X-Barry-Token") != TOKEN:
            self.send_response(403)
            self.end_headers()
            return
        count += 1
        body = json.dumps({"count": count}).encode()
        self.send_response(200)
        self.send_header("Content-Type", "application/json")
        self.send_header("Content-Length", str(len(body)))
        self.end_headers()
        self.wfile.write(body)

ThreadingHTTPServer(("127.0.0.1", PORT), Handler).serve_forever()

main.qml asks it:

import QtQuick

Rectangle {
    id: app
    required property var barry
    color: "black"

    Text {
        id: label
        anchors.centerIn: parent
        color: "white"
        font.pixelSize: 60 * app.barry.scale
        text: "Tap"
    }
    TapHandler {
        onTapped: app.barry.request("POST", "count", {}, function(status, reply) {
            label.text = status === 200 ? "Count: " + reply.count : "No answer (" + status + ")"
        })
    }
}

barry-app run starts the service too, so you can try this on a PC. The service's output shows in the same terminal.

Debugging

• Run the service on its own: BARRY_SERVICE_PORT=47900 BARRY_SERVICE_TOKEN=dev python3 service.py, then curl -H "X-Barry-Token: dev" -X POST http://127.0.0.1:47900/count.
• On the device, everything the service prints is in ~/.cache/barry_launcher/ID.log. Use python3 -u style output (Barry Launcher already turns buffering off) or flush=True.
• A service that crashes stays stopped until the app is opened again. Show the user that it isn't answering (status 0) rather than a blank screen.