When DEBUG is True, the template tag includes a small script. This script connects back to the development server and will automatically reload when runserver restarts, or a template is modified. (Template modification detection requires Django 3.2+.) The reload only happens in the most recently opened tab.
See the example app in the example/ directory of the GitHub repository. Start it up, and try modifying example/core/views.py or templates/index.html to see the reloading in action.
The template tag includes a listener script on each page. This listener script starts or connects to a SharedWorker, running a worker script. The worker script then connects to the events view in Django, using an EventSource to receive server-sent events.
This event source uses StreamingHttpResponse to send events to the worker. The view continues streaming events indefinitely, until disconnected. (This requires a thread and will not work if you use runserver’s --nothreading option.)
On a relevant event, the worker will reload the most recently connected tab. (It avoids reloading all tabs since that could be expensive.)
To reload when a template changes, django-browser-reload piggybacks on Django’s autoreloading infrastructure. An internal Django signal indicates when a template file has changed. The events view receives this signal and sends an event to the worker, which triggers a reload. There is no smart filtering - if any template file changes, the view is reloaded.
To reload when the server restarts, django-browser-reload uses a version ID. This ID is randomly generated when the view module is imported, so it will be different every time the server starts. When the server restarts, the worker’s EventSource reconnects with minimal delay. On connection, the events view sends the version ID, which the worker sees as different, so it triggers a reload.
The events view also sends the version ID every second to keep the connection alive.