How to automatically hot reload Flutter when Dart source files change

The scripts from this post are now on GitHub: https://github.com/kika/flutter_hotreloader

Dead simple:

brew install entr or use your OS package manager. Or go to http://eradman.com/entrproject/

flutter run -d <your device id> --pid-file /tmp/flutter.pid

cd yourprojectdir
find lib/ -name '*.dart' | \
    entr -p kill -USR1 $(cat /tmp/flutter.pid)

Bingo.

What does it do: entr reads a list of files from standard input and starts watching them for changes. Once it detects the change it executes a command passed to its command line. Flutter reacts to SIGUSR1 signal and hot reloads.

Although there’s a drawback in this simplistic approach: if you add a new Dart file and start making changes to it, the script above will not notice it. Because the list of files is created once at the start of the script and then fed to entr which then monitor them. If you need free white glove new files delivery do the following:

while true
do
    find lib/ -name '*.dart' | \
        entr -d -p kill -USR1 $(cat /tmp/flutter.pid)
done

The option -d will make entr exit every time a new file is detected in all the directories fed to it in the first place (in this case within lib/ hierarchy). And then the while loop will just restart the process all over again, scanning all the files and adding them to entr.

I actually use a more complex setup, because I use Redux in the project and hot reload doesn’t work good with changes to some redux related files (like middleware and such). So I call hot restart (SIGUSR2) for files related to state, and hot reload (SIGUSR1) for the rest.

Instead of kill I use a script, called hotreloader.sh which determines what type of file was changed and does the proper kill to cause either hot restart or hot reload

#!/bin/bash
set -euo pipefail
PIDFILE="/tmp/flutter.pid"
if [[ "${1-}" != "" && -e $PIDFILE ]]; then
    if [[ "$1" =~ \/state\/ ]]; then
        kill -USR2 $(cat $PIDFILE)
    else
        kill -USR1 $(cat $PIDFILE)
    fi
fi

And then I use the variation of the command above to start entr :

while true
do
    find lib/ -name '*.dart' | \
        entr -d -p ./hotreloader.sh /_
done