flutter-pi

A light-weight Flutter Engine Embedder for Raspberry Pi. Inspired by https://github.com/chinmaygarde/flutter_from_scratch. Flutter-pi also runs without X11, so you don't need to boot into Raspbian Desktop & have X11 and LXDE load up; just boot into the command-line.

You can now theoretically run every flutter app you want using flutter-pi, including apps using extensions & plugins, just that you'd have to build the platform side of the plugins you'd like to use yourself.

The difference between extensions and plugins is that extensions don't include any native code, they are just pure dart. Plugins (like the connectivity plugin) include platform-specific code.

Supported Platforms

Although flutter-pi is only tested on a Rasberry Pi 4 2GB, it should work fine on other linux platforms, with the following conditions:

support for hardware 3D acceleration. more precisely support for kernel-modesetting (KMS) and the direct rendering infrastructure (DRI)
CPU architecture is one of ARMv7, ARM64, x86 or x86 64bit.

This means flutter-pi won't work on a Pi Zero, Pi 1, or Pi 2. A Pi 3 works fine, even the 512MB A+ model.

If you encounter issues running flutter-pi on any of the supported platforms listed above, please report them to me and I'll fix them.

Running your App on the Raspberry Pi
1.1 Configuring your Raspberry Pi
1.2 Patching the App
1.3 Building the Asset bundle
1.4 Running your App with flutter-pi
Dependencies
2.1 flutter engine
2.2 graphics libs
2.3 fonts
Compiling flutter-pi (on the Raspberry Pi)
Performance
Keyboard Input
Touchscreen Latency

Running your App on the Raspberry Pi

Configuring your Raspberry Pi

Switching to Console mode

flutter-pi only works when Raspbian is in console mode (no X11 or Wayland server running). To switch the Pi into console mode, go to raspi-config -> Boot Options -> Desktop / CLI and select Console or Console (Autologin).

Enabling the V3D driver

flutter-pi doesn't support the legacy broadcom-proprietary graphics stack anymore. You need to make sure the V3D driver in raspi-config. Go to raspi-config -> Advanced Options -> GL Driver and select GL (Fake-KMS).

With this driver, it's best to give the GPU as little RAM as possible in raspi-config -> Advanced Options -> Memory Split, which is 16MB. This is because the V3D driver doesn't need GPU RAM anymore.

Fixing the GPU permissions

It seems like with newer versions of Raspbian, the pi user doesn't have sufficient permissions to directly access the GPU anymore. IIRC, this is because of some privilege escalation / arbitrary code execution problems of the GPU interface.

You can fix this by adding the pi user to the render group, but keep in mind that may be a security hazard:

usermod -a -G render pi

Then, restart your terminal session so the changes take effect. (reconnect if you're using ssh or else just reboot the Pi)

Otherwise, you'll need to run flutter-pi with sudo.

Patching the App

First, you need to override the default target platform in your flutter app, i.e. add the following line to your main method, before the runApp call:

debugDefaultTargetPlatformOverride = TargetPlatform.fuchsia;

The debugDefaultTargetPlatformOverride property is in the foundation library, so you need to import that.

Your main dart file should probably look similiar to this now:

import 'package:flutter/foundation.dart';

. . .

void main() {
  debugDefaultTargetPlatformOverride = TargetPlatform.fuchsia;
  runApp(MyApp());
}

. . .

Building the Asset bundle

Then to build the asset bundle, run the following commands. You need to use a flutter SDK that's compatible to the engine version you're using.

I'm using flutter_gallery in this example. (note that the flutter_gallery example does not work with flutter-pi, since it includes plugins that have no platform-side implementation for the raspberry pi yet)

cd flutter/examples/flutter_gallery
flutter build bundle

After that flutter/examples/flutter_gallery/build/flutter_assets would be a valid path to pass as an argument to flutter-pi.

Running your App with flutter-pi

USAGE:
  flutter-pi [options] <asset bundle path> [flutter engine options...]

OPTIONS:
  -i <glob pattern>   Appends all files matching this glob pattern
                      to the list of input (touchscreen, mouse, touchpad)
                      devices. Brace and tilde expansion is enabled.
                      Every file that matches this pattern, but is not
                      a valid touchscreen / -pad or mouse is silently
                      ignored.
                        If no -i options are given, all files matching
                      "/dev/input/event*" will be used as inputs.
                      This should be what you want in most cases.
                        Note that you need to properly escape each glob pattern
                      you use as a parameter so it isn't implicitly expanded
                      by your shell.

  -h                  Show this help and exit.

EXAMPLES:
  flutter-pi -i "/dev/input/event{0,1}" -i "/dev/input/event{2,3}" /home/helloworld_flutterassets
  flutter-pi -i "/dev/input/mouse*" /home/pi/helloworld_flutterassets
  flutter-pi /home/pi/helloworld_flutterassets

<asset bundle path> is the path of the flutter asset bundle directory (i.e. the directory containing kernel_blob.bin) of the flutter app you're trying to run.

[flutter engine options...] will be passed as commandline arguments to the flutter engine. You can find a list of commandline options for the flutter engine Here.

Dependencies

flutter engine

flutter-pi needs libflutter_engine.so and flutter_embedder.h to compile. It also needs the flutter engine's icudtl.dat at runtime. You have two options here:

you build the engine yourself. takes a lot of time, and it most probably won't work on the first try. But once you have it set up, you have unlimited freedom on which engine version you want to use. You can find some rough guidelines here. Andrew jones is working on some more detailed instructions.
you can use the pre-built engine binaries I am providing in the engine-binaries branch of this project.. I will only provide binaries for some engine versions though (most likely the stable ones).

graphics libs

Additionally, flutter-pi depends on mesa's OpenGL, OpenGL ES, EGL implementation and libdrm & libgbm. You can easily install those with sudo apt install libgl1-mesa-dev libgles2-mesa-dev libegl-mesa0 libdrm-dev libgbm-dev.

fonts

The flutter engine, by default, uses the Arial font. Since that doesn't come included with Raspbian, you need to install it using:

sudo apt install ttf-mscorefonts-installer fontconfig
sudo fc-cache

libgpiod (for the included GPIO plugin)

sudo apt-get install gpiod libgpiod-dev

Compiling flutter-pi (on the Raspberry Pi)

fetch all the dependencies, clone this repo and run

cd /path/to/the/cloned/flutter-pi/directory
make

The flutter-pi executable will then be located at this path: /path/to/the/cloned/flutter-pi/directory/out/flutter-pi

Performance

Performance is actually better than I expected. With most of the apps inside the flutter SDK -> examples -> catalog directory I get smooth 50-60fps.

Keyboard Input

Keyboard input is supported. There is one important limitation though. Text input (i.e. writing any kind of text/symbols to flutter input fields) only works when typing on the keyboard, which is attached to the terminal flutter-pi is running on. So, if you ssh into your Raspberry Pi to run flutter-pi, you have to enter text into your ssh terminal.

Raw Keyboard input (i.e. using tab to iterate through focus nodes) works with any keyboard attached to your Raspberry Pi.

converting raw key-codes to text symbols is not that easy (because of all the different keyboard layouts), so for text input flutter-pi basically uses stdin.

Touchscreen Latency

Due to the way the touchscreen driver works in raspbian, there's some delta between an actual touch of the touchscreen and a touch event arriving at userspace. The touchscreen driver in the raspbian kernel actually just repeatedly polls some buffer shared with the firmware running on the VideoCore, and the videocore repeatedly polls the touchscreen. (both at 60Hz) So on average, there's a delay of 17ms (minimum 0ms, maximum 34ms). If I have enough time in the future, I'll try to build a better touchscreen driver to lower the delay.

JeffyCN / flutter-pi