Bindings for LittelvGL

See also Micropython + LittlevGL blog post.
For advanced features, see Pure Micropython Display Driver blog post.
For questions and discussions - please use the forum: https://forum.littlevgl.com/c/micropython

Micropython

Micropython Binding for lvgl (LittlelvGL) provides an automatically generated Micropython module with classes and functions that allow the user access much of the lvgl library.
The module is generated automatically by the script gen_mpy.py.
This script reads, preprocesses and parses lvgl header files, and generates a C file lv_mpy.c which defines the Micropython module (API) for accessing lvgl from Micropython.
Micopython's build script (Makefile) should run gen_mpy.py automatically to generate and compile lv_mpy.c.

• If you would like to see an example of how a generated lv_mpy.c looks like, have a look at v_mpy_example.c. Note that its only exported (non static) symbol is mp_module_lvgl which should be registered in Micropython as a module.
• An example project that builds Micropython + lvgl + lvgl-bindings: lv_mpy

It's worth noting that the Mircopython Bindings module (lv_mpy.c) is dependant on lvgl configuration. lvgl is configured by lv_conf.h where different objects and features could be enabled or disabled. lvgl bindings are generated only for the enabled objects and features. Changing lv_conf.h requires re running gen_mpy.py, therfore it's useful to run it automatically in the build script.

Memory Management

When lvgl is built as a Micropython library, it is configured to allocate memory using Micropython memory allocation functions and take advantage of Micropython Garbage Collection ("gc").
This means that structs allocated for lvgl use don't need to be deallocated explicitly, gc takes care of that.
For this to work correctly, lvgl is configured to use gc and to use Micropython's memory allocation functions, and also register all lvgl "root" global variables to Micropython's gc.

From the user's perspective, structs can be created and will be collected by gc when they are no longer referenced.
However, lvgl screen objects (lv.obj with no parent) are automatically assigned to default display, therefor not collected by gc even when no longer explicitly referenced.
When you want to free a screen and all its decendants so gc could collect their memory, make sure you call screen.delete() when you no longer need it.

Concurrency

This implementation of Micropython Bindings to lvgl assumes that Micropython and lvgl are running on a single thread and on the same thread (or alternatively, running without multithreading at all).
No synchronization means (locks, mutexes) are taken.
However, asynchronous calls to lvgl still take place periodically for screen refresh and other lvgl tasks such as animation.

This is achieved by using the internal Micropython scheduler (that must be enabled), by calling mp_sched_schedule.
mp_sched_schedule is called when screen needs to be refreshed. lvgl expects the function lv_task_handler to be called periodically (see lvgl/README.md#porting). This is ususally handled in the display device driver.
Here is an example of calling lv_task_handler with mp_sched_schedule for refreshing lvgl. mp_lv_task_handler is scheduled to run on the same thread Micropython is running, and it calls both lv_task_handler for lvgl task handling and monitor_sdl_refr_core for refreshing the display and handling mouse events.

With REPL (interactive console), when waiting for the user input, asynchronous events can also happen. In this example we just call mp_handle_pending periodically when waiting for a keypress. mp_handle_pending takes care of dispatching asynchronous events registered with mp_sched_schedule.

Structs Classes and globals

The lvgl binding script parses lvgl headers and provides API to access lvgl classes (such as btn) and structs (such as color_t). All structs and classes are available under lvgl micropython module.

lvgl Class contains:

• functions (such as set_x)
• enums related to that class (such as STATE of a btn)

lvgl struct contains only attributes that can be read or written. For example:

c = lvgl.color_t()
c.ch.red = 0xff

structs can also be initialized from dict. For example, the example above can be written like this:

c = lvgl.color_t({'ch': {'red' : 0xff}})

All lvgl globals (functions, enums, types) are avaiable under lvgl module. For example, lvgl.SYMBOL is an "enum" of symbol strings, lvgl.anim_create will create animation etc.

Callbacks

In C a callback is a function pointer.
In Micropython we would also need to register a Micropython callable object for each callback.
Therefore in the Micropython binding we need to register both a function pointer and a Micropython object for every callback.

Therefore we defined a callback convention that expects lvgl headers to be defined in a certain way. Callbacks that are declared according to the convention would allow the binding to register a Micropython object next to the function pointer when registering a callback, and access that object when the callback is called.
The Micropython callable object is automatically saved in a user_data variable which is provided when registering or calling the callback.

The callback convetion assumes the following:

• There's a struct that contains a field called void * user_data.
• A pointer to that struct is provided as the first argument of a callback registration function.
• A pointer to that struct is provided as the first argument of the callback itself.

Another option is that the callback function pointer is just a field of a struct, in that case we expect the same struct to contain user_data field as well.

Another option is:

• A parameter called void * user_data is provided to the registration function as the last argument.
• The callback itself recieves void * as the last argument

In this case, the user should provide either None or a dict as the user_data argument of the registration function. The callback will recieve a Blob which can be casted to the dict in the last argument. (See async_call example below)

As long as the convention above is followed, the lvgl Micropython binding script would automatically set and use user_data when callbacks are set and used.

From the user perspective, any python callable object (such as python regular function, class function, lambda etc.) can be user as an lvgl callbacks. For example:

lvgl.anim_set_custom_exec_cb(anim, lambda anim, val, obj=obj: obj.set_y(val))

In this example an exec callback is registered for an animation anim, which would animate the y coordinate of obj.
An lvgl API function can also be used as a callback directly, so the example above could also be written like this:

lv.anim_set_exec_cb(anim, obj, obj.set_y)

lvgl callbacks that do not follow the Callback Convention cannot be used with micropython callable objects. A discussion related to adjusting lvgl callbacks to the convention: lvgl/lvgl#1036

The user_data field must not be used directly by the user, since it is used internally to hold pointers to Micropython objects.

Display and Input Drivers

LittlevGL can be configured to use different displays and different input devices. More information is available on LittlevGL documentation.
Registering a driver is essentially calling a registeration function (for example disp_drv_register) and passing a function pointer as a parameter (actually a struct that contains function pointers). The function pointer is used to access the actual display / input device.
When using LittlevGL with Micropython, it makes more sense to implement the display and input driver in C. However, the device registration is perfomed in the Micropython script to make it easy for the user to select and replace drivers without building the project and changing C files.
Technically, the driver can be written in either C or in pure Micropython using callbacks.

Example:

# init

import lvgl as lv
lv.init()

import SDL
SDL.init()

# Register SDL display driver.

disp_buf1 = lv.disp_buf_t()
buf1_1 = bytes(480*10)
disp_buf1.init(buf1_1, None, len(buf1_1)//4)
disp_drv = lv.disp_drv_t()
disp_drv.init()
disp_drv.buffer = disp_buf1
disp_drv.flush_cb = SDL.monitor_flush
disp_drv.hor_res = 480
disp_drv.ver_res = 320
disp_drv.register()

# Regsiter SDL mouse driver

indev_drv = lv.indev_drv_t()
indev_drv.init() 
indev_drv.type = lv.INDEV_TYPE.POINTER
indev_drv.read_cb = SDL.mouse_read
indev_drv.register()

In this example we import SDL. SDL module gives access to display and input device on a unix/linux machine. It contains several objects such as SDL.monitor_flush, which are wrappers around function pointers and can be registerd as LittlevGL display and input driver.
Behind the scences these objects implement the buffer protocol to give access to the function pointer bytes.

Starting from version 6.0, lvgl supports setting the display settings (width, length) on runtime. In this example they are set to 480x320. Color depth is set on compile time.

Currently supported drivers for Micropyton are

• SDL unix drivers (display and mouse)
• Linux Frame Buffer (/dev/fb0)
• ILI9341 driver for ESP32
• XPT2046 driver for ESP32
• Raw Resistive Touch for ESP32 (ADC connected to screen directly, no touch IC)

Driver code is under /driver directory.

Drivers can also be implemented in pure Micropython, by providing callbacks (disp_drv.flush_cb, indev_drv.read_cb etc.) Currently the supported ILI9341 and XPT2046 are pure micropython drivers

Adding Micropython Bindings to a project

An example project of "Micropython + lvgl + Bindings" is lv_mpy.
Here is a procedure for adding lvgl to an existing Micropython project. (The examples in this list are taken from lv_mpy):

• Add lv_bindings as a sub-module under lib.
• Add lv_conf.h in lib
• Edit the Makefile to run gen_mpy.py and build its product automatically. Here is an example.
• Register lvgl module and display/input drivers in Micropython as a builtin module. An example.
• Add lvgl roots to gc roots. An example.
• Configure lvgl to use Garbage Collection by setting several LV_MEM_CUSTOM_* and LV_GC_* macros example lv_conf.h was moved to lv_binding_micropython git module.
• Something I forgot? Please let me know.

gen_mpy.py syntax

usage: gen_mpy.py [-h] [-I <Include Path>] [-D <Macro Name>]
                  [-E <Preprocessed File>] [-M <Module name string>]
                  [-MP <Prefix string>] [-MD <MetaData File Name>]
                  input [input ...]

positional arguments:
  input

optional arguments:
  -h, --help            show this help message and exit
  -I <Include Path>, --include <Include Path>
                        Preprocesor include path
  -D <Macro Name>, --define <Macro Name>
                        Define preprocessor macro
  -E <Preprocessed File>, --external-preprocessing <Preprocessed File>
                        Prevent preprocessing. Assume input file is already
                        preprocessed
  -M <Module name string>, --module_name <Module name string>
                        Module name
  -MP <Prefix string>, --module_prefix <Prefix string>
                        Module prefix that starts every function name
  -MD <MetaData File Name>, --metadata <MetaData File Name>
                        Optional file to emit metadata (introspection)

Example:

python gen_mpy.py -MD lv_mpy_example.json -M lvgl -MP lv -I../../berkeley-db-1.xx/PORT/include -I../../lv_binding_micropython -I. -I../.. -Ibuild -I../../mp-readline -I ../../lv_binding_micropython/pycparser/utils/fake_libc_include ../../lv_binding_micropython/lvgl/lvgl.h

Binding other C libraries

The lvgl binding script can be used to bind other C libraries to Micropython.
I used it with lodepng and with parts of ESP-IDF.
For more details please read this blog post.

Micropython Bindings Usage

A simple example: advanced_demo.py. More examples can be found under /examples folder.

Importing and Initializing LittlelvGL

import lvgl as lv
lv.init()

Registering Display and Input drivers

import SDL
SDL.init()

# Register SDL display driver.

disp_buf1 = lv.disp_buf_t()
buf1_1 = bytes(480*10)
disp_buf1.init(buf1_1, None, len(buf1_1)//4)
disp_drv = lv.disp_drv_t()
disp_drv.init()
disp_drv.buffer = disp_buf1
disp_drv.flush_cb = SDL.monitor_flush
disp_drv.hor_res = 480
disp_drv.ver_res = 320
disp_drv.register()

# Regsiter SDL mouse driver

indev_drv = lv.indev_drv_t()
indev_drv.init() 
indev_drv.type = lv.INDEV_TYPE.POINTER
indev_drv.read_cb = SDL.mouse_read
indev_drv.register()

In this example, SDL display and input drivers are registered on a unix port of Micropython.

Here is an alternative example for ESP32 ILI9341 + XPT2046 drivers:

import lvgl as lv
import lvesp32

# Import ILI9341 driver and initialized it

from ili9341 import ili9341
disp = ili9341()

# Import XPT2046 driver and initalize it

from xpt2046 import xpt2046
touch = xpt2046()

By default, both ILI9341 and XPT2046 are initialized on the same SPI bus with the following parameters:

• ILI9341: miso=5, mosi=18, clk=19, cs=13, dc=12, rst=4, power=14, backlight=15, spihost=esp.HSPI_HOST, mhz=40, factor=4, hybrid=True
• XPT2046: cs=25, spihost=esp.HSPI_HOST, mhz=5, max_cmds=16, cal_x0 = 3783, cal_y0 = 3948, cal_x1 = 242, cal_y1 = 423, transpose = True, samples = 3

You can change any of these parameters on ili9341/xpt2046 constructor. You can also initalize them on different SPI buses if you want, by providing miso/mosi/clk parameters. Set them to -1 to use existing (initialized) spihost bus.

Creating a screen with a button and a label

scr = lv.obj()
btn = lv.btn(scr)
btn.align(lv.scr_act(), lv.ALIGN.CENTER, 0, 0)
label = lv.label(btn)
label.set_text("Button")

# Load the screen

lv.scr_load(scr)

Creating an instance of a struct

symbolstyle = lv.style_t(lv.style_plain)

symbolstyle would be an instance of lv_style_t initialized to the same value of lv_style_plain

Setting a field in a struct

symbolstyle.text.color = lv.color_hex(0xffffff)

symbolstyle.text.color would be initialized to the color struct returned by lv_color_hex

Setting a nested struct using dict

symbolstyle.text.color = {"red":0xff, "green":0xff, "blue":0xff}

Creating an instance of an object

self.tabview = lv.tabview(lv.scr_act())

The first argument to an object constructor is the parent object, the second is which element to copy this element from.
Both arguments are optional.

Calling an object method

self.symbol.align(self, lv.ALIGN.CENTER,0,0)

In this example lv.ALIGN is an enum and lv.ALIGN.CENTER is an enum member (an integer value).

Using callbacks

for btn, name in [(self.btn1, 'Play'), (self.btn2, 'Pause')]:
    btn.set_event_cb(lambda obj=None, event=-1, name=name: self.label.set_text('%s %s' % (name, get_member_name(lv.EVENT, event))))

Using callback with user_data argument:

def cb(user_data):
    print(user_data.cast()['value'])

lv.async_call(cb, {'value':42})

Listing available functions/memebers/constants etc.

print('\n'.join(dir(lvgl)))
print('\n'.join(dir(lvgl.btn)))
...