Coding style

File template

Use misc/lv_templ.c and misc/lv_templ.h

Naming conventions

  • Words are separated by '_'

  • In variable and function names use only lower case letters (e.g. height_tmp)

  • In enums and defines use only upper case letters (e.g. e.g. MAX_LINE_NUM)

  • Global names (API):

    • start with lv

    • followed by module name: button, label, style etc.

    • followed by the action (for functions): set, get, etc.

    • closed with the subject: name, size, state etc.

  • Typedefs

    • prefer typedef struct and typedef enum instead of struct name and enum name

    • always end typedef struct and typedef enum type names with _t

  • Abbreviations:

    • The following abbreviations are used and allowed:

      • dsc descriptor

      • param parameter

      • indev input device

      • anim animation

      • buf buffer

      • str string

      • min/max minimum/maximum

      • alloc allocate

      • ctrl control

      • pos position

    • Avoid adding new abbreviations

Coding guide

  • Functions:

    • Write function with single responsibility

    • Make the functions static where possible

  • Variables:

    • One line, one declaration (BAD: char x, y;)

    • Use <stdint.h> (uint8_t, int32_t etc)

    • Declare variables where needed (not all at function start)

    • Use the smallest required scope

    • Variables in a file (outside functions) are always static

    • Do not use global variables (use functions to set/get static variables)

Comments

Before every function have in .h files a comment like this:

/**
 * Return with the screen of an object
 * @param obj pointer to an object
 * @return pointer to a screen
 */
lv_obj_t * lv_obj_get_screen(lv_obj_t * obj);

Always use /*Something*/ format and NOT //Something

Write readable code to avoid descriptive comments like: x++; /*Add 1 to x*/. The code should show clearly what you are doing.

You should write why have you done this: x++; /*Because of closing '\0' of the string*/

Short "code summaries" of a few lines are accepted. E.g. /*Calculate the new coordinates*/

In comments use ` ` when referring to a variable. E.g. /*Update the value of `x_act`*/

API Conventions

To support the auto-generation of bindings, the LVGL C API must follow some coding conventions:

  • Use enums instead of macros. If inevitable to use defines export them with LV_EXPORT_CONST_INT(defined_value) right after the define.

  • In function arguments use type name[] declaration for array parameters instead of type * name

  • Use typed pointers instead of void* pointers

  • Widget constructor must follow the lv_<widget_name>_create(lv_obj_t * parent) pattern.

  • Widget members function must start with lv_<widget_name> and should receive lv_obj_t* as first argument which is a pointer to widget object itself.

  • struct APIs should follow the widgets' conventions. That is to receive a pointer to the struct as the first argument, and the prefix of the struct name should be used as the prefix of the function name too (e.g. lv_display_set_default(lv_display_t * disp))

  • Functions and structs which are not part of the public API must begin with underscore in order to mark them as "private".

  • Argument must be named in H files too.

  • Do not malloc into a static or global variables. Instead declare the variable in lv_global_t structure in lv_global.h and mark the variable with (LV_GLOBAL_DEFAULT()->variable) when it's used.

  • To register and use callbacks one of the following needs to be followed.

    • Pass a pointer to a struct as the first argument of both the registration function and the callback. That struct must contain void * user_data field.

    • The last argument of the registration function must be void * user_data and the same user_data needs to be passed as the last argument of the callback.

To learn more refer to the documentation of MicroPython.

Formatting

Here is example to show bracket placing and using of white spaces:

/**
 * Set a new text for a label. Memory will be allocated to store the text by the label.
 * @param label pointer to a label object
 * @param text '\0' terminated character string. NULL to refresh with the current text.
 */
void lv_label_set_text(lv_obj_t * label, const char * text)
{   /*Main brackets of functions in new line*/

    if(label == NULL) return; /*No bracket only if the command is inline with the if statement*/

    lv_obj_inv(label);

    lv_label_ext_t * ext = lv_obj_get_ext(label);

    /*Comment before a section*/
    if(text == ext->txt || text == NULL) {  /*Bracket of statements start inline*/
        lv_label_refr_text(label);
        return;
    }

    ...
}

Use 4 spaces indentation instead of tab.

You can use astyle to format the code. Run code-format.py from the scripts folder.

pre-commit

pre-commit is a multi-language package manager for pre-commit hooks. See the installation guide to get pre-commit python package installed into your development machine.

Once you have pre-commit installed you will need to set up the git hook scripts with:

pre-commit install

now pre-commit will run automatically on git commit!

Hooks

The format-source local hook (see .pre-commit-config.yaml) runs astyle on all the staged source and header files (that are not excluded, see exclude key of the hook configuration) before entering the commit message, if any file gets formatted by astyle you will need to add the change to the staging area and run git commit again.

The trailing-whitespace hook fixes trailing whitespaces on all of the files.

Skipping hooks

If you want to skip any particular hook you can do so with:

SKIP=name-of-the-hook git commit

Testing hooks

It's no necessary to do a commit to test the hooks, you can test hooks by adding the files into the staging area and run:

pre-commit run name-of-the-hook