Coding Style

Use misc/lv_templ.c and misc/lv_templ.

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. 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

• Editor:

  • Set editor to use 4 spaces for tab indentations (instead of tab characters).
  • Exception: the Kconfig file and any make files require leading tab characters on child items.

• Functions:

  • Write functions that use the single-responsibility principle.
  • Make functions static when not part of that object's public API (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 prototype in .h files, include a Doxygen-formatted comment like this:

/**
 * Brief description.  Add a blank line + additional paragraphs when more detail is needed.
 * @param  parent     brief description of argument.  Additional detail can appear
 *                    on subsequent lines.  List of accepted values:
 *                    - value one
 *                    - value two
 *                    - value three
 * @return   brief description of return value.
 */
type_name_t * lv_function_name(lv_obj_t * parent);

The normal comment prefix /** causes the comment to document the code member after the comment. When documenting a code member that is before the comment, such as a struct member, use /**< like this:

/**
 * Brief description of struct
 *
 * When more detail is needed, add a blank line then the detail.
 */
typedef struct {
    char      *text;    /**< Brief description of this member */
    uint16_t   length;  /**< Brief description of this member */
} lv_example_type_t;

• When commenting code, use block comments like this /* Description */, not end-of-line comments like this // Description.

• Include a space after the /* or /**< and before the */ for improved readability.

• 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 you did it: x++; /* Point to closing '\0' of string */

• Short "code summaries" of a few lines are accepted: /* Calculate new coordinates */

• In comments use back-quotes (...) when referring to a code element, such as a variable, type, or struct name: /* Update value of `x_act` */

• When adding or modifying comments, priorities are (in order of importance):

  1. clarity (the ease with which other programmers can understand your intention),
  2. readability (the ease with which other programmers can read your comments),
  3. brevity (the quality of using few words when speaking or writing).

• Blank lines within comments are desirable when they improve clarity and readability.

• Remember, when you are writing source code, you are not just teaching the computer what to do, but also teaching other programmers how it works, not only users of the API, but also future maintainers of your source code. Comments add information about what you were thinking when the code was written, and why you did things that way---information that cannot be conveyed by the source code alone.

Doxygen Comment Specifics

Doxygen is the first program in a chain that generates the online LVGL API documentation from the files in the LVGL repository. Doxygen detects files it should pay attention to by them having a @file command inside a Doxygen comment. Doxygen comments begin with a leading /**. It ignores comments that do not have exactly two *.

The following is an illustration of an API function prototype with documentation illustrating most of the Doxygen commands used in LVGL.

/**
 * Set alignment of Widgets placed in containers with LV_STYLE_FLEX_FLOW style.
 *
 * The values for the `..._place` arguments come from the `lv_flex_align_t`
 * enumeration and have the same meanings as they do for flex containers in CSS.
 * @param  obj                   pointer to flex container.  It must have
 *                               `LV_STYLE_FLEX_FLOW` style or nothing will happen.
 * @param  main_place            where to place items on main axis (in their track).
 *                               (Any value of `lv_flex_align_t`.)
 * @param  cross_place           where to place item in track on cross axis.
 *                               - `LV_FLEX_ALIGN_START`
 *                               - `LV_FLEX_ALIGN_END`
 *                               - `LV_FLEX_ALIGN_CENTER`
 * @param  track_cross_place     where to place tracks in cross direction.
 *                               (Any value of `lv_flex_align_t`.)
 * Example for a title bar layout:
 * @code{.c}
 *     lv_obj_set_flex_align(title_bar, LV_FLEX_ALIGN_START, LV_FLEX_ALIGN_CENTER, LV_FLEX_ALIGN_CENTER);
 * @endcode
 * @see
 *     - https://css-tricks.com/snippets/css/a-guide-to-flexbox/
 *     - see  `lv_obj_set_flex_grow()` for additional information.
 */
void lv_obj_set_flex_align(lv_obj_t * widget, lv_flex_align_t main_place, lv_flex_align_t cross_place,
                           lv_flex_align_t track_cross_place);

• Always start Doxygen comment with a brief description of the code element it documents.

• When more detail is needed, add a blank line below the brief description and add additional information that may be needed by LVGL API users, including preconditions for calling the function. Doxygen needs the blank line to separate "brief" from "detail" portions of the description.

• Describe function parameters with the @param command. When a function writes to the address contained by a pointer parameter, if not already obvious (e.g. when the parameter name contains the word "out"), include the direction in the command for clarity:

  @param[out] param_name description.

• Describe return values with the @return command.

• Add at least 2 spaces after Doxygen commands for improved readability.

• Use back-quotes (...) around code elements (variables, type names, function names). For type names and function names, Doxygen generates a hyperlink to that code member's documentation (when it exists) with or without the single back-quotes.

• Append empty "()" to function names. Doxygen will not generate a hyperlink to the function's documentation without this.

• Use proper grammar for clarity. Descriptions of parameters do not need periods after them unless they are full sentences.

• Align edges of text around lists of parameters for ease of reading.

• Lists (e.g. of accepted parameter values) can be created by using the '-' character. If the list needs to be numbered, numbers can also be used.

• Place example code in a code block by surrounding it with @code{.c} and @endcode commands. Doxygen doesn't need the {.c} part, but the downstream software does.

• Refer reader to additional information using the @see command. Doxygen adds a "See also" paragraph. The text following the @see command will be indented.

• If you create a new pair of .c and .h files (e.g. for a new driver), ensure a Doxygen comment like this is at the top of each new file. Doxygen will not parse the file without the @file command being present.

/**
 * @file filename.c
 *
 */

Supported Doxygen Commands

• @file tells Doxygen to parse this file and also supplies documentation about the file itself when applicable (everything following it in the same comment).
• @param name description documents name as a function parameter, and description is the text that follows it until Doxygen encounters a blank line or another Doxygen command.
• @return description documents the return value until Doxygen encounters a blank line or another Doxygen command.
• @code{.c}/@endcode surrounds code that should be placed in a code block. While Doxygen knows to use C color-coding of code blocks in a .C file, the downstream part of the documentation generation sequence does not, so the {.c} appendage to the @code command is necessary.
• @note text starts a paragraph where a note can be entered. The note ends with a blank line, the end of the comment, or another Doxygen command that starts a new section. If the note contains more than one paragraph, additional paragraphs can be added by using additional @note commands. At this writing, @par commands do not add additional paragraphs to notes as indicated in the Doxygen documentation.
• @see text generates a "See also" paragraph in a highlighted section, helpful when additional information about a topic can be found elsewhere.

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 as well (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 as well.

• 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 placement and use of white space:

/**
 * Set new text for a label.  Memory will be allocated by label to store text.
 *
 * @param  label  pointer to label Widget
 * @param  text   '\0' terminated character string.
 *                NULL to refresh with 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 command is inline with 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 starts on same line */
        lv_label_refr_text(label);
        return;
    }

    ...
}

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

Includes

Various subsystems of LVGL can be enabled or disabled by setting a macro in lv_conf.h to 1 or 0 respectively. The code files that contain the logic for those subsystems are often arranged so that there is an #if <ENABLING_MACRO_NAME> directive near the top of the file, and its matching #endif is at the end of the file. If you add or modify such a subsystem in LVGL, whenever possible, the only #include that should be above those conditional directives should be just enough to include the enabling/disabling macro. Specifically:

• in the .c file: the #include that includes the header with the closest correspondence to that .c file
• in the mated .h file: #include "lv_conf_internal.h"

which, itself includes lv_conf.h. See examples at lv_freetype.c, lv_freetype_private.h and lv_freetype.h.

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-commitinstalled you will need toset 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 is not 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