【LittlevGL】看看LVGL8的draw_ctx

调用"lv_disp_drv_init"函数初始化一个显示设备驱动，一个显示设备驱动的结构体内容如下：

typedef struct _lv_disp_drv_t {

    lv_coord_t hor_res;         /**< Horizontal resolution.*/
    lv_coord_t ver_res;         /**< Vertical resolution.*/

    lv_coord_t
    physical_hor_res;     /**< Horizontal resolution of the full / physical display. Set to -1 for fullscreen mode.*/
    lv_coord_t
    physical_ver_res;     /**< Vertical resolution of the full / physical display. Set to -1 for fullscreen mode.*/
    lv_coord_t
    offset_x;             /**< Horizontal offset from the full / physical display. Set to 0 for fullscreen mode.*/
    lv_coord_t offset_y;             /**< Vertical offset from the full / physical display. Set to 0 for fullscreen mode.*/

    /** Pointer to a buffer initialized with `lv_disp_draw_buf_init()`.
     * LVGL will use this buffer(s) to draw the screens contents*/
    lv_disp_draw_buf_t * draw_buf;

    uint32_t direct_mode : 1;        /**< 1: Use screen-sized buffers and draw to absolute coordinates*/
    uint32_t full_refresh : 1;       /**< 1: Always make the whole screen redrawn*/
    uint32_t sw_rotate : 1;          /**< 1: use software rotation (slower)*/
    uint32_t antialiasing : 1;       /**< 1: anti-aliasing is enabled on this display.*/
    uint32_t rotated : 2;            /**< 1: turn the display by 90 degree. @warning Does not update coordinates for you!*/
    uint32_t screen_transp : 1;      /**Handle if the screen doesn't have a solid (opa == LV_OPA_COVER) background.
                                       * Use only if required because it's slower.*/

    uint32_t dpi : 10;              /** DPI (dot per inch) of the display. Default value is `LV_DPI_DEF`.*/

    /** MANDATORY: Write the internal buffer (draw_buf) to the display. 'lv_disp_flush_ready()' has to be
     * called when finished*/
    void (*flush_cb)(struct _lv_disp_drv_t * disp_drv, const lv_area_t * area, lv_color_t * color_p);

    /** OPTIONAL: Extend the invalidated areas to match with the display drivers requirements
     * E.g. round `y` to, 8, 16 ..) on a monochrome display*/
    void (*rounder_cb)(struct _lv_disp_drv_t * disp_drv, lv_area_t * area);

    /** OPTIONAL: Set a pixel in a buffer according to the special requirements of the display
     * Can be used for color format not supported in LittelvGL. E.g. 2 bit -> 4 gray scales
     * @note Much slower then drawing with supported color formats.*/
    void (*set_px_cb)(struct _lv_disp_drv_t * disp_drv, uint8_t * buf, lv_coord_t buf_w, lv_coord_t x, lv_coord_t y,
                      lv_color_t color, lv_opa_t opa);

    void (*clear_cb)(struct _lv_disp_drv_t * disp_drv, uint8_t * buf, uint32_t size);


    /** OPTIONAL: Called after every refresh cycle to tell the rendering and flushing time + the
     * number of flushed pixels*/
    void (*monitor_cb)(struct _lv_disp_drv_t * disp_drv, uint32_t time, uint32_t px);

    /** OPTIONAL: Called periodically while lvgl waits for operation to be completed.
     * For example flushing or GPU
     * User can execute very simple tasks here or yield the task*/
    void (*wait_cb)(struct _lv_disp_drv_t * disp_drv);

    /** OPTIONAL: Called when lvgl needs any CPU cache that affects rendering to be cleaned*/
    void (*clean_dcache_cb)(struct _lv_disp_drv_t * disp_drv);

    /** OPTIONAL: called when driver parameters are updated */
    void (*drv_update_cb)(struct _lv_disp_drv_t * disp_drv);

    /** On CHROMA_KEYED images this color will be transparent.
     * `LV_COLOR_CHROMA_KEY` by default. (lv_conf.h)*/
    lv_color_t color_chroma_key;

    lv_draw_ctx_t * draw_ctx;
    void (*draw_ctx_init)(struct _lv_disp_drv_t * disp_drv, lv_draw_ctx_t * draw_ctx);
    void (*draw_ctx_deinit)(struct _lv_disp_drv_t * disp_drv, lv_draw_ctx_t * draw_ctx);
    size_t draw_ctx_size;

#if LV_USE_USER_DATA
    void * user_data; /**< Custom display driver user data*/
#endif

} lv_disp_drv_t;

前面的是一些常规的显示设置和显存写回调函数，后面有一个“draw_ctx”字段，这个是一个”绘制语境“结构体，里面包含了最常用几何图形、图片、字符的绘制函数：

typedef struct _lv_draw_ctx_t  {
    /**
     *  Pointer to a buffer to draw into
     */
    void * buf;

    /**
     * The position and size of `buf` (absolute coordinates)
     */
    lv_area_t * buf_area;

    /**
     * The current clip area with absolute coordinates, always the same or smaller than `buf_area`
     */
    const lv_area_t * clip_area;


    void (*draw_rect)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_rect_dsc_t * dsc, const lv_area_t * coords);

    void (*draw_arc)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_arc_dsc_t * dsc, const lv_point_t * center,
                     uint16_t radius,  uint16_t start_angle, uint16_t end_angle);

    void (*draw_img_decoded)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_img_dsc_t * dsc,
                             const lv_area_t * coords, const uint8_t * map_p, lv_img_cf_t color_format);

    lv_res_t (*draw_img)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_img_dsc_t * draw_dsc,
                         const lv_area_t * coords, const void * src);

    void (*draw_letter)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_label_dsc_t * dsc,  const lv_point_t * pos_p,
                        uint32_t letter);


    void (*draw_line)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_line_dsc_t * dsc, const lv_point_t * point1,
                      const lv_point_t * point2);


    void (*draw_polygon)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_rect_dsc_t * draw_dsc,
                         const lv_point_t * points, uint16_t point_cnt);

    /**
     * Replace the buffer with a rect without decoration like radius or borders
     */
    void (*draw_bg)(struct _lv_draw_ctx_t * draw_ctx, const lv_draw_rect_dsc_t * draw_dsc, const lv_area_t * coords);

    /**
     * Wait until all background operations are finished. (E.g. GPU operations)
     */
    void (*wait_for_finish)(struct _lv_draw_ctx_t * draw_ctx);

    /**
     * Copy an area from buffer to an other
     * @param draw_ctx      pointer to a draw context
     * @param dest_buf      copy the buffer into this buffer
     * @param dest_stride   the width of the dest_buf in pixels
     * @param dest_area     the destination area
     * @param src_buf       copy from this buffer
     * @param src_stride    the width of src_buf in pixels
     * @param src_area      the source area.
     *
     * @note dest_area and src_area must have the same width and height
     *       but can have different x and y position.
     * @note dest_area and src_area must be clipped to the real dimensions of the buffers
     */
    void (*buffer_copy)(struct _lv_draw_ctx_t * draw_ctx, void * dest_buf, lv_coord_t dest_stride,
                        const lv_area_t * dest_area,
                        void * src_buf, lv_coord_t src_stride, const lv_area_t * src_area);
#if LV_USE_USER_DATA
    void * user_data;
#endif

} lv_draw_ctx_t;

“draw_ctx”后面还有一个“draw_ctx_init”字段，在"lv_disp_drv_init"函数中，如果没有使用硬件GPU的话，会被默认初始化成“lv_draw_sw_init_ctx”函数，这个函数中会将“draw_ctx”字段中的所有绘制回调函数都初始化成软绘（软件绘制）函数，软绘函数都是使用CPU进行绘制处理，如果有GPU的话，可以使用GPU绘制函数初始化“draw_ctx”。

以“draw_ctx”里的“draw_letter”函数为例，这是字符绘制函数，软绘字符绘制函数为“lv_draw_sw_letter”，在这个函数里面会调用“lv_font_get_glyph_dsc”函数用以获取glyph descriptor（字符描述符），如果获取描述符失败，则会显示一个placeholder（占位符，是一个方框框，如下图），表示这个字符是字库里面没有的字符。否则，就会从字库里面获取字符的点阵数据并绘制。