LVGL仪表盘逆时针

背景

LVGL 8.3 自带的lv_meter控件，目前只支持顺时针显示，但是，项目上刚好用到了逆时针。官网论坛有讨论通过arc + lv_meter 组合的方式实现。这个方法在模拟器上可以，但是放到板子上，刻度显示不出来，只显示一个完整的弧，这不是我要的效果，具体为何如此，深入研究了一番，没找到原因。唯一的差别在模拟器使用的SDL，板子使用的FB。

思路

鉴于arc + lv_meter 组合的方案没有通过，转而换个思路。由于arc是支持逆时针的，那就先研究arc的实现方式，然后在lv_meter代码中修改。按照这个思路，又研究了一番源码，找到了方法。

实现

lv_meter_indicator_t

typedef struct
    {
        lv_meter_scale_t *scale;
        lv_meter_indicator_type_t type;
        lv_opa_t opa;
        int32_t start_value;
        int32_t end_value;
        union
        {
            // ... ...
            struct
            {
                uint16_t width;
                const void *src;
                lv_color_t color;
                int16_t r_mod;
                uint16_t mode; // 0: clockwise; 1: counter clockwise
            } arc;
            // ... ...
        } type_data;
    } lv_meter_indicator_t;

增加mode字段，用于描述顺时针还是逆时针。

lv_meter_add_arc

lv_meter_indicator_t *lv_meter_add_arc(lv_obj_t *obj, lv_meter_scale_t *scale, uint16_t width, lv_color_t color,
                                       int16_t r_mod, uint16_t mode)
{
    LV_ASSERT_OBJ(obj, MY_CLASS);
    lv_meter_t *meter = (lv_meter_t *)obj;
    lv_meter_indicator_t *indic = _lv_ll_ins_head(&meter->indicator_ll);
    LV_ASSERT_MALLOC(indic);
    lv_memset_00(indic, sizeof(lv_meter_indicator_t));
    indic->scale = scale;
    indic->opa = LV_OPA_COVER;

    indic->type = LV_METER_INDICATOR_TYPE_ARC;
    indic->type_data.arc.width = width;
    indic->type_data.arc.color = color;
    indic->type_data.arc.r_mod = r_mod;
    indic->type_data.arc.mode = mode;

    lv_obj_invalidate(obj);
    return indic;
}

增加输入参数，用于配置mode。

draw_arcs

static void draw_arcs(lv_obj_t *obj, lv_draw_ctx_t *draw_ctx, const lv_area_t *scale_area)
{
    // ... ...
     _LV_LL_READ_BACK(&meter->indicator_ll, indic)
    {
        lv_meter_scale_t *scale = indic->scale;
        int32_t start = scale->rotation;
        int32_t end = scale->rotation + scale->angle_range;
        int32_t start_angle;
        int32_t end_angle;

        if (indic->type_data.arc.mode == 0)
        {
            start_angle = lv_map(indic->start_value, scale->min, scale->max, start, end);
            end_angle = lv_map(indic->end_value, scale->min, scale->max, start, end);
        }
        else
        {
            start_angle = lv_map(indic->start_value, scale->min, scale->max, end, start);
            end_angle = lv_map(indic->end_value, scale->min, scale->max, end, start);
        }

        arc_dsc.start_angle = start_angle;
        arc_dsc.end_angle = end_angle;
        part_draw_dsc.radius = r_out + indic->type_data.arc.r_mod;
        part_draw_dsc.sub_part_ptr = indic;
        part_draw_dsc.p1 = &scale_center;

        lv_event_send(obj, LV_EVENT_DRAW_PART_BEGIN, &part_draw_dsc);
        if (indic->type_data.arc.mode == 0)
        {
            lv_draw_arc(draw_ctx, &arc_dsc, &scale_center, part_draw_dsc.radius, start_angle, end_angle);
        }
        else
        {
            lv_draw_arc(draw_ctx, &arc_dsc, &scale_center, part_draw_dsc.radius, end_angle, start_angle);
        }
        lv_event_send(obj, LV_EVENT_DRAW_PART_END, &part_draw_dsc);
    }
    // ... ...
}

根据mode值，进行不同设置。

inv_arc

static void inv_arc(lv_obj_t *obj, lv_meter_indicator_t *indic, int32_t old_value, int32_t new_value)
{
    bool rounded = lv_obj_get_style_arc_rounded(obj, LV_PART_ITEMS);

    lv_area_t scale_area;
    lv_obj_get_content_coords(obj, &scale_area);

    lv_coord_t r_out = lv_area_get_width(&scale_area) / 2;
    lv_point_t scale_center;
    scale_center.x = scale_area.x1 + r_out;
    scale_center.y = scale_area.y1 + r_out;

    r_out += indic->type_data.arc.r_mod;

    lv_meter_scale_t *scale = indic->scale;
    int32_t start = scale->rotation;
    int32_t end = scale->rotation + scale->angle_range;
    int32_t start_angle;
    int32_t end_angle;

    if (indic->type_data.arc.mode == 0)
    {
        start_angle = lv_map(old_value, scale->min, scale->max, start, end);
        end_angle = lv_map(new_value, scale->min, scale->max, start, end);
    }
    else
    {
        start_angle = lv_map(old_value, scale->min, scale->max, end, start);
        end_angle = lv_map(new_value, scale->min, scale->max, end, start);
    }

    lv_area_t a;
    lv_draw_arc_get_area(scale_center.x, scale_center.y, r_out, LV_MIN(start_angle, end_angle), LV_MAX(start_angle, end_angle), indic->type_data.arc.width, rounded, &a);
    lv_obj_invalidate_area(obj, &a);
}

根据mode值，进行不同设置。