Need for Drawing Events in lv_scale

[vwheeler63]

Problem to solve

When lv_meter went away in LVGL v9.0.0 in favor of lv_scale, some important drawing events went away, and in previous firmware projects, I made heavy use of those events in a tachometer (rev counter in British), for example to:

• change the text of the major-tick labels about to be rendered -- div by 1000 so "5" = "5000 RPM", and
• to change the color of the needle just before it was rendered, to match a user-configured color representing the power range of the engine.

The drawing events sent by lv_meter were both powerful and valuable. They were well documented in the v8.4 documentation, and they came with vital drawing descriptor objects attached to the event (e argument) which gave write access to the text pointer, color and other things relevant to the drawing about to happen:

It would be a powerful addition to lv_scale to implement these drawing events for the same reasons that they were powerful and valuable for lv_meter.

Success criteria

That the documentation of events for lv_meter in LVGL v8.4 will apply in full to the current version. Since it is a feature ADDITION, no API will be broken.

Solution outline

• All the events with drawing descriptors be harvested out of the lv_meter and added to lv_scale.
• Documentation of events from v8.4 be copy/pasted into current documentation for lv_scale.

Rabbit holes

None known.

Testing

• In one of the lv_scale (or perhaps in a new example, so examples don't get too complex), demonstrate changing actual text and color of major-tick labels, and
• perhaps this same (new) example can demonstrate changing something about the needle (e.g. color?)

using these drawing events.

Teaching

• Documentation of events from v8.4 be copy/pasted into current documentation for lv_scale.

Considerations

The code that populated the descriptors for the documented in events in lv_meter were quite straightforward and can be copied nearly verbatim if the drawing descriptors have not changed between LVGL v8.4 and v9.3.

[kisvegabor]

Aren't it way simpler in with scale?

change the text of the major-tick labels about to be rendered -- div by 1000 so "5" = "5000 RPM"

    static const char * texts[] = {"a", "b", "c", "d", "e", "f", "g", NULL};
    lv_scale_set_text_src(scale, texts);

to change the color of the needle just before it was rendered, to match a user-configured color representing the power range of the engine.

    lv_obj_t * needle_line = lv_line_create(scale_line);
    
    //Later
    lv_scale_set_line_needle_value(scale, line, 100, value);
    lv_obj_set_style_line_color(needle_line, value_to_color(value), LV_PART_MAIN);

[vwheeler63]

Aren't it way simpler in with scale?

Indeed, those DO handle what I was using them for.... In which case, the "need" for them drops to near zero....

[kisvegabor]

Great! Note that you can still use the DRAW_TASK_ADDED event to adjust e.g. all the tick lines one by one.

[vwheeler63]

Great! Note that you can still use the DRAW_TASK_ADDED event to adjust e.g. all the tick lines one by one.

Excellent! In that case, I will close this since the need appears to have been filled in other ways! 👍