fix(display): load screen from matching display

[TheOfficialMrBlah]

If the function "lv_screen_load_anim()" is used to change the screen with a defined delay and the default display is changed within the delay in a multi-display setup, the wrong screen is loaded as the "previous" screen.

Fix: lv_screen_active() replaced with lv_display_get_screen_active()
(lv_display.c)

lvgl_bug_load_screen.mp4

The bug can be seen in the video:
the top left display is set as the default display. When starting the animation from the lower display and the upper right display, the active screen is loaded from the upper left display.

Kind regards and have a nice day! :)

[liamHowatt]

Nice!

Here and here too, I believe.

diff --git a/src/display/lv_display.c b/src/display/lv_display.c
index 4c061d095..4eda20a3c 100644
--- a/src/display/lv_display.c
+++ b/src/display/lv_display.c
@@ -570,7 +570,7 @@ void lv_screen_load_anim(lv_obj_t * new_scr, lv_screen_load_anim_t anim_type, ui
                          bool auto_del)
 {
     lv_display_t * d = lv_obj_get_display(new_scr);
-    lv_obj_t * act_scr = lv_screen_active();
+    lv_obj_t * act_scr = lv_display_get_screen_active(d);
 
     if(act_scr == new_scr || d->scr_to_load == new_scr) {
         return;
@@ -586,7 +586,7 @@ void lv_screen_load_anim(lv_obj_t * new_scr, lv_screen_load_anim_t anim_type, ui
         if(d->del_prev) {
             lv_obj_delete(act_scr);
         }
-        act_scr = lv_screen_active(); /*Active screen changed.*/
+        act_scr = lv_display_get_screen_active(d); /*Active screen changed.*/
 
         scr_load_internal(d->scr_to_load);
     }

Based on this small test

static void scr_load_anim_issue(lv_display_t * d1, lv_display_t * d2)
{
    lv_display_set_default(d1);

    lv_obj_t * d1_s1 = lv_display_get_screen_active(d1);
    lv_label_set_text(lv_label_create(d1_s1), "d1_s1");

    lv_obj_t * d2_s1 = lv_display_get_screen_active(d2);
    lv_label_set_text(lv_label_create(d2_s1), "d2_s1");

    lv_display_set_default(d2);
    lv_obj_t * d2_s2 = lv_obj_create(NULL);
    lv_display_set_default(d1);
    lv_label_set_text(lv_label_create(d2_s2), "d2_s2");

    lv_screen_load_anim(d2_s2, LV_SCR_LOAD_ANIM_OUT_RIGHT, 3000, 1000, false);
}

Also I think d->act_scr would be fine instead of lv_display_get_screen_active(d).

[TheOfficialMrBlah]

Thank you for your answer!

Here and here too, I believe.

At the calls, in the function "lv_screen_load_anim()" I was unsure, because when creating objects they are always created on the current default display.

But I would also find it more logical and better to change it here as well, since no object is created but one (screen) is passed, and the screen belongs to a display.

Also I think d->act_scr would be fine instead of lv_display_get_screen_active(d).

For "lv_display_get_screen_active()" I wanted to keep it similar to the original call, but of course these are unnecessary function calls.

I'll add/update this quickly.

[liamHowatt]

Thank you!

I went through the same thought process. For consistency it would make sense for the functions to operate on the active display, but the screen being passed belongs to a display which may not be the default one, and if it's not, it's an error to try to load it to a different display.

As I say that though, I have some doubts because I'm not sure if/why screens can't/shouldn't be moved between displays. The other reviewers may be able to check the claim. There's another way this could be fixed, which is to change lv_display_t * d = lv_obj_get_display(new_scr); to lv_display_t * d = lv_display_get_default(); so that lv_screen_load_anim would operate on the current default display and load whatever screen was passed.

[kisvegabor]

Nice catch! Thank you for opening this PR!

Loading the same screen on others displays is an interesting idea, but it would require more consideration. E.g. is the display has a list for its screens it might lead to all sort of issues, if we load a screen on a foreign display.

Anyway, this PR looks good. 🙂 If there is an interest to discuss "cross-loading" screens, please open an issue for it.