fix(animimage): add NULL pointer check

[yinntian]

Description of the feature or fix

When dsc sets an incorrect NULL value, it can cause the animation's callback function to incorrectly take an array subscript value for the NULL value and cause the program to crash, which should prompt an error message and return.

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[XuNeo]

Could you also add a commit to fix the value error in lv_animimg_set_src?

I think it should be lv_anim_set_values(&animimg->anim, 0, (int32_t)num);

https://github.com/lvgl/lvgl/pull/6206/files#diff-e02308a3e1e1189a9fb9cc6f0e8408f740eb84b5685ee8c7e5bc17b6c72a2543L69-L76

[liamHowatt]

@XuNeo the lv_example_animimg_1 now looks more correct after removing the - 1.

[kisvegabor]

If you modify lv_example_animimg_1 like this:

lv_animimg_set_repeat_count(animimg0, 1);

On the last frame it will set image[0] again due to this in index_change:

static void index_change(lv_obj_t * obj, int32_t index)
{
    int32_t idx;
    lv_animimg_t * animimg = (lv_animimg_t *)obj;

    idx = index % animimg->pic_count;

For good timing we should really remove the -1 but use this in index_change:

if(index >= animimg->pic_count) index =  animimg->pic_count - 1; 

[XuNeo]

if(index >= animimg->pic_count) index = animimg->pic_count - 1;

This is better. The anim value is mapped directly to the image index, thus lv_anim_set_values(&animimg->anim, 0, (int32_t)num - 1); sounds correct. if(index >= animimg->pic_count) index = animimg->pic_count - 1; makes sure if animation has overshoot, the index is still in range.

@yinntian Could you please update the PR accordingly?

[yinntian]

@XuNeo
Now here's the state of my attempts:

lv_anim_set_values(&animimg->anim, 0, (int32_t)num);

static void index_change(lv_obj_t * obj, int32_t index)
{
    int32_t idx;
    lv_animimg_t * animimg = (lv_animimg_t *)obj;

    if(animimg->dsc == NULL){
        return ;
    }
    
    if(index >= animimg->pic_count){
        index = animimg->pic_count - 1;
    }
    idx = index % animimg->pic_count;
    
    // debug code -------
    static int debug_count = -2;
    printf("%d ", idx);
    debug_count++;
    if(debug_count % 4 == 0){
        printf("\n");
    }
    //-------

    lv_image_set_src(obj, animimg->dsc[idx]);
}

Looks like the animation is normal

But by the output message, the change of 'idx' is a cycle of 0, 1, 2, 2, but the ‘2’ output will immediately output ‘0’, is it possible that there is something wrong with anim in this behavior?

[kisvegabor]

idx = index % animimg->pic_count; is not required. It looks like this for a 3 second animation with 3 images:

  0s        1s          2s         3s (stop)
  img0      img1        img2       img2
   ---------- ----------- ----------
       1s         1s          1s

As you can see this way each image were shown for 1 sec.