feat(examples): add infinite scroll example (#7388)

examples/scroll/index.rst

@@ -30,4 +30,9 @@ Translate on scroll
 .. lv_example:: scroll/lv_example_scroll_6
   :language: c
 
+Infinite scrolling
+------------------
+.. lv_example:: scroll/lv_example_scroll_7
+  :language: c
+
 

examples/scroll/lv_example_scroll.h

@@ -31,6 +31,7 @@ void lv_example_scroll_3(void);
 void lv_example_scroll_4(void);
 void lv_example_scroll_5(void);
 void lv_example_scroll_6(void);
+void lv_example_scroll_7(void);
 
 /**********************
  *      MACROS

examples/scroll/lv_example_scroll_7.c

@@ -0,0 +1,123 @@
+#include "../lv_examples.h"
+#if LV_BUILD_EXAMPLES && LV_USE_FLEX
+
+static lv_obj_t * high_label;
+static lv_obj_t * low_label;
+static int32_t top_num;
+static int32_t bottom_num;
+
+static lv_obj_t * load_item(lv_obj_t * parent, int32_t num)
+{
+    lv_obj_t * new = lv_obj_create(parent);
+    lv_obj_set_size(new, LV_PCT(100), LV_SIZE_CONTENT);
+    lv_obj_t * label = lv_label_create(new);
+    lv_label_set_text_fmt(label, "%"PRId32, num);
+    return new;
+}
+
+static void update_scroll(lv_obj_t * obj)
+{
+    int32_t top_num_original = top_num;
+    int32_t bottom_num_original = bottom_num;
+
+    /* load items we're getting close to */
+    while(bottom_num > -30 && lv_obj_get_scroll_bottom(obj) < 200) {
+        bottom_num -= 1;
+        load_item(obj, bottom_num);
+        lv_obj_update_layout(obj);
+        LV_LOG_USER("loaded bottom num: %"PRId32, bottom_num);
+    }
+    while(top_num < 30 && lv_obj_get_scroll_top(obj) < 200) {
+        top_num += 1;
+        int32_t bottom_before = lv_obj_get_scroll_bottom(obj);
+        lv_obj_t * new_item = load_item(obj, top_num);
+        lv_obj_move_to_index(new_item, 0);
+        lv_obj_update_layout(obj);
+        int32_t bottom_after = lv_obj_get_scroll_bottom(obj);
+        lv_obj_scroll_by(obj, 0, bottom_before - bottom_after, LV_ANIM_OFF);
+        LV_LOG_USER("loaded top num: %"PRId32, top_num);
+    }
+
+    /* delete far-away items */
+    while(lv_obj_get_scroll_bottom(obj) > 600) {
+        bottom_num += 1;
+        lv_obj_t * child = lv_obj_get_child(obj, -1);
+        lv_obj_delete(child);
+        lv_obj_update_layout(obj);
+        LV_LOG_USER("deleted bottom num: %"PRId32, bottom_num);
+    }
+    while(lv_obj_get_scroll_top(obj) > 600) {
+        top_num -= 1;
+        int32_t bottom_before = lv_obj_get_scroll_bottom(obj);
+        lv_obj_t * child = lv_obj_get_child(obj, 0);
+        lv_obj_delete(child);
+        lv_obj_update_layout(obj);
+        int32_t bottom_after = lv_obj_get_scroll_bottom(obj);
+        lv_obj_scroll_by(obj, 0, bottom_before - bottom_after, LV_ANIM_OFF);
+        LV_LOG_USER("deleted top num: %"PRId32, top_num);
+    }
+
+    if(top_num != top_num_original) {
+        lv_label_set_text_fmt(high_label, "current largest\nloaded value:\n%"PRId32, top_num);
+    }
+    if(bottom_num != bottom_num_original) {
+        lv_label_set_text_fmt(low_label, "current smallest\nloaded value:\n%"PRId32, bottom_num);
+    }
+}
+
+static void scroll_cb(lv_event_t * e)
+{
+    lv_obj_t * obj = lv_event_get_target_obj(e);
+    update_scroll(obj);
+}
+
+static void checkbox_cb(lv_event_t * e)
+{
+    lv_obj_t * checkbox = lv_event_get_target_obj(e);
+    lv_obj_t * obj = lv_event_get_user_data(e);
+    bool checked = lv_obj_has_state(checkbox, LV_STATE_CHECKED);
+    lv_obj_set_style_opa(obj, checked ? LV_OPA_COVER : LV_OPA_TRANSP, LV_PART_SCROLLBAR);
+}
+
+/**
+ * Dynamically load widgets while scrolling
+ */
+void lv_example_scroll_7(void)
+{
+    lv_obj_t * scr = lv_screen_active();
+    lv_obj_t * obj = lv_obj_create(scr);
+    lv_obj_set_size(obj, 160, 220);
+    lv_obj_align(obj, LV_ALIGN_RIGHT_MID, -10, 0);
+    lv_obj_set_flex_flow(obj, LV_FLEX_FLOW_COLUMN);
+    lv_obj_set_style_opa(obj, LV_OPA_TRANSP, LV_PART_SCROLLBAR);
+
+    high_label = lv_label_create(scr);
+    lv_label_set_text_static(high_label, "current largest\nloaded value:");
+    lv_obj_align(high_label, LV_ALIGN_TOP_LEFT, 10, 10);
+
+    lv_obj_t * checkbox = lv_checkbox_create(scr);
+    lv_checkbox_set_text_static(checkbox, "show\nscrollbar");
+    lv_obj_align(checkbox, LV_ALIGN_LEFT_MID, 10, 0);
+    lv_obj_add_event_cb(checkbox, checkbox_cb, LV_EVENT_VALUE_CHANGED, obj);
+
+    low_label = lv_label_create(scr);
+    lv_label_set_text_static(low_label, "current smallest\nloaded value:");
+    lv_obj_align(low_label, LV_ALIGN_BOTTOM_LEFT, 10, -10);
+
+    /* These counters hold the the highest/lowest number currently shown.
+     * Since no numbers are show yet, set them such that they will be
+     * correct after the first value is added.
+     * I.e., if a value is added at the top, `top_num` will be incremented
+     * and its new value (4) will be added to the top (`top_num`=4, `bottom_num`=4).
+     * If a value is added to the bottom, `bottom_num` will be decremented
+     * and its new value (3) will be added to the bottom (`top_num`=3, `bottom_num`=3).
+     */
+    top_num = 3;
+    bottom_num = 4;
+
+    lv_obj_update_layout(obj);
+    update_scroll(obj);
+    lv_obj_add_event_cb(obj, scroll_cb, LV_EVENT_SCROLL, NULL);
+}
+
+#endif