The LittlevGL (LVGL) library provides a powerful and flexible framework for creating embedded graphical user interfaces (GUIs). One common task in GUI development is determining the index or position of a selected item within a list. This seemingly simple operation can have subtle variations depending on the type of list view used and the specifics of your implementation. This article delves into various methods for retrieving the position of a selected item in an LVGL list, covering both simple and custom list views, and providing detailed explanations and code examples.
Understanding LVGL List Views
Before diving into the methods, let's clarify the types of list views in LVGL. The core component is `lv_list`. This can be used in its simplest form to display a list of text labels. However, for more complex applications, developers often create custom list views by defining a custom `lv_obj` type and using it as the list's item. This allows for the inclusion of more sophisticated elements like images, icons, and multiple text labels within each list item. The approach to getting the selected item's position differs subtly between these two scenarios.
I. Simple List Views (Text-Only)
In a simple list view, each item consists solely of a text label. Obtaining the selected item's position is relatively straightforward. LVGL provides the `lv_list_get_selected_btn()` function. This function returns a pointer to the button object representing the selected item. While you don't directly get the index, you can traverse the list's children to find the index of this button.
Method 1: Iterating through Children
This method involves iterating through the children of the `lv_list` object and comparing each child with the selected button obtained from `lv_list_get_selected_btn()`.
#include "lvgl.h"
// ... other code ...
lv_obj_t * list = lv_list_create(lv_scr_act(), NULL);
// ... Add items to the list ...
lv_obj_t * selected_btn = lv_list_get_selected_btn(list);
if (selected_btn != NULL) {
int i = 0;
lv_obj_t * child;
lv_obj_t * next_child;
for (child = lv_obj_get_child(list, 0); child != NULL; child = next_child) {
next_child = lv_obj_get_next_sibling(child);
if (child == selected_btn) {
printf("Selected item index: %d\n", i);
break;
}
i++;
} else {
printf("No item selected\n");
// ... rest of your code ...
This code iterates through the children of the `lv_list` object. `lv_obj_get_child(list, 0)` gets the first child, and `lv_obj_get_next_sibling(child)` gets the next sibling. The loop continues until the selected button is found or the end of the children list is reached. The index `i` keeps track of the position. This approach is simple and works well for small lists, but it can be less efficient for very large lists.
Method 2: Using a Custom Data Structure (for larger lists)
For larger lists, a more efficient approach involves maintaining a separate data structure that maps the index to the corresponding list item. This avoids the need for iterative searching.
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