API for traversing hierarchy depth-first

[SanderMertens]

A common pattern for applications dealing with hierarchies is to access the data in a depth-first way. As asked by @teksoc-1 in #63 :

@SanderMertens On the subject of Flecs hierarchies, is there a public API to get all the children of a given parent? I poked around and wasn't seeing it, but seemed like a natural API to have, unless I'm missing something.

[SanderMertens]

@teksoc-1 Currently you can get the children for a given parent by creating an EcsManual system that matches the components of the children you want to see (e.g. TransformationMatrix), and then call ecs_run_w_filter, where you specify the parent as the filter. To get the full tree, you can invoke the system recursively.

In the next major version of Flecs this will change however, as I'm changing the way children are stored to solve some of the biggest problems that todays implementation has. There will be an API for traversing entities depth-first, but I don't know yet what it will look like.

[teksoc-1]

@SanderMertens Ahh ok, thank you for the response.

[SanderMertens]

The API to do depth-first traversal is implemented on the v2 branch, and looks like this:

void print_tree(
    ecs_world_t *world, 
    ecs_entity_t entity) 
{
    printf("%*s%s\n", indent * 2, "", ecs_get_name(world, entity));

    indent ++;

    // Get an iterator to the children of the current entity
    ecs_view_t it = ecs_tree_iter(world, entity);

    // Iterate all the tables that contain children for the parent
    while (ecs_tree_next(&it)) {
        for (int i = 0; i < it.count; i ++) {
            // Print the child, and recursively iterate
            print_tree(world, it.entities[i]);
        }
    }

    indent --;
}