confused with auto sync points

[yohanip]

Hello there great library!
i got in c++

#include <cstdio>
#include <flecs.h>

struct SceneA {};
struct SceneB {};
struct SceneC {};

flecs::entity sa, sb, sc;

void system_a(flecs::iter &it, size_t row) {
    auto world = it.world();
    printf("world:%p, System A run\n", &world);
    sa.mut(it).destruct();
    sb = world.entity("B").add<SceneB>();
}

void system_b(flecs::iter &it, size_t row) {  
    auto world = it.world();
    printf("world:%p, System B run\n", &world);
    sb.mut(it).destruct();
    sc = world.entity("C").add<SceneC>();
}

void system_c(flecs::iter &it, size_t row) {
    auto world = it.world();
    printf("world:%p, System C run\n", &world);
    sc.mut(it).destruct();
    sa= it.world().entity("A").add<SceneA>();
}

int main() {
    flecs::world world;

    sa = world.entity("A").add<SceneA>();
    
    world.set_target_fps(0.5);

    // the following will call A,B,C system on first world.progress (automatically add sync points)
    // and then on the 2nd world.progress it will only call A system
    // and then on the 3rd world.progress it will only call B system
    // and then on the 4th world.progress it will only call C system
    world.system().with<SceneA>().each(system_a);
    world.system().with<SceneB>().each(system_b);
    world.system().with<SceneC>().each(system_c); 

    while(true) {        
        world.progress();
        printf("--->\n");
    }

    return 0;
}

the question..
why on the first world.progress it add automatic sync points (i think..)
but the next one doesn't?
what if i need on each progress it should only run one system at a time

if i modify it to

    world.system().write<SceneA>().each(system_a);
    world.system().write<SceneB>().each(system_b);
    world.system().write<SceneC>().each(system_c);

now it prints A, B, C on each world.progress

thank you very much in advance

[SanderMertens]

Sync points are only inserted when:

• a system marks a component as read/write
• a subsequent system reads or writes that component

To see which sync points got inserted, do:

flecs::log::set_level(1); // will print pipeline changes, "merge" is a sync point

or connect your application to the explorer by doing this in the code:

world.set(flecs::Rest{});

and then going to https://www.flecs.dev/explorer