queries/run_callback

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  #include <run_callback.h>
  #include <iostream>
   
  // This example shows how to use the run callback, which provides more direct
  // control over the iteration of a query. It also makes it easier to run code
  // before and after the iteration.
   
  struct Position { 
      double x, y;
  };
   
  struct Velocity { 
      double x, y;
  };
   
  int main(int, char *[]) {
      flecs::world ecs;
   
      // Create a query for Position, Velocity.
      flecs::query<Position, const Velocity> q = 
          ecs.query<Position, const Velocity>();
   
      // Create a few test entities for a Position, Velocity query
      ecs.entity("e1")
          .set<Position>({10, 20})
          .set<Velocity>({1, 2});
   
      ecs.entity("e2")
          .set<Position>({10, 20})
          .set<Velocity>({3, 4});
   
      // This entity will not match as it does not have Position, Velocity
      ecs.entity("e3")
          .set<Position>({10, 20});
   
      // Iterate the query
      q.run([](flecs::iter& it) {
          std::cout << "Move begin\n";
   
          while (it.next()) {
              auto p = it.field<Position>(0);
              auto v = it.field<const Velocity>(1);
   
              for (auto i : it) {
                  flecs::entity e = it.entity(i);
                  p[i].x += v[i].x;
                  p[i].y += v[i].y;
                  std::cout << e.name() 
                              << ": {" << p[i].x << ", " << p[i].y << "}\n";
              }
          }
   
          std::cout << "Move end\n";
      });
   
      // Output
      //  Move begin
      //  e1: {11, 22}
      //  e2: {13, 24}
      //  Move end
  }