how to delay a function that is called in a while loop without delaying the loop

Question

imagine I have something like this

void color(int a)
{
  if (a > 10)
  {
    return;
  }
    square[a].red();
    sleep(1second);
    color(a+1);
}
while (programIsRunning())
{
  color(1);
  updateProgram();
}

but with something that actually requires a recursive function. how can I call this recursive function to color the squares one by one. because on its own its too fast and if the program is being updated every frame. they instantly get colored when I want them to get colored one by one (with a delay).

The question already has answers here. https://stackoverflow.com/questions/4184468/sleep-for-milliseconds — Sai Suman Chitturi, Jul 17 '21 at 16:01
If you have access to C++20, you might want to have a look at coroutines, it could be what you want. — ShadowMitia, Jul 17 '21 at 16:09

score 0 · Accepted Answer · 2021-07-18T16:16:58.827

sleep() will cause the current thread to stop. That makes it a bad candidate for human-perceptible delays from the main thread.

You "could" have a thread that only handles that process, but threads are expensive, and creating/managing one just to color squares in a sequence is completely overkill.

Instead, you could do something along the lines of: Every time the program updates, check if it's he appropriate time to color the next square.

const std::chrono::duration<double> color_delay{0.1};

auto last_color_time = std::chrono::steady_clock::now();
bool coloring_squares = true;

while (programIsRunning()) {

  if (coloring_squares) {
    auto now = std::chrono::steady_clock::now();

    // This will "catch up" as needed.
    while (now - last_color_time >= color_delay) {
      last_color_time += color_delay;
      coloring_squares = color_next_square();
    }
  }

  updateProgram();
}

How color_next_square() works is up to you. You could possibly "pre-bake" a list of squares to color using your recursive function, and iterate through it.

Also, obviously, this example just uses the code you posted. You'll want to organise all this as part of updateProgram(), possibly in some sort of class SquareAnim {}; stateful wrapper.

N.B. If your program has little jitter, i.e. it has consistent time between updates, and the delay is low, using the following instead can lead to a slightly smoother animation:

if (now - last_color_time >= color_delay) {
  last_color_time = now;
  // ...

how to delay a function that is called in a while loop without delaying the loop

1 Answers1