Unity async / coroutine

public void Start()
{
    StartCoroutine(CountdownCoroutine(10));
}

public IEnumerator CountdownCoroutine(int value)
{
    int counter = value;
    var waitOneSecond = new WaitForSeconds(1.0F);

    while (counter > 0)
    {
        Debug.Log($"Countdown: {counter}");
            
        yield return waitOneSecond;
        counter--;
    }
}

Since the new WaitForSecond() call creates a new object every time it is run, it is a good idea to cache this object once at the beginning and then re-use it in every loop, like this example does with the waitOneSecond variable.

public void Start()
{
    CountdownAsync(10);
}

public async void CountdownAsync(int value)
{
    int counter = value;

    while (counter > 0)
    {
        Debug.Log($"Countdown: {counter}");
            
        await Task.Delay(1000);
        counter--;
    }
}

/* Not directly supported */
        

A coroutine cannot directly return data, because it is already returning the IEnumerator.

You can work around this limitation by writing the result into a global variable, but this is not ideal since it hides what the method is actually doing.

public async Start()
{
    int result = await CountdownAsync();
    
    Debug.Log($"Result: {result}");
}
            
public async Task<int> CountdownAsync(int value)
{
    int counter = value;

    while (counter > 0)
    {
        counter--;
        await Task.Yield();
    }

    return Random.Range(0, 1000);
}

public IEnumerator Start()
{
    yield return CountdownCoroutine(10);
    yield return CountdownCoroutine(20);
    yield return CountdownCoroutine(30);
}

public IEnumerator CountdownCoroutine(int value)
{
    int counter = value;

    while (counter > 0)
    {
        counter--;
        yield return null;
    }
}

public async Start()
{
    await CountdownAsync(10);
    await CountdownAsync(20);
    await CountdownAsync(30);
}

public async Task CountdownAsync(int value)
{
    int counter = value;

    while (counter > 0)
    {
        counter--;
        await Task.Yield();
    }
}

/* Not directly supported */
        

public async Start()
{
    List<Task> tasks = new List<Task>();

    tasks.Add(Countdown(10));
    tasks.Add(Countdown(20));
    tasks.Add(Countdown(30));

    await Task.WhenAll(tasks);
}

public async Task Countdown(int value)
{
    int counter = value;

    while (counter > 0)
    {
        counter--;
        await Task.Yield();
    }

    Debug.Log($"Finish {value} - 0");
}