A sample which runs coroutine method within Update()

IEnumeratorSample.cs

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

///
/// Add any IEnumerator function to the queue which to be executed in the Update() 
///
public class IEnumeratorSample : MonoBehaviour 
{
    Queue<IEnumerator> queue = new Queue<IEnumerator>();
    IEnumerator action = null;

    void Start () 
    {
        queue.Enqueue(Process());
    }

    bool keydown = false;

    void Update () 
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            keydown = true;
        }
        
        if (Input.GetKeyUp(KeyCode.Space))
            keydown = false;

        FireAction(queue);
    }

    void FireAction(Queue<IEnumerator> t)
    {
        if (action == null || action.MoveNext() == false)
        {
            if (queue.Count == 0)
                action = null;
            else
            {
                action = t.Dequeue();
                if (action.MoveNext() == false)
                    action = null;
            }
        }
    }

    IEnumerator Process()
    {
        while(!keydown)
            yield return 0;
        keydown = false;
        Debug.Log("first");

        while (!keydown)
            yield return 0;
        keydown = false;
        Debug.Log("second");

        while (!keydown)
            yield return 0;
        keydown = false;
        Debug.Log("third");

        while (!keydown)
            yield return 0;
        keydown = false;
        Debug.Log("fourth");

        yield return 0;
    }
}