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Coroutine and simple examples


What is coroutine, TL;DR:

Simply put, coroutine in Unity is a piece of code (normally a method), and within that code there is a place to pause at each frame, and run again in the next frame until certain condition is met.

When will need coroutine:

There are a couple of scenarios:

  • When you have a large computing job which can't be finished in one frame, such as path finding. Normally we split the function into different frames.
  • When you have a trigger event, nothing will be done until it is triggered. e.g. when a game object moves to a specific location.
  • Asynchronous jobs, such as fetching resources online, etc


The following example shows how to set up a timer using coroutine:

private float countDown = 0;

public IEnumerator StartCountdown(float countdownValue = 10) {
	countDown = countdownValue;
	while (countDown > 0) {
		Debug.Log("Countdown: " + countDown);
		yield return new WaitForSeconds(1.0f);

// to use
// or 

// output
Countdown: 10
Countdown: 9
Countdown: 8
Countdown: 7
Countdown: 6
Countdown: 5
Countdown: 4
Countdown: 3
Countdown: 2
Countdown: 1

Return value of coroutine

Coroutine returns an IEnumerator, which is an iterator. The following are some different uses of coroutine:

// Continue after a specified time delay (2s in here), after all Update 
// functions have been called for the frame
yield return new WaitForSeconds(2);

// Continue after all FixedUpdate has been called on all scripts
yield return new WaitForFixedUpdate();