How to Utilize yield return in Unity: Examples and Best Practices

When working with Unity’s scripting system, coroutines are an essential feature for handling time-dependent tasks such as animations, delays, or even timed events. Typically, you may use yield return new WaitForSeconds() for basic timing, but there are other uses for yield return that offer greater flexibility, particularly when passing information between classes or managing tasks more efficiently.

In this post, we’ll explore how to effectively use yield return in Unity, go beyond just waiting, and discuss how it can help you structure your code in a cleaner, more modular way.

Understanding `yield return` in Coroutines

In Unity, a coroutine is a special type of function that allows you to pause its execution and return control to Unity’s game loop, allowing the rest of the game to continue running. The function then resumes from where it left off after the specified condition is met, such as a time delay or completion of a task.

yield return is what controls the flow of the coroutine. For example:

IEnumerator ExampleCoroutine()
{
    yield return new WaitForSeconds(2f);  // Waits for 2 seconds
    Debug.Log("2 seconds have passed!");
}

Going Beyond `WaitForSeconds`

While yield return new WaitForSeconds() is a popular choice, there are other ways to use yield return that provide more functionality. One common pattern is using yield return with custom conditions or objects, enabling complex interactions between game components.

Here are a few examples to expand your use of yield return.

1. Using `yield return` with Custom Wait Conditions

In certain cases, you might want to wait for something other than a set time period, such as waiting for an event or condition to be met. This is where you can use custom objects like WaitUntil or WaitWhile:

IEnumerator WaitForCondition()
{
    yield return new WaitUntil(() => playerHealth <= 0);  // Wait until player health reaches 0
    Debug.Log("Player has died!");
}

Alternatively, use WaitWhile to wait until a condition is false:

IEnumerator WaitForMovementStop()
{
    yield return new WaitWhile(() => player.isMoving);  // Wait until the player stops moving
    Debug.Log("Player has stopped moving.");
}

2. Passing Information Between Coroutines and Classes

Using yield return for passing information between coroutines is also possible. This is a great use case. If you want to return data from a coroutine to another class or method in a more organized way than using global variables, you can use yield return to send back results.

For example, let’s say you have a coroutine that performs a calculation and returns the result to another class:

// Coroutine in one class that sends data back
IEnumerator CalculateAndReturnResult()
{
    float result = 10f + 5f;
    yield return result;  // Yield the result back
}

// Class that calls the coroutine
void Start()
{
    StartCoroutine(HandleCalculation());
}

IEnumerator HandleCalculation()
{
    // Capture the result when the coroutine finishes
    float calculationResult = (float)yield return StartCoroutine(CalculateAndReturnResult());
    Debug.Log("The result of the calculation is: " + calculationResult);
}

In this example, you’re using yield return to pass the result of a coroutine back into the calling function, which allows for a cleaner and more modular approach compared to global variables.

3. Handling Nested Coroutines

Another powerful use of yield return is handling nested coroutines. You can call one coroutine from another and use yield return to wait for the completion of one before continuing with the next.

For example:

IEnumerator OuterCoroutine()
{
    Debug.Log("Starting outer coroutine...");
    yield return StartCoroutine(InnerCoroutine());  // Wait for InnerCoroutine to finish
    Debug.Log("Outer coroutine finished.");
}

IEnumerator InnerCoroutine()
{
    Debug.Log("Starting inner coroutine...");
    yield return new WaitForSeconds(2f);  // Wait for 2 seconds
    Debug.Log("Inner coroutine finished.");
}

Best Practices for Using `yield return`

Use yield return for Asynchronous Operations: Coroutines are great for asynchronous tasks that need to run in the background. Use yield return when you need to pause a task, like waiting for a button press, loading data, or handling animations.
Avoid Overusing Global Variables: By utilizing yield return to pass data between coroutines, you can avoid the need for global variables, which can lead to more maintainable and organized code.
Keep Coroutines Lightweight: Coroutines are lightweight, but if you chain too many of them or nest them deeply, they can make the code harder to debug. Keep your coroutines simple and try to break up complex logic into smaller, reusable coroutines.
Use the Right Wait Condition: Use WaitUntil and WaitWhile when you need to wait for specific conditions, such as waiting for a player’s health to drop to zero or a certain event to trigger. These tools allow you to create more dynamic and responsive coroutines.

Conclusion

yield return is a versatile tool in Unity that can be used in a variety of ways, from simple delays to complex workflows involving multiple coroutines. By using yield return to pass information, wait for conditions, and manage asynchronous tasks, you can create more modular, efficient, and clean code for your Unity projects.

Experiment with these different techniques in your own projects, and you’ll quickly discover just how powerful coroutines and yield return can be!

Understanding yield return in Coroutines

Going Beyond WaitForSeconds

1. Using yield return with Custom Wait Conditions