Unity async vs. coroutines

Key takeaways:

  • Coroutines in Unity are frame-based methods for sequential execution over multiple frames.

  • Coroutines are ideal for delays, animations, and incremental game logic.

  • Coroutines lack error handling, value returns, and multithreading support.

  • Async functions use C#'s async and await for asynchronous operations without blocking the main thread.

  • Async functions handle background tasks like file I/O and web requests efficiently.

  • Coroutines are part of Unity’s MonoBehaviour, while async functions are a general C# feature.

  • Async functions support error handling and return values using Task<T>.

  • Coroutines stop if their associated MonoBehaviour is destroyed, while async functions require manual lifetime management.

  • Coroutines work on the main thread, while async functions can leverage multithreading.

  • Coroutines excel in frame-based tasks; async functions are better for I/O-heavy workflows.

In game development, triggering actions of a series of frames or having multiple actions based on the previous action is very important. This sequential behavior can be achieved in Unity and C# using coroutines and async functions. However, often, the question that comes up is which is better than the other? Let’s look at both these methods in a bit of detail.

Let’s have a quick look at what they are, and then we can move on to the main differences between them.

What are coroutines?

In Unity, coroutines are special methods that allow you to execute code over multiple frames without blocking the main thread. Coroutines are primarily used for tasks that take time, like waiting for an animation to finish, handling delays, or performing tasks incrementally. They are implemented using the IEnumerator interface and are started using StartCoroutine.

Example

The code shown below gives a thorough use of how coroutines can be used:

using UnityEngine;



public class CoroutineExample : MonoBehaviour

{

    private void Start()

    {

        StartCoroutine(WaitAndPrint());

    }



    private IEnumerator WaitAndPrint()

    {

        Debug.Log("Before Wait");

        yield return new WaitForSeconds(2); // Wait for 2 seconds

        Debug.Log("After Wait");

    }

}

Pros and cons

Coroutines in Unity are lightweight and well-suited for frame-based tasks such as delays, animations, or incremental logic. They offer seamless integration with Unity APIs and low memory overhead. However, they lack flexibility for background processing, cannot directly return values, and have no built-in error handling or cancellation, making them less suitable for complex workflows or CPU-intensive operations.

When do we use coroutines?

Coroutines in Unity are best used for frame-based tasks that require sequential execution without blocking the main thread. They are ideal for implementing delays (WaitForSeconds), animations, or gradual transitions like fading, scaling, or movement over time. Additionally, they work well for game mechanics such as cooldown timers, wave spawning, or scripted events. Coroutines are particularly effective when you need to break a larger task into smaller steps spread across multiple frames, ensuring smooth gameplay performance.

What are async functions?

In Unity, async refers to asynchronous programming using the async and await keywords in C#. This allows you to run long-running tasks, like downloading data from a server, loading assets, or performing computations, without blocking the main thread (which handles rendering and gameplay).

Example

The code shown below gives an example of how async functions are used:

using System.Threading.Tasks;

using UnityEngine;



public class AsyncExample : MonoBehaviour

{

    private async void Start()

    {

        Debug.Log("Before Task");

        await LoadAssetsAsync();

        Debug.Log("After Task");

    }



    private async Task LoadAssetsAsync()

    {

        // Simulate a delay

        await Task.Delay(2000);

        Debug.Log("Assets Loaded");

    }

}

Pros and cons

Async functions in Unity offer a modern, readable way to handle asynchronous tasks, such as loading assets, making web requests, or performing file I/O, without blocking the main thread. They allow background processing, support return values via Task, and simplify error handling with try-catch. However, async functions come with limitations, such as restricted use of Unity APIs (which must run on the main thread), slightly higher memory overhead due to generated state machines, and potential complexity when managing cancellations or debugging. They are powerful for non-Unity tasks but must be used carefully in game development to avoid threading conflicts.

When do we use async functions

Async functions in Unity are best used for tasks that involve waiting for external operations without blocking the main thread, such as loading assets, making network requests, performing file I/O, or running heavy computations. They excel in scenarios where background processing is needed, like fetching data from APIs, downloading files, or handling database queries. Async functions are particularly useful when working with third-party libraries or APIs that support asynchronous programming, enabling smoother and more efficient workflows. However, they should be avoided for tasks that require direct Unity API calls, as these must always run on the main thread.

Differences in coroutines and async functions

There are several differences in coroutines and async functions. Let us look at the following table, which displays some of them:

Coroutines

Async Functions

Unity API feature

A C# feature

A coroutine cannot return a value

An async function can return a value

A coroutine stops if its object is destroyed

An async function does not stop if its object is destroyed

Cannot use try/catch statements for error handling

Can use try/catch statements for error handling

Does not have multithreading support

Has multithreading support

Let us look at these points in a bit of detail:

  • Accessibility: A coroutine is a part of the MonoBehaviour class, which is a part of the Unity API. This means that coroutines are only accessible to those classes that inherit from the MonoBehaviour class, which effectively limits the type of classes that can use this tool. An async function, however, can be used in any class as it is a part of C#.

  • Returning a value: A coroutine returns an instance of IEnumerator, which is used by Unity’s runtime to manage the coroutine. It cannot return any other value at any point. On the other hand, an async function has a Task<T> type, which represents an operation that returns a variable of type T upon completion. This value will be available once the task is completed and can be used with the await keyword.

  • Lifetime management: A coroutine is tightly coupled to the MonoBehaviour class that started it. If its MonoBehaviour is destroyed or disabled, the coroutine will stop as well. Furthermore, re-enabling the object will not resume the coroutine either. However, an async function runs on a dedicated scheduler and requires manual lifetime management. It will not stop if the script instance is destroyed or disabled.

  • Error handling: Coroutines cannot use C#’s try/catch statements as yield statements cannot be wrapped by a try/catch block. Async functions, on the other hand, can call await commands from within try/catch blocks.

  • Multithreading support: Coroutines do not have multithreading support and run on the main thread. Async functions, by nature, run on multiple threads as they are asynchronous.

Quiz

Let’s test our understanding with a short quiz.

1

What is true about coroutines in Unity?

A)

Coroutines can run on multiple threads.

B)

Coroutines can return values directly.

C)

Coroutines are part of Unity’s MonoBehaviour class.

D)

Coroutines can handle errors with try/catch blocks.

Question 1 of 30 attempted

Conclusion

Conclusively, while both coroutines and async functions can implement the sequential execution of commands, they are generally used for different purposes. Coroutines execute a function over several frames, whereas async functions wait for I/O operations to complete. Both are very good techniques to learn and should be used according to their strengths.

Frequently asked questions

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Does async work in Unity?

Yes, Unity supports asynchronous programming using the async and await keywords. This allows for non-blocking operations, improving performance and responsiveness, especially when dealing with long-running tasks or I/O operations.

Are coroutines good in Unity?

Coroutines are a powerful tool in Unity for controlling the flow of execution over multiple frames. They are often used for timing-based actions, sequential tasks, and asynchronous operations. However, they can be less efficient than async/await in some cases, especially when dealing with true asynchronous operations that involve background threads.

What are the disadvantages of coroutines?

While coroutines are useful, they have some limitations:

  • Performance overhead: Coroutines can introduce performance overhead due to the context switching involved in their execution.
  • Limited asynchronous capabilities: Coroutines are primarily designed for sequential asynchronous operations within the main thread. They are not as well-suited for true parallel processing or background tasks.
  • Complexity: Coroutines can sometimes make code more complex, especially when dealing with nested coroutines or intricate timing scenarios.
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