Скачать или смотреть How to Return a Variable Inside of Lambda in Kotlin

Learn how to handle `lambda` expressions in Kotlin and return variables effectively using suspend functions and coroutines.
---
This video is based on the question https://stackoverflow.com/q/69784604/ asked by the user 'sooyeon' ( https://stackoverflow.com/u/16626322/ ) and on the answer https://stackoverflow.com/a/69785017/ provided by the user 'headofmobile' ( https://stackoverflow.com/u/11675125/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: How can I return a variable inside of lambda

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/l...
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/... ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Understanding Lambdas in Kotlin: How to Return a Variable Inside

In Kotlin, lambdas are a powerful tool for writing concise and efficient code, but they can also introduce challenges when it comes to returning variables. If you've ever written a piece of code that attempts to return a value from a function, only to find that you can’t access your desired variable because it’s inside a lambda, you’re not alone. This issue is common, especially in asynchronous programming.

In this guide, we will address how to return a variable inside a lambda by utilizing Kotlin's coroutines. With this approach, we can maintain the performance of our code while ensuring we have access to the data we need.

The Problem: Returning a Variable from a Lambda

Let's take a look at a scenario where you may run into problems. Consider the following function that aims to calculate the average rating from a list of reviews, which is retrieved from a database asynchronously.

[[See Video to Reveal this Text or Code Snippet]]

In this code, the function tries to return average_rating immediately after calling the asynchronous function get(), which completes its work later. This results in average_rating being returned before it has been calculated, leading to unexpected results.

Why This Happens

Asynchronous Execution: The function get() starts a process that runs asynchronously. This means that the code inside the lambda (where we compute our average rating) executes later, after the rest of the function has already returned.

Scope Issues: The average_rating variable is declared outside the lambda, but because the computation is delayed, its value at the time of return is still 0.0f, which is not what's intended.

The Solution: Using Coroutines and Suspend Functions

To solve this issue, we need to adopt a different approach. We can utilize Kotlin’s coroutines and create a suspend function to wait for the asynchronous operation to complete before returning the calculated value.

Updated Code Example

Here is an example of how to refactor the original function using a suspend function:

[[See Video to Reveal this Text or Code Snippet]]

Breaking It Down

Suspend Function: By declaring the function as a suspend function, we indicate that it may be paused and resumed, allowing us to wait for asynchronous results.

suspendCancellableCoroutine: This function allows us to write asynchronous code that can be canceled, helping to handle execution flow cleanly.

Continuation: Inside the lambda, we call continuation.resumeWith(Result.success(average_rating)), which effectively returns the calculated average rating once it's computed.

Conclusion

By leveraging Kotlin's coroutines and suspend functions, we can effectively handle asynchronous operations and return values computed in lambdas.

This approach not only resolves the issue of returning a variable but also enhances the overall structure and readability of your code.

If you're dealing with asynchronous tasks in your Kotlin projects, don’t hesitate to incorporate coroutines for a more intuitive coding experience!