Скачать или смотреть Understanding Kotlin's Rendezvous Channel: Why Does send() Not Suspend?

A deep dive into why Kotlin's rendezvous channels behave the way they do when sending values, and how to resolve common issues with coroutines.
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Understanding Kotlin's Rendezvous Channel: Why Does send() Not Suspend?

Kotlin's coroutines provide a powerful way to manage asynchronous programming, yet they can often lead to confusion, especially when dealing with channels. In this post, we will address a common question: Why doesn't sending on a rendezvous channel suspend if nobody is reading? Let's analyze a sample Kotlin program to illustrate the problem and then break down the solution step-by-step.

The Problem: Unending Execution

Consider the following Kotlin code snippet that utilizes a rendezvous channel:

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

In this code, we create a producer coroutine that generates an infinite number of messages. However, you may notice that the program does not terminate. This can be puzzling: You might expect that sending on the channel would suspend until a receiver is available, but oddly, it seems to keep sending indefinitely.

What’s Going Wrong?

The confusion arises from the interaction between coroutines and channels. Here's what's happening:

Every time you send a message using output.send(line), you are essentially telling the producer coroutine to send the message down the channel.

However, the call to output.send() is wrapped in another coroutine (specifically, it's invoked within the async block). This async operation does not suspend, it simply launches and forgets—effectively turning into a fire-and-forget operation.

Key Insight

The line output.send(line) does indeed suspend if the channel is not being read, but since it’s contained within an async block, the producer loop continues to run without waiting for the send operation to complete. Thus, the loop does not pause as expected.

The Solution: Making it Suspend

To fix this issue, we need to ensure that the invoke() method, which is used to send values, behaves properly with respect to suspending. Below are the changes needed to achieve this:

Step 1: Modify the invoke Method

Let's change the invoke() method in the ChannelDestination class to properly suspend:

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

Step 2: Make the Producer Function Suspend

Next, we need to adjust the producer() function to ensure it can pause when necessary:

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

Final Thoughts

Now, when the producer function runs, it will correctly suspend while waiting for the channel to be ready for sending messages. Changes like these are crucial in ensuring that your coroutines work harmoniously and prevent unintended behaviors, like infinite loops.

With these adjustments, your code will be able to handle sending messages in a more expected manner, terminating when it should under appropriate conditions.



In conclusion, understanding how async works in conjunction with sending on channels is key to properly managing coroutine behavior in Kotlin. Always remember that launching a new coroutine does not mean it will suspend—it simply allows the operation to continue without waiting for the previous task to complete. Happy coding!