Waiting for Coroutines in Kotlin
Waiting for Coroutines in Kotlin
In this article, we discuss how to wait for Coroutines in Kotlin to better understand how to work with asynchronous tasks.
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Coroutines allow you to execute tasks asynchronously without blocking the calling thread, such as the main thread. Great, but sometimes you will need to wait for all of these tasks to finish. In this post, we will look at how to wait for a coroutine to finish using
await will not be covered here, as I will cover that in a later post. For now, read the Kotlin docs - Concurrent using async if that is what you are interested in.
How to Get a Job
No, you didn’t misread the heading, I will show you how to get a job:
There, you now have a job. I have solved the world’s employment problems…
Ok, seriously, a
Job is returned when you start a coroutine. In fact, from what I understand anyway, the words job and coroutine are interchangeable in this context. Really, they are the same thing, as seen below:
AbstractCoroutine implements the
Job interface. So a coroutine is a job.
What to Do Once You Have a Job
You can control a coroutine through the functions available on the
Job interface. Here are some of the functions (there are many more):
Job operations will be explored in future posts.
Waiting for a Coroutine
To wait for a coroutine to finish, you can call
join is a suspending function, meaning that the coroutine calling it will be suspended until it is told to resume. At the point of suspension, the executing thread is released to any other available coroutines (that are sharing that thread or thread pool).
Below is a short example:
In the snippet above, a coroutine is launched, and a
Job is returned.
join is then called on the created job/coroutine to wait for it to finish before resuming. This leads to the following output:
The main thread is blocked while it waits for the job/coroutine to finish. Note, that the main thread is used by
runBlocking, while the child is launched using one of the default thread pools.
Just like futures and threads, many coroutines can be created and waited for through the use of
join. This is also made slightly easier by the convenient
joinAll extension function:
Something similar to the following would be output:
I wonder if you can guess what
joinAll’s implementation looks like. I don’t have any way to hide text, so I’ll just assume you actually guessed. I have added it below:
Joining Is Not Always Required
Joining all jobs running inside a coroutine is not a requirement to ensure their completion is waited for. By using the
coroutineScope builder (a function), you can create a parent/child relationship between coroutines. More precisely, the
coroutineScope builder will only progress once all of the coroutines inside it have completed. This is effectively doing a
joinAll on all the jobs inside the
The example below attempts to show this:
Which outputs something like this:
Each parent coroutine had to wait until their children finished. This was enabled by using
coroutineScope, ensuring each coroutine launched inside it had completed before moving on.
I only touched on this subject to show that joining is not the only way to wait for jobs to finish. For some more information, have a look at the Kotlin docs - Scope builder.
Sometimes you will want to wait for several coroutines to complete before moving on. This behavior can be achieved using
joinAll provided by the
Job interface. Doing so will suspend the calling coroutine until the joined jobs have concluded. Similar behavior can also be obtained via the
coroutineScope builder, ensuring that all jobs launched within it have ended before continuing.
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