When and how to call suspend function from Android Activity?

Question

I have a suspend function that calls POST request to the server. I want to configure some text in the activity to show the information I received from the server.

suspend fun retrieveInfo():String 

I tried calling inside onCreate, onResume but crashes runtime.

runBlocking { 
    retrieveInfo()
}

    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.google.augmentedimage/com.google.AugmentedImageActivity}: android.os.NetworkOnMainThreadException
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3086)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3229)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)

Where am I suppose to put these suspend calls (in which part of lifecycle of activity)? Should I be using something other than runBlocking?

Solution 1

By default runBlocking runs the suspending code block in the thread runBlocking was called on.

So if you called runBlocking from the Activity callback your suspending block will be executed on the main thread, from which you cannot access network (query a server).

You need to switch a dispatcher in your coroutine block for that call. The simplest fix for your code would be to move the execution to the Dispatchers.IO.

runBlocking {
    withContext(Dispatchers.IO) {
        retrieveInfo()
    }
}

---

That being said, I suggest two things (not related directly to your question):

1. Read Coroutines on Android (this part and the following ones)

2. Don't use runBlocking for your case, but define a correct job and use job.launch{}

Solution 2

If you want to write in activity:

class MyActivity : AppCompatActivity() {
private val scope = CoroutineScope(newSingleThreadContext("name"))

fun doSomething() {
    scope.launch { ... }
  }
}

Comments

[Taseer]

Launch a coroutine from your main thread. CoroutineScope(Dispatchers.Main).launch { withContext(Dispatchers.IO) { myFun() }

[John O'Reilly]

Generally the favoured approach for something like this is to use a ViewModel and then use it's built in viewModelScope (some more details in medium.com/androiddevelopers/…)

[user3087615]

Your quick solution of runBlocking with IO context allows the code to run. However, unless the API call is fast enough, it still gives Unable to start activity ComponentInfo java.net.SocketTimeoutException

[Bartek Lipinski]

@user3087615 that's a whole another issue unrelated to the coroutine itself. You need to have a proper error handling for your api call.

[Bartek Lipinski]

@user3087615 also you really should follow the second point of my answer because calling runBlocking in Activity lifecycle methods is a really bad idea.

[user3087615]

is there any good documentation on how to create a job and launch? I am new to kotlin and struggling to find good documentation. If you could show a short example or point to documentation, it would be awesome.

[Bartek Lipinski]

@user3087615 my post consists of a great post on the subject

[Slava]

Not really, please, provide a better one :(

[Akito]

newSingleThreadContext is obsolete.