为什么我的 CoroutineScope 在它有 Firebase 数据并完成之前不停止代码？

Question

I created a CoroutineScope to get the data from Firebase before expanding a Card and showing that data in a listView . But the card expands while the CoroutineScope is still getting the data from Firebase and tries to display the listView with an empty list.

Here is my Expand Function inside the OnClickListener ( StartPageActivity.customClassList is a List in a Object that is defined earlier):

        var list : List<CustomClass> = ArrayList()

        CoroutineScope(Dispatchers.IO).launch {
            var customList: List<CustomClass> = ArrayList()
            StartPageActivity.customClassExpandList.forEach {
                if (it.title.contains(CustomClass.title)) {
                    customList += getFirebaseData(it.date)
                    if (customList.size == 12) {
                        list = customList
                    }
                }
            }
        }

        val listAdapter = MyListAdapter(context, list)

        listView.adapter = listAdapter

        listView.visibility = View.VISIBLE
        listView.dividerHeight = 0
        listView.layoutParams.height = calculateHeight(listView, 12) 

Here is my getFirebaseData function:

suspend fun getFirebaseDate(date : LocalDate) : CustomClass = withContext(Dispatchers.IO){
    val customClass = CustomClass("$date", date, "Empty", false)
             FirebaseFirestore.getInstance().collection("users").document(FirebaseAuth.getInstance().currentUser!!.uid).collection("customClass")
        .document(date.toString()).get().addOnCompleteListener { task ->
            if (task.isSuccessful) {
                val document = task.result
                if (document.exists()) {
                    goal.description = document["description"].toString()
                    goal.title = document["tile"].toString()
                }
            }
        }.await()
    return@withContext customClass
}

The getFirebaseData function works and it returns the customClass ; this is also added to my customList . But this happens while the code tries to build the expanded listView with the list, that is initiated before the CoroutineScope .

I tried to run the Code after the CoroutineScope inside that scope, but it doesn't allow that and returns an error.

I also tried adding multiple suspend functions, but that also has not fixed my problem.

I also tried putting the forEach function in a separate suspend function, but my problem still occurred.

Answer 1

Launching a coroutine queues it up to run in parallel or in the background, and the current function continues running without waiting for it. See here for more explanation.

To get the behavior you're expecting, you would need to use runBlocking instead of CoroutineScope(Dispatchers.IO).launch . However, this is not a viable solution for a.network operation. The entire device will be completely frozen to user input and screen refreshes/animation until the coroutine is finished. If the.network operation takes more than a few milliseconds, this will be unacceptable to the user, because they can't even back out of your app if they change their mind, among many other potential problems.

You must design your app to gracefully handle the fact that a.network operation might take a while. This is typically done by showing some kind of loading indicator in the UI, and then you can start your coroutine, and then inside the coroutine you can update the list view when the results are ready.

Two other issues with your use of coroutines:

1. You should not create a new CoroutineScope like that. It will leak your fragment. Use viewLifecycleOwner.lifecycleScope.launch . And don't use Dispatchers.IO , because that will cause glitches and crashes if you start working with UI in your coroutine. You don't need Dispatchers.IO unless you're directly calling blocking code inside your coroutine, and if you do, you should wrap only those parts of the coroutine with withContext(Dispatchers.IO) { } .

2. In your suspend function, you should not mix using listeners and await() . That is convoluted and error prone. You're also neglecting the failure condition. If you don't use try / catch when using await() , you risk a crash or unhandled error (depending on how your coroutine scope is set up). Incidentally, you don't need to specify Dispatchers.IO here (like I mentioned above) because await() is not a blocking function. It's a suspending function. Your function should look something like this instead. I'm guessing that you are using CustomClass to communicate whether the retrieval was successful.

suspend fun getFirebaseDate(date : LocalDate) : CustomClass = try {
    val document = FirebaseFirestore.getInstance()
        .collection("users")
        .document(FirebaseAuth.getInstance().currentUser!!.uid)
        .collection("customClass")
        .document(date.toString())
        .await()
    require(document.exists()) // Throw exception if it doesn't. Catch block will handle it.
    goal.description = document["description"].toString()
    goal.title = document["tile"].toString()
    CustomClass("$date", date, "Empty", false)
} catch (e: Exception) {
    CustomClass("$date", date, "Empty", true)
}