Android app programming:

So let's say I trigger an asynchronous task in the onResume() block of my fragment that has 2 Java callback methods on completion - onSuccess() and onFailure(). Meanwhile, there's a button on the screen that the user can click any time. The catch is, on button click I need to wait for the earlier asynchronous task to complete before executing button click code.

I need to show a progress bar on button click while the asynchronous task completes (if it hasn't completed already), but not sure how to code it such that on button click I am aware that the button has been clicked and also the async task hasn't completed yet and I need to wait for it to complete before proceeding further.

I am thinking I need to use LiveData and/or Kotlin coroutines in this process, but not sure how to proceed with it.

Can anyone please guide me in the right direction on this? I am trying my best to understand asynchronous programming.

CodePudding user response：

If you don't use coroutines, etc. right now then you can implement this easily with just a few booleans and ifs. Assuming all onSuccess(), onFailure() and onBtnClick() are invoked on the main thread, this should do the trick:

private var firstTaskRunning = false
private var scheduledSecondTask = false

fun startFirstTask() {
    firstTaskRunning = true
    // start first task
}

fun onSuccess() {
    onFirstTaskFinished()
}

fun onFailure() {
    onFirstTaskFinished()
}

fun onFirstTaskFinished() {
    firstTaskRunning = false
    
    if (scheduledSecondTask) {
        scheduledSecondTask = false
        hideBtnProgress()
        secondTask()
    }
}

fun onBtnClick() {
    if (firstTaskRunning) {
        scheduledSecondTask = true
        showBtnProgress()
    } else {
        secondTask()
    }
}

You should also think about other corner cases like for example: first task was scheduled while it is already running; button is clicked while second task is running, etc.

CodePudding user response：

Define a lateinit var at the top of your Activity, private lateinit var asyncJob: Job

override fun onResume() {
    super.onResume()
    asyncJob = lifecycleScope.launch {
        // Do the work
    }
}

fun onButtonClicked() {
    while (asyncJob.isActive) showProgressIndicator()
    hideProgressIndicator()
    // Handle the button click
}

Keep in mind that lifecycleScope's default thread is main, if you want a coroutine to run on a background thread, provide an adequate dispatcher to the launch(dispatcher) {...} function.

If you're using a Fragment instead of an Activity use viewLifecycleOwner.lifecycleScope.launch instead of lifecycleScope.launch

lifecycleScope is an extension function and can be found in androidx.lifecycle:lifecycle-runtime-ktx:version