How does a task manager transmit the execution result?

It depends on the specific task manager.

1. AsyncTask: Deprecated, but used to pass results via the onPostExecute() method, which runs on the main UI thread.
2. ExecutorService + Callbacks: Often used in conjunction with callback interfaces. The result is passed to a callback method, which can be invoked on the desired thread (e.g., using Handler or Activity#runOnUiThread).
3. RxJava/RxKotlin: Uses data streams (Observables, Flows) and operators for processing and passing results. Results are received by subscribers (Observer, Consumer, Subscriber, Collector).
4. Kotlin Coroutines + Flow: The result of a suspend function is directly returned to the caller (in a suspend context). For data streams, Flow is used, and results are received in a collector (collect).
5. WorkManager: Passes results via LiveData<WorkInfo> or Flow<WorkInfo>, which contain the task status and result data (OutputData).

Example with coroutines:

// Task running on a different thread
suspend fun doBackgroundWork(): Result {
    // ... perform work ...
    return Result("Successful execution result")
}

// Calling the task and getting the result
suspend fun processResult() {
    val result = withContext(Dispatchers.IO) {
        doBackgroundWork() // Call suspend function on background thread
    }
    // Here `result` contains the execution result after task completion
    updateUI(result) // Use the result on the UI thread
}

data class Result(val data: String)

Example with WorkManager:

// Defining a WorkRequest
val workRequest = OneTimeWorkRequestBuilder<MyWorker>()
    .build()

// Enqueue the task
WorkManager.getInstance(context).enqueue(workRequest)

// Track status and result
WorkManager.getInstance(context).getWorkInfoByIdLiveData(workRequest.id)
    .observe(lifecycleOwner, Observer { workInfo ->
        if (workInfo != null && workInfo.state == WorkInfo.State.SUCCEEDED) {
            val outputData = workInfo.outputData
            val resultString = outputData.getString("MY_RESULT_KEY")
            // Here `resultString` contains the task result
            handleResult(resultString)
        }
    })

// Inside MyWorker
class MyWorker(appContext: Context, workerParams: WorkerParameters)
    : Worker(appContext, workerParams) {

    override fun doWork(): Result {
        // ... perform work ...
        val outputData = workDataOf("MY_RESULT_KEY" to "Data from worker")
        return Result.success(outputData) // Pass the result
    }
}