I have a scenario where I need to create a sequential chain using RxJava in Kotlin. Specifically, I want to create a chain that starts with a Completable and then proceeds to a Single operation.
Here is the relevant code snippet:
class TestRepository {
fun performCompletableTask(): Completable {
Timber.v("Migration performCompletableTask")
return Completable.fromAction {
// Perform some operation here
for (i in 1..100) {
// Do something with the number i
// Example: Print the number i
Timber.v("$i ")
}
}
}
fun performSingleStringTask(): Single<String> {
Timber.v("Migration performSingleStringTask")
return Single.fromCallable {
// Perform some operation here
val stringBuilder = StringBuilder()
for (i in 1..100) {
// Append the number i to the string builder
stringBuilder.append(i).append(" ")
}
// Return the final string
stringBuilder.toString()
}
}
fun performSingleIntTask(): Single<Int> {
Timber.v("Migration performSingleIntTask")
return Single.fromCallable {
// Perform some operation here
var sum = 0
for (i in 1..100) {
// Accumulate the sum of numbers
sum += i
}
// Return the final sum
sum
}
}
}
fun testMigration() {
Timber.v("Performing migration...")
val completable = testRepository.performCompletableTask()
.andThen(testRepository.performSingleStringTask())
.flatMap {
Timber.v("flatMap Single<String> result: $it")
testRepository.performSingleIntTask()
}
val subscribe = completable
.subscribe({
Timber.v("subscribe Single<Int> result: $it")
Timber.v("Migration completed.")
}, { error ->
Timber.e(error, "Error occurred during migration: ${error.message}")
})
}
In the given code, both performCompletableTask() and performSingleStringTask() are called simultaneously. However, I want to modify the chain to ensure that performSingleStringTask() is called only after performCompletableTask() has completed execution.
Can you suggest how I can modify the code to achieve sequential execution in the RxJava chain? I appreciate any guidance or example code you can provide. Thank you!
performSingleStringTask()
already is only executed after performCompletableTask()
completes; that is how andThen
works. It just looks like they run simultaneously because you put the logging outside of the fromAction
and fromCallable
operators. Unlike most other RxJava operators, andThen
does not take a callback, it takes an Rx type directly, so the performSingleStringTask()
method is called immediately after performSingleStringTask()
is called, and both are called before suscribe
is called. However, the callbacks you passed to the fromAction
and fromCallable
operators still happen sequentially, starting from the point subscribe
is called. So the order of operations is:
performCompletableTask()
is called, logs a message, and returns a Completable
performSingleStringTask()
is called logs a message, and returns a Single
, which is passed as a parameter to andThen()
.flatMap
is called and is passed a lambda functionsubscribe
is called, which triggers the Completable.fromAction()
callback to run and emit onComplete
when its finished.Single.fromCallable
callback runs and emits onSuccess
when its finished.