Kotlin - How to read from file asynchronously? - kotlin

Question

Is there any kotlin idiomatic way to read a file content's asynchronously? I couldn't find anything in documentation.

Answer 1

A least as of Java 7 (which is where Android is stuck), there isn't any API that would tap into the low-level async file IO support (like io_uring). There is a class called AsynchronousFileChannel, but, as its docs state,
An AsynchronousFileChannel is associated with a thread pool to which tasks are submitted to handle I/O events and dispatch to completion handlers that consume the results of I/O operations on the channel.
That makes it no better than the following, bog-standard Kotlin idiom:
launch {
val contents = withContext(Dispatchers.IO) {
FileInputStream("filename.txt").use { it.readBytes() }
}
processContents(contents)
}
go_on_with_other_stuff_while_file_is_loading()
This uses Kotlin's own dedicated IO thread pool and unblocks the UI thread. If you're on Android, that is your actual concern, anyway.

Answer 2

Java NIO Asynchronous Channel is the tool you want.
Check out this AsynchronousFileChannel.aRead extension function from coroutine example:
suspend fun AsynchronousFileChannel.aRead(buf: ByteBuffer): Int =
suspendCoroutine { cont ->
read(buf, 0L, Unit, object : CompletionHandler<Int, Unit> {
override fun completed(bytesRead: Int, attachment: Unit) {
cont.resume(bytesRead)
}
override fun failed(exception: Throwable, attachment: Unit) {
cont.resumeWithException(exception)
}
})
}
You just open an AsynchronousFileChannel then call this aRead() in a coroutine,
val channel = AsynchronousFileChannel.open(Paths.get(fileName))
try {
val buf = ByteBuffer.allocate(4096)
val bytesRead = channel.aRead(buf)
} finally {
channel.close()
}
It's an essential function, don't know why it is not part of coroutine-core lib.

Answer 3

javasync/RxIo uses Java NIO Asynchronous Channel to provide a non-blocking API to read and write a file content's asynchronously, including kotlin idiomatic way. Next you have two examples: one reading/writing in bulk through coroutines, and other iterating lines through an asynchronous Kotlin Flow:
suspend fun copyNio(from: String, to: String) {
val data = Path(from).readText() // suspension point
Path(to).writeText(data) // suspension point
}
fun printLinesFrom(filename: String) {
Path(filename)
.lines() // Flow<String>
.onEach(::println)
.collect() // block if you want to wait for completion
}
Disclaimer I am the author and main contributor of javasync/RxIo