How to know which methods in `<http://java.io|java.io>` should be called using `<http://Dispatchers....

# coroutines

[Mark]

How to know which methods in <http://java.io|java.io> should be called using <http://Dispatchers.IO|Dispatchers.IO> dispatcher? For example File.exists File.canRead etc

[louiscad]

Think about whether it's just reading into the memory (RAM), or whether it might need to read into the storage (for example to check whether the file exists or has read rights).

[yschimke]

Any file operation, even ones you think should be almost free, can be delayed under certain load or system faults. So any file system ones.

[yschimke]

And Dispatchers.IO is special, it shares threads with Default. So it's not a thread switch onto it usually. I believe it ends up being an accounting change effectively.

[louiscad]

name, nameWithoutExtension, path, and the constructor of File/Path should be the only exceptions that don't actually touch the filesystem and can be looked into.

[Mark]

Thank you all. Here is my situation. Suppose I have this:

suspend fun someFun() {
	someSuspendingFun() // suppose I know that this uses <http://Dispatchers.IO|Dispatchers.IO>
	if (someFile.exists()) {
		...
	}
}

and I realise that File.exists might touch the file system. Should I solve this by:

suspend fun someFun() = withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
	someSuspendingFun()
	if (someFile.exists()) {
		...
	}
}

or:

suspend fun someFun() {
	someSuspendingFun()
	withContext(<http://Dispatchers.IO|Dispatchers.IO>) {
		if (someFile.exists()) {
			...
		}
	}
}

[yschimke]

Probably doesn't matter. I'd probably do the second if you are already on default, so you don't accidentally have expensive CPU work on the IO dispatcher.