Support retrieving resources as flows

[paxbun]

Changes

• Added fun getResourceAsFlow(String, Int): Flow<ByteArray>, which retrieves the resource as a flow of byte array chunks.
  • On Android & JVM targets, the flow uses InputStream.
  • On Apple targets, the flow uses NSInputStream.
  • On web targets, the flow uses ReadableStreamBYOBReader.
• Added ResourceIOException, which wraps internal IO exceptions thrown while reading resources.
• Modified the Gradle plugin so the generated Res class contains getAsFlow(String, Int), which invokes getResourceAsFlow.
• Added a section using Res.getAsFlow and kotlinx.io in the files tab of the resource demo app.
• The default requested size of chunks is 8192, which is the value of Segment.SIZE of kotlinx.io or okio.

Motivation

I am making an app that uses deep learning. Although the actual app downloads the DL model from our server, we embed the DL model into our app as a resource for internal testing. I have been handling this by implementing resource reading with platform-specific APIs. I tried refactoring this part using the new resource API, but it didn't work because the app crashed with an OOM error when I used Res.readBytes.

To prevent such an error, I need to read resources as chunks, so I need a proper IO API here. Since kotlinx.io already provides platform-agonistic IO, I tried to implement this with kotlinx.io, but kotlinx.io only provides synchronous IO and is incompatible with web targets, so I failed.

So, I came up with the idea of using Flow for this, as Flow is the only first-party Kotlin API to do something sequentially and asynchronously, as far as I know.

Example

LaunchedEffect(Unit) {
  withContext(Dispatchers.IO) {
    // copy the resource content to someFileStream
    someFileStream.use { stream ->
      Res.getAsFlow("files/bigFile.bin").collect { chunk ->
        stream.write(chunk);
      }
    }
  }
}