How can I use a while loop in Kotlin to iterate through a BufferedReader and append to a StringBuilder?

There are many different extensions in the Kotlin standard library you could make use of here for inputstreams, files, readers, etc. For this use case, Reader.forEachLine would be the simplest to use:

jsonReader.forEachLine { line ->
    jsonBuilder.append(line)
}

This lets you process an Reader line by line, and it automatically closes the Reader when the iteration is over.

You might also want to take a look at readLines, lineSequence, and useLines which could help you in more complex situations.

Unfortunately, the best way I've found to do this is with a while(true):

var audioDataString = ""
        val jsonReader = BufferedReader(InputStreamReader(context.resources.openRawResource(resourceName)))
        val jsonBuilder = StringBuilder()
        var line: String? = null
        while (true) {
            line = jsonReader.readLine() ?: break
            jsonBuilder.append(line).append("")
        }

Just in case you don't know, the elvis operator (?:) checks if what's on the left is null and carries out the action on the right if it is.

In Kotlin, variable assignments are not considered expressions. But this is actually intended to improve the code quality. All you need is to create an iterable reader object. And you can do that using the generateSequence function:

for (line in generateSequence(::readLine)) {
    /** here you can use the line just read,
     *  and you can even add other line reading
     *  calls within this block */
}

This works for any reader that signalizes the end of sequence with null. The above example reads from stdin, but jsonReader::readLine would work as well.