To watch file changes

FileWatcher.kt

import java.io.File
import java.nio.file.*

class FileWatcher(watchFile: String) {
    private val folderPath: Path
    private val watchFile: String

    init {
        val filePath = Paths.get(watchFile)
        val isRegularFile = Files.isRegularFile(filePath)
        require(isRegularFile) {
            // Do not allow this to be a folder since we want to watch files
            "$watchFile is not a regular file"
        }

        // This is always a folder
        folderPath = filePath.parent

        // Keep this relative to the watched folder
        this.watchFile = watchFile.replace(folderPath.toString() + File.separator, "")
    }

    @Throws(Exception::class)
    fun watchFile(
        onModified: () -> Unit,
    ) {
        // We obtain the file system of the Path
        val fileSystem = folderPath.fileSystem
        fileSystem.newWatchService().use { service ->
            // We watch for modification events
            folderPath.register(service, StandardWatchEventKinds.ENTRY_MODIFY)

            // Start the infinite polling loop
            while (true) {
                // Wait for the next event
                val watchKey: WatchKey = service.take()
                for (watchEvent: WatchEvent<*> in watchKey.pollEvents()) {
                    // Get the type of the event
                    val kind: WatchEvent.Kind<*> = watchEvent.kind()
                    if (kind === StandardWatchEventKinds.ENTRY_MODIFY) {
                        val watchEventPath: Path = watchEvent.context() as Path

                        // Call this if the right file is involved
                        if ((watchEventPath.toString() == watchFile)) {
                            onModified()
                        }
                    }
                }
                if (!watchKey.reset()) {
                    // Exit if no longer valid
                    break
                }
            }
        }
    }
}