Introduced a simple way to work with files inside frontend

The usecase of this code is something like this: let image that user is making a protonmail or any another application where content inside frontend (=browser storage) some secret user's data that shouldn't be exposed to backend. Secret key for example or anything like that. This code allows to developer to save any `Array[Byte]` from frontend as simple call `FileService.save()` and load any uploaded file to `Array[Byte]` as `FileService.load()`. Unfortunately scalatags doesn't support `download` attribute and I need to make it by hand. I've opened a PR: com-lihaoyi/scalatags#212 to introduce it, but it might be a while until it is included to release.

Author: catap

core/.js/src/main/scala/io/udash/utils/FileService.scala

@@ -0,0 +1,67 @@
+package io.udash.utils
+
+import java.io.IOException
+
+import org.scalajs.dom._
+import org.scalajs.dom.html.Anchor
+import scalatags.JsDom
+
+import scala.scalajs.js
+import scala.concurrent.{Future, Promise}
+import scala.util.Try
+
+object FileService {
+
+ final val OctetStreamType = "application/octet-stream"
+
+ /**
+ * Converts specified bytes array to string that contains URL
+ * that representing the array given in the parameter with optionally specified mime-type.
+ *
+ * Keep in mind that returned URL should be revoked via `org.scalajs.dom.revokeObjectURL(url)`.
+ */
+ def bytesAsURL(bytes: Array[Byte], mimeType: String = OctetStreamType): String = {
+ import js.typedarray._
+
+ val jsBytes = js.Array[js.Any](bytes.toTypedArray)
+ val blob = new Blob(jsBytes, BlobPropertyBag(mimeType))
+ URL.createObjectURL(blob)
+ }
+
+ /**
+ * Create an anchor element that on click downloads byte array as a file with specified name.
+ *
+ * Keep in mind that anchor's href URL should be revoked via `org.scalajs.dom.revokeObjectURL(url)`.
+ */
+ def save(filename: String, bytes: Array[Byte], mimeType: String = OctetStreamType): JsDom.TypedTag[Anchor] = {
+ import JsDom.all._
+
+ val download = attr("download")
+ a(href := bytesAsURL(bytes, mimeType), download := filename)
+ }
+
+ /**
+ * Asynchronously convert specified file to bytes array.
+ */
+ def load(file: File): Future[Array[Byte]] = {
+ import js.typedarray._
+
+ val fileReader = new FileReader()
+ val promise = Promise[Array[Byte]]()
+
+ fileReader.onerror = (e: Event) =>
+ promise.failure(new IOException(e.toString))
+
+ fileReader.onabort = (e: Event) =>
+ promise.failure(new IOException(e.toString))
+
+ fileReader.onload = (_: UIEvent) =>
+ promise.complete(Try(
+ new Int8Array(fileReader.result.asInstanceOf[ArrayBuffer]).toArray
+ ))
+
+ fileReader.readAsArrayBuffer(file)
+
+ promise.future
+ }
+}