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 convert any `Array[Byte]` from frontend to URL
as simple call `FileService.asURL`, create an anchor to download it as
`FileService.asAnchor` and asynchronously convert any uploaded `File` to
`Array[Byte]` as `FileService.asBytesArray`, or to `InputStream` via
`FileService.asInputStream`.

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.

`FileService.asInputStream` is using `FileReaderSync` that is also missed inside
scala-js-dom. I've opened a PR:
scala-js/scala-js-dom#424 but it might be a while. Also,
this is draft API but it is supported by majority of modern browsers:
https://developer.mozilla.org/en-US/docs/Web/API/FileReaderSync#Browser_Compatibility

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

@@ -0,0 +1,115 @@
+package io.udash.utils
+
+import java.io.{IOException, InputStream}
+
+import org.scalajs.dom._
+import org.scalajs.dom.html.Anchor
+import org.scalajs.dom.raw.Blob
+import scalatags.JsDom
+
+import scala.scalajs.js
+import scala.concurrent.{Future, Promise}
+import scala.scalajs.js.annotation.JSGlobal
+import scala.scalajs.js.typedarray.ArrayBuffer
+import scala.util.Try
+
+@js.native
+@JSGlobal
+sealed class FileReaderSync() extends js.Object {
+  def readAsArrayBuffer(blob: Blob): ArrayBuffer = js.native
+}
+
+sealed class FileBufferedInputStream(val file: File) extends InputStream {
+  val fileReaderSync = new FileReaderSync()
+
+  var filePos: Int = 0
+  var pos: Int = 0
+
+  var buffer: Array[Byte] = Array.empty
+
+  override def read(): Int = {
+    if (pos >= buffer.length) {
+      import js.typedarray._
+
+      if (filePos >= file.size) {
+        return -1
+      }
+
+      val len = math.min(filePos + 1024, file.size.toInt)
+      val slice = file.slice(filePos, len)
+      buffer = new Int8Array(fileReaderSync.readAsArrayBuffer(slice)).toArray
+      filePos += buffer.length
+      pos = 0
+    }
+    val r = buffer(pos).toInt & 0xff
+    pos += 1
+    r
+  }
+}
+
+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 asURL(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 asAnchor(filename: String, bytes: Array[Byte], mimeType: String = OctetStreamType): JsDom.TypedTag[Anchor] = {
+    import JsDom.all._
+
+    val download = attr("download")
+    a(href := asURL(bytes, mimeType), download := filename)
+  }
+
+  /**
+   * Asynchronously convert specified file to bytes array.
+   */
+  def asBytesArray(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
+  }
+
+  /**
+   * Convert specified file to InputStream with blocking I/O
+   *
+   * Because it is using synchronous I/O that could potentially this API can be used only inside worker.
+   *
+   * This method is using FileReaderSync that is part of Working Draft File API.
+   * Anyway it is supported for majority of modern browsers
+   */
+  def asInputStream(file: File): InputStream =
+    new FileBufferedInputStream(file)
+}

guide/guide/.js/src/main/scala/io/udash/web/guide/views/frontend/FrontendFilesView.scala

@@ -24,6 +24,8 @@ class FrontendFilesView extends View {
     ),
     p("You can find a working demo application in the ", a(href := References.UdashFilesDemoRepo, target := "_blank")("Udash Demos"), " repositiory."),
     h3("Frontend forms"),
+    p(i("FileService"), " is an object that allows to convert ", i("Array[Byte]")," to URL, save it as file from frontend ",
+      " and asynchronously convert ", i("File"), " to ", i("Array[Byte]"), "."),
     p(i("FileInput"), " is the file HTML input wrapper providing a property containing selected files. "),
     fileInputSnippet,
     p("Take a look at the following live demo:"),

guide/guide/.js/src/main/scala/io/udash/web/guide/views/frontend/demos/FileInputDemo.scala

@@ -1,6 +1,7 @@
 package io.udash.web.guide.views.frontend.demos
 
 import io.udash.css.CssView
+import io.udash.utils.FileService
 import io.udash.web.guide.demos.AutoDemo
 import io.udash.web.guide.styles.partials.GuideStyles
 import scalatags.JsDom.all._
@@ -9,8 +10,11 @@ object FileInputDemo extends AutoDemo with CssView {
 
   private val (rendered, source) = {
     import io.udash._
-    import org.scalajs.dom.File
+    import org.scalajs.dom.{File, URL}
     import scalatags.JsDom.all._
+    import org.scalajs.dom.window
+
+    import scala.concurrent.ExecutionContext.Implicits.global
 
     val acceptMultipleFiles = Property(true)
     val selectedFiles = SeqProperty.blank[File]
@@ -19,7 +23,27 @@ object FileInputDemo extends AutoDemo with CssView {
       FileInput(selectedFiles, acceptMultipleFiles)("files"),
       h4("Selected files"),
       ul(repeat(selectedFiles)(file => {
-        li(file.get.name).render
+        val content = Property(Array.empty[Byte])
+
+        window.setTimeout(() =>
+          FileService.asBytesArray(file.get) foreach { bytes =>
+            content.set(bytes)
+          }, 3000)
+
+        val name = file.get.name
+        li(showIfElse(content.transform(_.isEmpty))(
+          span(name).render,
+          {
+            val anchor = FileService.asAnchor(name, content.get)(name).render
+
+            window.setTimeout(() => {
+              content.set(Array.empty[Byte])
+              URL.revokeObjectURL(anchor.href)
+            }, 10000)
+
+            anchor
+          }
+        )).render
       }))
     )
   }.withSourceCode