FileService: a way to work with file at 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 `Seq[Array[Byte]]` from
frontend to URL as simple call `FileService.createURL`, asynchronously
convert any uploaded `File` to `Future[Array[Byte]]` as
`FileService.asBytesArray`, or to `Array[Byte]` via
`FileService.asSyncBytesArray` inside worker.

Unfortunately scalatags doesn't support `download` attribute and I need
to make it by hand. I've opened a PR[^1] to introduce it, but it might be a
while until it is included to release.

`FileService.asSyncBytesArray` is using `FileReaderSync` that is also
missed inside scala-js-dom. I've opened a PR[^2] but it might be a
while. Also, this draft API but it is supported by majority of modern
browsers[^3].

[^1]: com-lihaoyi/scalatags#212

[^2]: scala-js/scala-js-dom#424

[^3]: https://caniuse.com/?search=FileReaderSync

Author: catap

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

@@ -0,0 +1,133 @@
+package io.udash.utils
+
+import com.avsystem.commons.misc.AbstractCase
+
+import java.io.IOException
+import org.scalajs.dom._
+import org.scalajs.dom.raw.Blob
+
+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
+private[utils] final class FileReaderSync() extends js.Object {
+  def readAsArrayBuffer(blob: Blob): ArrayBuffer = js.native
+}
+
+final case class CloseableUrl(value: String) extends AbstractCase with AutoCloseable {
+  override def close(): Unit = {
+    URL.revokeObjectURL(value)
+  }
+}
+
+object FileService {
+
+  final val OctetStreamType = "application/octet-stream"
+
+  /**
+   *  Converts specified bytes arrays to string that contains URL
+   *  that representing the array given in the parameter with specified mime-type.
+   *
+   *  Keep in mind that returned URL should be closed.
+   */
+  def createURL(bytesArrays: Seq[Array[Byte]], mimeType: String): CloseableUrl = {
+    import js.typedarray._
+
+    val jsBytesArrays = js.Array[js.Any](bytesArrays.map(_.toTypedArray) :_ *)
+    val blob = new Blob(jsBytesArrays, BlobPropertyBag(mimeType))
+    CloseableUrl(URL.createObjectURL(blob))
+  }
+
+  /**
+   *  Converts specified bytes arrays to string that contains URL
+   *  that representing the array given in the parameter with `application/octet-stream` mime-type.
+   *
+   *  Keep in mind that returned URL should be closed.
+   */
+  def createURL(bytesArrays: Seq[Array[Byte]]): CloseableUrl =
+    createURL(bytesArrays, OctetStreamType)
+
+  /**
+   *  Converts specified bytes array to string that contains URL
+   *  that representing the array given in the parameter with specified mime-type.
+   *
+   *  Keep in mind that returned URL should be closed.
+   */
+  def createURL(byteArray: Array[Byte], mimeType: String): CloseableUrl =
+    createURL(Seq(byteArray), mimeType)
+
+  /**
+   *  Converts specified bytes array to string that contains URL
+   *  that representing the array given in the parameter with `application/octet-stream` mime-type.
+   *
+   *  Keep in mind that returned URL should be closed.
+   */
+  def createURL(byteArray: Array[Byte]): CloseableUrl =
+    createURL(Seq(byteArray), OctetStreamType)
+
+  /**
+   * Asynchronously convert specified part of file to bytes array.
+   */
+  def asBytesArray(file: File, start: Double, end: Double): 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
+      ))
+
+    val slice = file.slice(start, end)
+    fileReader.readAsArrayBuffer(slice)
+
+    promise.future
+  }
+
+  /**
+   * Asynchronously convert specified file to bytes array.
+   */
+  def asBytesArray(file: File): Future[Array[Byte]] =
+    asBytesArray(file, 0, file.size)
+
+  /**
+   * Synchronously convert specified part of file to bytes array.
+   *
+   * Because it is using synchronous I/O 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 asSyncBytesArray(file: File, start: Double, end: Double): Array[Byte] = {
+    import js.typedarray._
+
+    val fileReaderSync = new FileReaderSync()
+    val slice = file.slice(start, end)
+
+    val int8Array = new Int8Array(fileReaderSync.readAsArrayBuffer(slice))
+
+    int8Array.toArray
+  }
+
+  /**
+   * Synchronously convert file to bytes array.
+   *
+   * Because it is using synchronous I/O 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 asSyncBytesArray(file: File): Array[Byte] =
+    asSyncBytesArray(file, 0, file.size)
+}

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("Future[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._
@@ -12,14 +13,35 @@ object FileInputDemo extends AutoDemo with CssView {
     import org.scalajs.dom.File
     import scalatags.JsDom.all._
 
-    val acceptMultipleFiles = Property(true)
+    import scala.concurrent.ExecutionContext.Implicits.global
+
+    val acceptMultipleFiles = true.toProperty
     val selectedFiles = SeqProperty.blank[File]
 
     div(
       FileInput(selectedFiles, acceptMultipleFiles)("files"),
       h4("Selected files"),
       ul(repeat(selectedFiles)(file => {
-        li(file.get.name).render
+        val content = Property(Array.empty[Byte])
+
+        FileService.asBytesArray(file.get) foreach { bytes =>
+          content.set(bytes)
+        }
+
+        val name = file.get.name
+        li(showIfElse(content.transform(_.isEmpty))(
+          span(name).render,
+          {
+            val url = FileService.createURL(content.get)
+            val download = a(href := url.value, attr("download") := name)(name)
+            val revoke = a(href := "#", onclick := { () =>
+              content.set(Array.empty[Byte])
+              url.close()
+            })("revoke")
+
+            Seq(download, span(" or "), revoke).render
+          }
+        )).render
       }))
     )
   }.withSourceCode