File input and Dart - file-upload

Question

I'm trying out Dart, but I cant figure out, how to send an image from the user to the server. I have my input-tag, and i can reach this in the DART code, but i cant seem to read from it. Im trying something like:
InputElement ie = document.query('#myinputelement');
ie.on.change.add((event){<br/>
InputElement iee = document.query('#myinputelement');<br/>
FileList mfl = iee.files;<br/>
File myFile = mlf.item(0);<br/>
FileReader fr = new FileReader();
fr.readAsBinaryString(myFile);
String result = fr.result; //this is always empty
});
With the html containing:
<input type="file" id="myinputelement">
I really hope you cant help me, im kinda stuck. I might just be missing how to do the onload for the filereader, or maybe im doing it totally wrong.

Answer 1

The FileReader API is asynchronous so you need to use event handlers.
var input = window.document.querySelector('#upload');
Element log = query("#log");
input.addEventListener("change", (e) {
FileList files = input.files;
Expect.isTrue(files.length > 0);
File file = files.item(0);
FileReader reader = new FileReader();
reader.onLoad = (fileEvent) {
print("file read");
log.innerHTML = "file content is ${reader.result}";
};
reader.onerror = (evt) => print("error ${reader.error.code}");
reader.readAsText(file);
});
you also need to allow file uploads from to your browser, which can be done in Chrome by starting it with the flag --allow-file-access-from-files

Answer 2

This is how to read a file using dart:html.
document.querySelector('#myinputelement`).onChange.listen((changeEvent) {
List fileInput = document.querySelector('#myinputelement').files;
if (fileInput.length > 1) {
// More than one file got selected somehow, could be a browser bug.
// Unless the "multiple" attribute is set on the input element, of course
}
else if (fileInput.isEmpty) {
// This could happen if the browser allows emptying an upload field
}
FileReader reader = new FileReader();
reader.onLoad.listen((fileEvent) {
String fileContent = reader.result;
// Code doing stuff with fileContent goes here!
});
reader.onError.listen((itWentWrongEvent) {
// Handle the error
});
reader.readAsText(fileInput[0]);
});

Answer 3

It's not necessary (any more) to use dart:dom FileReader instead of the one from dart:html.
Your code should work if you add an event listener to the file reader, like this:
FileReader fr = new FileReader();
fr.on.load.add((fe) => doSomethingToString(fe.target.result));
fr.readAsBinaryString(myFile);

Answer 4

My attempt
void fileSelected(Event event) async {
final files = (event.target as FileUploadInputElement).files;
if (files.isNotEmpty) {
final reader = new FileReader();
// ignore: unawaited_futures
reader.onError.first.then((evt) => print('error ${reader.error.code}'));
final resultReceived = reader.onLoad.first;
reader.readAsArrayBuffer(files.first);
await resultReceived;
imageReference.fileSelected(reader.result as List<int>);
}
}

Answer 5

Thanks to the help from this post, I got it to work. I still utilized my event handler in the input tag and made sure that I DID NOT import both dart:io and dart:html, only dart:html is needed.
This is what my final AppComponent looked like.
import 'dart:html';
import 'package:angular/angular.dart';
#Component(
selector: 'my-app',
styleUrls: ['app_component.css'],
templateUrl: 'app_component.html',
directives: [coreDirectives],
)
class AppComponent {
// Stores contents of file upon load
String contents;
AppComponent();
void fileUpload(event) {
// Get tag and the file
InputElement input = window.document.getElementById("fileUpload");
File file = input.files[0];
// File reader and event handler for end of loading
FileReader reader = FileReader();
reader.readAsText(file);
reader.onLoad.listen((fileEvent) {
contents = reader.result;
});
}
}
This is what my template looks like:
<h1>File upload test</h1>
<input type="file" (change)="fileUpload($event)" id="fileUpload">
<div *ngIf="contents != null">
<p>Hi! These are the contents of your file:</p>
<p>{{contents}}</p>
</div>