Flutter web upload file cancel event - flutter

Question

I develop a web app in Flutter and I want to load a file from file system. In order to do that I use the following code:
static Future<Uint8List> chooseImage(dynamic parent, dynamic provider) async {
Uint8List uploadedImage;
final completer = Completer<List<String>>();
InputElement uploadInput = FileUploadInputElement();
uploadInput.accept = 'image/*';
uploadInput.click();
uploadInput.addEventListener('change', (e) async {
final files = uploadInput.files;
Iterable<Future<String>> resultsFutures = files.map((file) {
final reader = FileReader();
reader.readAsDataUrl(file);
reader.onError.listen((error) => completer.completeError(error));
return reader.onLoad.first.then((_) async {
String result = reader.result as String;
uploadedImage = base64Decode(result.substring(22, result.length));
return reader.result as String;
});
});
final results = await Future.wait(resultsFutures);
completer.complete(results);
});
document.body.append(uploadInput);
final List<String> images = await completer.future;
parent.setState(() {
parent.pickedImage = uploadedImage;
});
uploadInput.remove();
return uploadedImage;
}
In my app I need to handle the case when the user press the Cancel button in this pop-up:
I have added listener for: onFocus, onSuspen, onSubmit, onEnded, onAbort but none of these events are triggered when that cancel button is pressed.
How can I handle the pop-up cancelation?

Answer 1

I've already answered to a similar question here
You can already find a way to manage this kind of event in the web implementation of the package file_picker.
Because depending of the browser you are using the cancel event might be managed differently the most generic solution would be to rely on a listener on the focus event from the window so when you will lose the focus on your file picker window without any data loaded, it will complete your future with a null value.
Code Sample
import 'dart:html' as html;
import 'dart:async';
Future<html.File?> pickFile(String type) async {
final completer = Completer<List<html.File>?>();
final input = html.FileUploadInputElement() as html.InputElement;
input.accept = '$type/*';
var changeEventTriggered = false;
void changeEventListener(html.Event e) {
if (changeEventTriggered) return;
changeEventTriggered = true;
final files = input.files!;
final resultFuture = files.map<Future<html.File>>((file) async {
final reader = html.FileReader();
reader.readAsDataUrl(file);
reader.onError.listen(completer.completeError);
return file;
});
Future.wait(resultFuture).then((results) => completer.complete(results));
}
void cancelledEventListener(html.Event e) {
html.window.removeEventListener('focus', cancelledEventListener);
// This listener is called before the input changed event,
// and the `uploadInput.files` value is still null
// Wait for results from js to dart
Future.delayed(Duration(milliseconds: 500)).then((value) {
if (!changeEventTriggered) {
changeEventTriggered = true;
completer.complete(null);
}
});
}
input.onChange.listen(changeEventListener);
input.addEventListener('change', changeEventListener);
// Listen focus event for cancelled
html.window.addEventListener('focus', cancelledEventListener);
input.click();
final results = await completer.future;
if (results == null || results.isEmpty) return null;
return results.first;
}
Sources
file_picker implementation