File input and Dart

问题:

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.

回答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

回答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]);
});

回答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);

回答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>);
    }
  }

回答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>