How to write a `ByteData` instance to a File in Dart? - dart

Question

I am using Flutter to load an "asset" into a File so that a native application can access it.
This is how I load the asset:
final dbBytes = await rootBundle.load('assets/file');
This returns an instance of ByteData.
How can I write this to a dart.io.File instance?

Answer 1

ByteData is an abstraction for:
A fixed-length, random-access sequence of bytes that also provides
random and unaligned access to the fixed-width integers and floating
point numbers represented by those bytes.
As Gunter mentioned in the comments, you can use File.writeAsBytes. It does require a bit of API work to get from ByteData to a List<int>, however.
import 'dart:async';
import 'dart:io';
import 'dart:typed_data';
Future<void> writeToFile(ByteData data, String path) {
final buffer = data.buffer;
return new File(path).writeAsBytes(
buffer.asUint8List(data.offsetInBytes, data.lengthInBytes));
}
I've also filed an issue to make the docs on Flutter more clear for this use case.

Answer 2

you need to have path_provider package installed, then
This should work :
import 'dart:async';
import 'dart:io';
import 'dart:typed_data';
import 'package:path_provider/path_provider.dart';
final dbBytes = await rootBundle.load('assets/file'); // <= your ByteData
//=======================
Future<File> writeToFile(ByteData data) async {
final buffer = data.buffer;
Directory tempDir = await getTemporaryDirectory();
String tempPath = tempDir.path;
var filePath = tempPath + '/file_01.tmp'; // file_01.tmp is dump file, can be anything
return new File(filePath).writeAsBytes(
buffer.asUint8List(data.offsetInBytes, data.lengthInBytes));
}
//======================
to get your file :
var file;
try {
file = await writeToFile(dbBytes); // <= returns File
} catch(e) {
// catch errors here
}
Hope this helps,
Thank you.

Answer 3

to search flutter ByteData to List<int> then found here, but not fully answer my question:
how to convert ByteData to List<int> ?
after self investigate, solution is:
use .cast<int>()
ByteData audioByteData = await rootBundle.load(audioAssetsFullPath);
Uint8List audioUint8List = audioByteData.buffer.asUint8List(audioByteData.offsetInBytes, audioByteData.lengthInBytes);
List<int> audioListInt = audioUint8List.cast<int>();
or 2. use .map
ByteData audioByteData = await rootBundle.load(audioAssetsFullPath);
Uint8List audioUint8List = audioByteData.buffer.asUint8List(audioByteData.offsetInBytes, audioByteData.lengthInBytes);
List<int> audioListInt = audioUint8List.map((eachUint8) => eachUint8.toInt()).toList();

Answer 4

For those looking to write bytes (aka Uint8List) instead of ByteData please note that ByteData is a wrapper for Uint8List.
From /runtime/lib/typed_data.patch:
#patch
class ByteData implements TypedData {
#patch
#pragma("vm:entry-point")
factory ByteData(int length) {
final list = new Uint8List(length) as _TypedList;
_rangeCheck(list.lengthInBytes, 0, length);
return new _ByteDataView(list, 0, length);
}
#patch
class Uint8List {
#patch
#pragma("vm:exact-result-type", _Uint8List)
factory Uint8List(int length) native "TypedData_Uint8Array_new";
}
If you are using the latter type you can use the answer provided by Rami and modify the return as follow:
import 'dart:async';
import 'dart:io';
import 'dart:typed_data';
import 'package:path_provider/path_provider.dart';
Future<File> writeToFile(Uint8List data) async {
(...)
return new File(filePath).writeAsBytes(data);
}