dart 自己写一个简单的文件编码器

 // import 'dart:convert';
 import 'dart:io';

 main() async {
 　　var src = File('./lib/convert/source.txt');
 　　var output = File('./lib/convert/output.txt');

 　　var ins = src.openRead();
 　　var encoded = convert(ins, 128);
 　　var outs = output.openWrite();
 　　await encoded.pipe(outs);
 　　outs.close();

 　　// the second transform.
 　　var output2 = File('./lib/convert/output2.txt');
 　　var outs2 = output2.openWrite();
 　　await convert(output.openRead(), -128).pipe(outs2);
 　　outs2.close();
 }

 Stream<List<int>> convert(Stream<List<int>> source,
 　　　　[int key = 128, int base = 256]) async* {
 　　await for (var chunk in source) {
 　　　　yield encode(chunk, key, base);
 　　}
 }

 List<int> encode(List<int> original, [int key = 128, int base = 256]) {
     var encoded = List<int>(original.length);
     for (var i = 0; i < original.length; i++) {
 　　encoded[i] = (original[i] + key) % base;
     }
     return encoded;
 }