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;
}