Async and Practical

Temp File Round Trip

Dart can reserve a fresh temporary directory for short-lived files. This program writes a small text file, reads it back, summarizes the result, and uses a finally block to delete the directory even if the body throws.

Program

Play the program to round-trip two lines of text through a temp file.

file_temp_roundtrip.dart
Replay: real traced execution (multi-file project)
import 'dart:io';

void main() {
  var tempRoot = Directory.systemTemp;
  var dir = tempRoot.createTempSync('egtry_dart_');
  try {
    var path = '${dir.path}/note.txt';
    var file = File(path);
    var payload = 'Ada\nLin\n';
    file.writeAsStringSync(payload);
    var text = file.readAsStringSync();
    var lines = text.trim().split('\n');
    var count = lines.length;
    var first = lines[0];
    print('lines=$count first=$first');
  } finally {
    dir.deleteSync(recursive: true);
  }
}

  1. tempRoot ← system temp

    3void main() {4  var tempRoot = Directory.systemTemp;5  var dir = tempRoot.createTempSync('egtry_dart_');
    values this stepsystem temptempRoot

  2. dir ← temp directory

    4var tempRoot = Directory.systemTemp;5var dir = tempRoot.createTempSync('egtry_dart_');6try {
    values this steptemp directorydir

  3. path ← <temp>/note.txt

    6try {7  var path = '${dir.path}/note.txt';8  var file = File(path);
    values this step<temp>/note.txtpath

  4. file ← File handle

    7var path = '${dir.path}/note.txt';8var file = File(path);9var payload = 'Ada\nLin\n';
    values this stepFile handlefile<temp>/note.txtpath

  5. payload ← Ada\nLin\n

    8var file = File(path);9var payload = 'Ada\nLin\n';10file.writeAsStringSync(payload);
    values this stepAda\nLin\npayload

  6. content ← written

    9var payload = 'Ada\nLin\n';10file.writeAsStringSync(payload);11var text = file.readAsStringSync();
    values this stepwrittencontentAda\nLin\npayload

  7. text ← Ada\nLin\n

    10file.writeAsStringSync(payload);11var text = file.readAsStringSync();12var lines = text.trim().split('\n');
    values this stepAda\nLin\ntextwrittencontent

  8. lines ← [Ada, Lin]

    11var text = file.readAsStringSync();12var lines = text.trim().split('\n');13var count = lines.length;
    values this step[Ada, Lin]linesAda\nLin\ntext

  9. count ← 2

    12var lines = text.trim().split('\n');13var count = lines.length;14var first = lines[0];
    values this step2count[Ada, Lin]lines

  10. first ← Ada

    13var count = lines.length;14var first = lines[0];15print('lines=$count first=$first');
    values this stepAdafirstAdalines[0]

  11. print('lines=$count first=$first');

    14  var first = lines[0];15  print('lines=$count first=$first');16} finally {
    outputlines=2 first=Ada
    values this step2countAdafirst

  12. dir ← removed

    16} finally {17  dir.deleteSync(recursive: true);18}
    values this stepremoveddir
Directory.systemTemp `createTempSync('prefix')` returns a fresh unique temp directory the program can write inside.
write/read round trip `writeAsStringSync` saves the text, `readAsStringSync` returns the same bytes back as a `String`.
finally cleanup `finally { dir.deleteSync(recursive: true); }` removes the directory even if the body throws.