Capturing stdout result of `flutter test integration_test`

Question

For CI, I need to write a program that runs flutter test integration_test and captures the stdout result. In python the code would be:

import subprocess


process = subprocess.Popen(
    f'flutter test integration_test',
    shell=True,
    universal_newlines=True,
    stderr=subprocess.PIPE,
    stdout=subprocess.PIPE)

print('starting')

for line in process.stdout:
    print(f'[from python]: {line}')

If you run the above code in the example directory of a plugin package(for example path_provider) with an emulator, every output line will be prefixed with [from python] as expected:

...

[from python]: 00:17 +0: getTemporaryDirectory                                                                                                                                                                        

[from python]: 00:17 +1: getTemporaryDirectory                                                                                                                                                                        

[from python]: 00:17 +1: getApplicationDocumentsDirectory                                                                                                                                                             

[from python]: 00:18 +1: getApplicationDocumentsDirectory                                                                                                                                                             

[from python]: 00:18 +2: getApplicationDocumentsDirectory                                                                                                                                                             

...

However I need to write the program in dart, the code I wrote is:

import 'dart:io';

Future<void> main() async {
  print('starting');

  final process = await Process.start(
      'flutter', <String>['test', 'integration_test'],
      runInShell: true);
  await for (final line in process.stdout) {
    print('[from dart]: ${String.fromCharCodes(line)}');
  }
}

When I run the above code in the same example directory, some lines are not prefixed with [from dart], which means some stdout results were not captured by the dart program but are directly written to console.

[from dart]: â  Built build/app/outputs/flutter-apk/app-debug.apk.

00:15 +0: loading /home/hakkyu/mydir/project/flutter/flutter_plugins/packages/path_provider/path_provider/example/integration_test/path_provider_test.dart                                             
[from dart]: Installing build/app/outputs/flutter-apk/app.apk...             
00:16 +0: loading /home/hakkyu/mydir/project/flutter/flutter_plugins/packages/path_provider/path_provider/example/integration_test/path_provider_test.dart                                             
[from dart]:    492ms

00:17 +0: loading /home/hakkyu/mydir/project/flutter/flutter_plugins/packages/path_provider/path_provider/example/integration_test/path_provider_test.dart                                             
00:17 +0: getTemporaryDirectory                                                                                                                                                                        
00:17 +1: getTemporaryDirectory                                                                                                                                                                        
00:17 +1: getApplicationDocumentsDirectory                                                                                                                                                             
00:17 +2: getApplicationDocumentsDirectory                                                                                                                                                             
00:17 +2: getApplicationSupportDirectory                                                                                                                                                               
00:17 +3: getApplicationSupportDirectory                                                                                                                                                               
00:17 +3: getLibraryDirectory                                                                                                                                                                          
00:17 +4: getLibraryDirectory                                                                                                                                                                          
00:17 +4: getExternalStorageDirectory

If I haven't made any mistakes in the code, I can only assume that there are implementation differences in "process" between python and dart. I have no clue what to look into at this point. What could be causing this issue?

Solution

Give this a try:

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

Future<void> main() async {
  print('starting');

  final process = await Process.start(
      'flutter', <String>['test', 'integration_test'],
      runInShell: true);
  await for (final line
      in process.stdout.transform(Utf8Decoder()).transform(LineSplitter())) {
    print('[from dart]: $line');
  }
}

.transform(Utf8Decoder()) will convert bytes into Strings. .transform(LineSplitter()) will split the strings up if there are newlines, I am not too familiar with python, but I am guessing this is what the universal_newlines=True option does.

Comments

[Hakkyu Kim]

This worked! Thank you! Was newline not splitting correctly because the results of process.stdout were in bytes?

[Hakkyu Kim]

Ah, I guess that's it. I can see similar issue when I remove universal_newlines=True in python.