Logging your app's print statements to a local file for later processing.

run_logged.dart

import 'dart:async';
import 'dart:core';
import 'dart:io';

import 'package:flutter/material.dart';
import 'package:path/path.dart' as p;
import 'package:path_provider/path_provider.dart';

void main() => runLoggedApp(App());

// View the log with:
// adb shell run-as <your-android-package> cat /data/data/<your-android-package>/app_flutter/log.txt
void runLoggedApp(Widget app, [String filename = 'log.txt']) async {
  final log = File(p.join((await getApplicationDocumentsDirectory()).path, filename)).openWrite(mode: FileMode.append);
  runZoned(() {
    runApp(app);
  }, zoneSpecification: ZoneSpecification(
    print: (Zone self, ZoneDelegate parent, Zone zone, String line) {
      log.writeln('${DateTime.now().toIso8601String()} | $line');
      log.flush();
      parent.print(zone, line);
    },
  ));
}

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('error!');
    return Container();
  }
}

Ran into some issues with the flush method, this works for me:

import 'dart:async';
import 'dart:io';

import 'package:flutter/material.dart';

import 'package:path/path.dart' as p;
import 'package:path_provider/path_provider.dart';

void runLoggedApp(Widget app, [String filename = 'log.txt']) async {
  final dir = await getApplicationDocumentsDirectory();
  final file = File(p.join(dir.path, filename));
  if (!await file.exists()) await file.create();
  final log = file.openWrite(mode: FileMode.append);
  runZoned(() {
    runApp(app);
  }, zoneSpecification: ZoneSpecification(
    print: (Zone self, ZoneDelegate parent, Zone zone, String line) {
      log.writeln('${DateTime.now().toIso8601String()} | $line');
      // await log.flush();
      parent.print(zone, line);
    },
  ));
}