`DocumentFile.findFile` causes ANR

[clragon]

Describe the bug

Using DocumentFile.findFile inside of a folder with many files will somehow use so much CPU that it hangs the main UI thread.

To Reproduce

• Compile, install and run the sample code below on an Android device
• Select a folder with many images
• Trigger the ANR
• Attempt to click increment the counter
• Observe that the increment button cannot be clicked because the App no longer responds to user input

Note that the sample code actually uses DocumentFile.child but this triggers an unreasonably costly findFile regardless.

Sample code

import 'package:flutter/material.dart';
import 'package:shared_storage/shared_storage.dart';

void main() => runApp(const App());

class App extends StatelessWidget {
  const App({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(
          brightness: Brightness.dark,
          seedColor: Colors.deepPurple,
        ),
        useMaterial3: true,
      ),
      home: const Home(),
    );
  }
}

class Home extends StatefulWidget {
  const Home({super.key});

  @override
  State<Home> createState() => _HomeState();
}

class _HomeState extends State<Home> {
  int count = 0;
  DocumentFile? folder;
  bool busy = false;
  bool hangs = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: const Text('findFile ANR'),
      ),
      body: Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            Text(
              '$count',
              style: Theme.of(context).textTheme.headlineMedium,
            ),
            const SizedBox(width: 32),
            ElevatedButton(
              onPressed: () => setState(() => count++),
              child: const Text('increment'),
            ),
            const SizedBox(width: 32),
            AnimatedOpacity(
              duration: const Duration(milliseconds: 200),
              opacity: hangs ? 1 : 0,
              child: Text(
                'try incrementing now',
                style: Theme.of(context).textTheme.bodySmall,
              ),
            ),
            const SizedBox(width: 32),
            Row(
              mainAxisSize: MainAxisSize.min,
              children: [
                ElevatedButton(
                  onPressed: !busy
                      ? () async {
                          setState(() => busy = true);
                          folder = await (await openDocumentTree())
                              ?.toDocumentFile();
                          setState(() => busy = false);
                        }
                      : null,
                  child: const Text('select'),
                ),
                const SizedBox(width: 16),
                ElevatedButton(
                  onPressed: !busy && folder != null
                      ? () async {
                          setState(() {
                            busy = true;
                            hangs = true;
                          });
                          await folder!.child('any.png');
                          setState(() {
                            busy = false;
                            hangs = false;
                          });
                        }
                      : null,
                  child: const Text('trigger ANR'),
                ),
              ],
            ),
          ],
        ),
      ),
    );
  }
}

Expected behavior

Using DocumentFile.findFile should not hang the UI thread.

Screenshots

Here is a video demonstration of the issue:

https://github.com/alexrintt/shared-storage/assets/11785085/21d9f96f-279c-4ce2-aa1b-4d50ffbde3c6

Smartphone:

• Device: Samsung Galaxy S20 FE
• OS: Android 13
• Flutter 3.13.0, shared_storage 0.8.0

[clragon]

Further, I have found out that I can work around this issue with the following code:

await Uri.parse('${folder.uri}%2F${Uri.encodeComponent(fileName)}')).toDocumentFile();

which is basically just manually constructing the same URI and then converting it into a DocumentFile. For whatever reason, this does not trigger a findFile call and therefore doesnt run into the ANR issue.

However, I would obviously strongly prefer constructing files with the intended APIs and not manually.

[alexrintt]

Hi! I could reproduce here as well, thank you very much for the detailed issue.

findFiles is flawed by design, it's a brute force search implemented by the android sdk (a simple linear search) which is a performance disaster when we have a folder with several files (which is the case for the vast majority of the modern devices).

---

About your workaround, it won't work with some edgy content providers because sometimes the URI doesn't follow the parent/child.ext naming pattern but a provider_storage_id/document_id pattern.

I'll try to fix it using content providers and manually query the storage DB, but in the worst scenario we can just run the findFiles method in a background thread (it will still be slow but will fix the main thread blocking).