Dart FileSystemModifyEvent

last modified April 4, 2025

The FileSystemModifyEvent class in Dart represents file system modification events. It's part of the dart:html library for web applications.

This event is triggered when files or directories are modified in the watched directory. It provides details about the changes that occurred in the file system.

Basic Definition

FileSystemModifyEvent is an event object for file system changes. It extends Event and contains information about modified entries.

Key properties include path for changed file paths and type for the modification type (create, modify, or delete).

Basic FileSystemModifyEvent Usage

This example shows how to listen for file system modification events.

import 'dart:html';

void main() {
  DirectoryEntry dir;
  
  window.requestFileSystem(1024 * 1024).then((fs) {
    dir = fs.root;
    
    dir.onChange.listen((FileSystemModifyEvent event) {
      print('Change detected in: ${event.path}');
      print('Change type: ${event.type}');
    });
  });
}

We request a file system and set up an event listener on the root directory. The listener prints details when any modification occurs in the file system.

$ dart main.dart
Change detected in: /example.txt
Change type: modify

Filtering Specific Event Types

This example demonstrates filtering for specific modification types.

import 'dart:html';

void main() {
  DirectoryEntry dir;
  
  window.requestFileSystem(1024 * 1024).then((fs) {
    dir = fs.root;
    
    dir.onChange.listen((FileSystemModifyEvent event) {
      if (event.type == 'create') {
        print('New file created: ${event.path}');
      } else if (event.type == 'delete') {
        print('File deleted: ${event.path}');
      }
    });
  });
}

We filter events to only handle file creation and deletion. This helps focus on specific changes while ignoring others like modifications.

$ dart main.dart
New file created: /test.txt
File deleted: /old.txt

Monitoring Specific Files

This example shows how to watch for changes to specific files only.

import 'dart:html';

void main() {
  DirectoryEntry dir;
  const targetFile = '/config.json';
  
  window.requestFileSystem(1024 * 1024).then((fs) {
    dir = fs.root;
    
    dir.onChange.listen((FileSystemModifyEvent event) {
      if (event.path == targetFile) {
        print('Config file changed: ${event.type}');
        // Reload config or take appropriate action
      }
    });
  });
}

We check the event path against our target file path. This creates a focused watcher that only responds to changes in the specified configuration file.

$ dart main.dart
Config file changed: modify

Handling Multiple Changes

This example demonstrates processing multiple changes from a single event.

import 'dart:html';

void main() {
  DirectoryEntry dir;
  
  window.requestFileSystem(1024 * 1024).then((fs) {
    dir = fs.root;
    
    dir.onChange.listen((FileSystemModifyEvent event) {
      print('Batch changes detected:');
      event.changes.forEach((change) {
        print('  ${change.path} - ${change.type}');
      });
    });
  });
}

Some implementations may batch multiple changes into a single event. We iterate through all changes in the event to handle each modification individually.

$ dart main.dart
Batch changes detected:
  /file1.txt - modify
  /temp/file2.txt - create

Error Handling in File Monitoring

This example adds error handling to file system monitoring.

import 'dart:html';

void main() {
  DirectoryEntry dir;
  
  window.requestFileSystem(1024 * 1024).then((fs) {
    dir = fs.root;
    
    var subscription = dir.onChange.listen(
      (FileSystemModifyEvent event) {
        print('Change: ${event.path}');
      },
      onError: (e) => print('Error: $e'),
      cancelOnError: false
    );
    
    // Later: subscription.cancel();
  }).catchError((e) => print('Filesystem error: $e'));
}

We add error handlers for both the filesystem request and event listening. The subscription can be cancelled later when monitoring is no longer needed.

$ dart main.dart
Change: /update.txt
Error: FileSystemError {code: 5, name: "InvalidModificationError"}

Best Practices

• Specific paths: Filter events to only watch relevant files
• Error handling: Always implement error handlers
• Resource cleanup: Cancel subscriptions when done
• Performance: Avoid heavy processing in event handlers
• Batch handling: Be prepared for multiple changes per event

Source

Dart FileSystemModifyEvent Documentation

This tutorial covered Dart's FileSystemModifyEvent class with practical examples showing basic usage, filtering, error handling, and monitoring techniques.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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