window_proc_delegate

A Flutter plugin that allows you to hook into Windows WindowProc messages from Dart code.

Features

  • Register WindowProc delegates from Dart
  • Intercept and handle Windows messages (WM_* messages)
  • Multiple delegates support with priority handling
  • Clean API for registering and unregistering delegates

Platform Support

Platform Supported
Windows ✅
Linux ❌
macOS ❌
Android ❌
iOS ❌

Getting Started

Add this to your pubspec.yaml:

dependencies:
  window_proc_delegate:

Usage

Basic Example

import 'package:window_proc_delegate/window_proc_delegate.dart';

// Register a delegate to handle WindowProc messages
int delegateId = registerWindowProcDelegate(
  (int hwnd, int message, int wParam, int lParam) {
    // Check for WM_ACTIVATEAPP (0x001C)
    if (message == 0x001C) {
      print('Window activation changed: $wParam');
    }
    
    // Return null to let other delegates process the message
    return null;
  },
);

// Later, unregister the delegate
unregisterWindowProcDelegate(delegateId);

Intercepting Messages

You can intercept and prevent default handling of messages by returning a result value:

int delegateId = registerWindowProcDelegate(
  (int hwnd, int message, int wParam, int lParam) {
    // Intercept WM_CLOSE (0x0010) to prevent window closing
    if (message == 0x0010) {
      print('Window close prevented!');
      return 0; // Return a result to handle the message
    }
    
    return null; // Let other delegates process the message
  },
);

Setting Custom Result Values

You can set a custom result value for handled messages:

int delegateId = registerWindowProcDelegate(
  (int hwnd, int message, int wParam, int lParam) {
    if (message == 0x0084) { // WM_NCHITTEST
      // Return custom result
      return 2; // HTCAPTION - makes the window draggable
    }
    
    return null;
  },
);

Callback Parameters

The callback receives the following parameters:

  • hwnd (int): Handle to the window
  • message (int): The message identifier (WM_* constant)
  • wParam (int): Additional message-specific information
  • lParam (int): Additional message-specific information

Return value:

  • Return an int value to handle the message and use that as the result
  • Return null to let other delegates process the message

API

registerWindowProcDelegate(WindowProcDelegateCallback delegate)

Registers a WindowProc delegate callback. Returns an ID that can be used to unregister the delegate later.

unregisterWindowProcDelegate(int id)

Unregisters a previously registered delegate by its ID.

Common Windows Messages

Here are some commonly used Windows messages:

Message Value Description
WM_CLOSE 0x0010 Window is being closed
WM_ACTIVATEAPP 0x001C Window activation state changed
WM_NCHITTEST 0x0084 Hit test (for custom window dragging)
WM_SYSCOMMAND 0x0112 System command
WM_SIZE 0x0005 Window size changed
WM_MOVE 0x0003 Window position changed

For a complete list, see the Windows Message documentation.

Implementation Details

This plugin uses FFI (Foreign Function Interface) to communicate between Dart and native Windows code. It uses NativeCallable.isolateLocal to register Dart callbacks that can be called from native code.

The plugin maintains a list of delegates in Dart and dispatches WindowProc messages to each delegate in order until one handles the message (returns true).

Example App

See the example directory for a complete example application that demonstrates:

  • Registering WindowProc delegates
  • Logging received messages
  • Intercepting specific messages

Getting Started

This project is a Flutter plug-in package, a specialized package that includes platform-specific implementation code for Windows.

For help getting started with Flutter development, view the online documentation, which offers tutorials, samples, guidance on mobile development, and a full API reference.

Libraries

window_proc_delegate