app_links

In AndroidManifest.xml

<!-- App Link sample -->
<intent-filter android:autoVerify="true">
    <action android:name="android.intent.action.VIEW" />
    <category android:name="android.intent.category.DEFAULT" />
    <category android:name="android.intent.category.BROWSABLE" />
    <data android:scheme="http" android:host="www.example.com" android:pathPrefix="/foo" />
    <data android:scheme="https" android:host="www.example.com" android:pathPrefix="/foo" />
</intent-filter>

<!-- Deep Link sample -->
<intent-filter>
    <action android:name="android.intent.action.VIEW" />
    <category android:name="android.intent.category.DEFAULT" />
    <category android:name="android.intent.category.BROWSABLE" />
    <!-- Add optional android:host to distinguish your app
          from others in case of conflicting scheme name -->
    <data android:scheme="sample" android:host="open.my.app" />
    <!-- <data android:scheme="sample" /> -->
</intent-filter>

import app_links

@UIApplicationMain
@objc class AppDelegate: FlutterAppDelegate {
  override func application(
    _ application: UIApplication,
    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
  ) -> Bool {
    GeneratedPluginRegistrant.register(with: self)

    // Retrieve the link from parameters
    if let url = AppLinks.shared.getLink(launchOptions: launchOptions) {
      // We have a link, propagate it to your Flutter app
      AppLinks.shared.handleLink(url: url)
      return true // Returning true will stop the propagation to other packages
    }

    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
  }
}

import app_links

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    func scene(_ scene: UIScene, openURLContexts URLContexts: Set<UIOpenURLContext>) {
      for context in URLContexts {
        AppLinks.shared.handleLink(url: context.url)
      }
    }
    
    func scene(_ scene: UIScene, continue userActivity: NSUserActivity) {
      if let url = userActivity.webpageURL {
        AppLinks.shared.handleLink(url: url)
      }
    }
}

Add this inclusion in <PROJECT_DIR>\windows\runner\main.cpp

#include "app_links/app_links_plugin_c_api.h"

Add this method before wWinMain

bool SendAppLinkToInstance(const std::wstring& title) {
  // Find our exact window
  HWND hwnd = ::FindWindow(L"FLUTTER_RUNNER_WIN32_WINDOW", title.c_str());

  if (hwnd) {
    // Dispatch new link to current window
    SendAppLink(hwnd);

    // (Optional) Restore our window to front in same state
    WINDOWPLACEMENT place = { sizeof(WINDOWPLACEMENT) };
    GetWindowPlacement(hwnd, &place);

    switch(place.showCmd) {
      case SW_SHOWMAXIMIZED:
          ShowWindow(hwnd, SW_SHOWMAXIMIZED);
          break;
      case SW_SHOWMINIMIZED:
          ShowWindow(hwnd, SW_RESTORE);
          break;
      default:
          ShowWindow(hwnd, SW_NORMAL);
          break;
    }

    SetWindowPos(0, HWND_TOP, 0, 0, 0, 0, SWP_SHOWWINDOW | SWP_NOSIZE | SWP_NOMOVE);
    SetForegroundWindow(hwnd);
    // END Restore

    // Window has been found, don't create another one.
    return true;
  }

  return false;
}

Add the call to the previous method in wWinMain

int APIENTRY wWinMain(_In_ HINSTANCE instance, _In_opt_ HINSTANCE prev,
                      _In_ wchar_t *command_line, _In_ int show_command) {
  // Replace "example" with the generated title found as parameter of `window.Create` in this file.
  // You may ignore the result if you need to create another window.
  if (SendAppLinkToInstance(L"example")) {
    return EXIT_SUCCESS;
  }

...

Great!

Now you can register your own scheme. On Windows, URL protocols are setup in the Windows registry.

This package won't do it for you (and will never sorry).

You may achieve it with using the win32_registry package with the following code to register the URL protocol:

Future<void> register(String scheme) async {
  String appPath = Platform.resolvedExecutable;

  String protocolRegKey = 'Software\\Classes\\$scheme';
  RegistryValue protocolRegValue = const RegistryValue(
    'URL Protocol',
    RegistryValueType.string,
    '',
  );
  String protocolCmdRegKey = 'shell\\open\\command';
  RegistryValue protocolCmdRegValue = RegistryValue(
    '',
    RegistryValueType.string,
    '"$appPath" "%1"',
  );

  final regKey = Registry.currentUser.createKey(protocolRegKey);
  regKey.createValue(protocolRegValue);
  regKey.createKey(protocolCmdRegKey).createValue(protocolCmdRegValue);
}

Add this XML chapter in your macos/Runner/Info.plist inside <plist version="1.0"><dict> chapter:

<key>CFBundleURLTypes</key>
<array>
    <dict>
        <key>CFBundleURLName</key>
        <!-- abstract name for this URL type (you can leave it blank) -->
        <string>sample_name</string>
        <key>CFBundleURLSchemes</key>
        <array>
            <!-- your schemes -->
            <string>sample</string>
        </array>
    </dict>
</array>

Done!

Apply the following changes to your linux/my_application.cc file:

diff --git a/example/linux/my_application.cc b/example/linux/my_application.cc
index 0ba8f43..f07f765 100644
--- a/example/linux/my_application.cc
+++ b/example/linux/my_application.cc
@@ -17,6 +17,13 @@ G_DEFINE_TYPE(MyApplication, my_application, GTK_TYPE_APPLICATION)
 // Implements GApplication::activate.
 static void my_application_activate(GApplication* application) {
   MyApplication* self = MY_APPLICATION(application);
+
+  GList* windows = gtk_application_get_windows(GTK_APPLICATION(application));
+  if (windows) {
+    gtk_window_present(GTK_WINDOW(windows->data));
+    return;
+  }
+
   GtkWindow* window =
       GTK_WINDOW(gtk_application_window_new(GTK_APPLICATION(application)));

@@ -78,7 +85,7 @@ static gboolean my_application_local_command_line(GApplication* application, gch
   g_application_activate(application);
   *exit_status = 0;

-  return TRUE;
+  return FALSE;
 }

 // Implements GObject::dispose.
@@ -99,6 +106,6 @@ static void my_application_init(MyApplication* self) {}
 MyApplication* my_application_new() {
   return MY_APPLICATION(g_object_new(my_application_get_type(),
                                      "application-id", APPLICATION_ID,
-                                     "flags", G_APPLICATION_NON_UNIQUE,
+                                     "flags", G_APPLICATION_HANDLES_COMMAND_LINE | G_APPLICATION_HANDLES_OPEN,
                                      nullptr));

Notes:

• Please ensure your APPLICATION_ID is the same as your desktop file name if you're using Flathub.
• Please ensure you have added this section in your snapcraft.yaml file if you're using SnapStore:

slots:
  dbus-appflowy:
    interface: dbus
    bus: session
    name: `APPLICATION_ID`

• You can refer to these two repositories for more details: FlatHub setup and Snapcraft setup.

Done!