use assumeInteractiveReceiver + add comments for non-interactive use

example/lib/payjoin_library.dart

@@ -5,6 +5,7 @@ import 'dart:typed_data';
 import 'package:flutter/cupertino.dart';
 import 'package:payjoin_flutter/common.dart' as common;
 import 'package:payjoin_flutter/receive/v1.dart' as v1;
+import 'package:payjoin_flutter/receive/v2.dart';
 import 'package:payjoin_flutter/send.dart' as send;
 import 'package:payjoin_flutter/uri.dart' as pj_uri;
 
@@ -33,19 +34,42 @@ class PayJoinLibrary {
     });
     final unchecked = await v1.UncheckedProposal.fromRequest(
         body: body.toList(), query: '', headers: headers);
-    final provisionalProposal = await handleUnckedProposal(unchecked, isOwned);
+    final provisionalProposal =
+        await handleUncheckedProposal(unchecked, isOwned);
     return provisionalProposal;
   }
 
-  Future<v1.ProvisionalProposal> handleUnckedProposal(
+  Future<v1.ProvisionalProposal> handleUncheckedProposal(
       v1.UncheckedProposal uncheckedProposal,
       Future<bool> Function(Uint8List) isOwned) async {
-    // in a payment processor where the sender could go offline, this is where you schedule to broadcast the original_tx
-    var _ = await uncheckedProposal.extractTxToScheduleBroadcast();
-    final inputsOwned = await uncheckedProposal.checkBroadcastSuitability(
-        canBroadcast: (e) async {
-      return true;
-    });
+    // A consumer wallet has some manual interaction to initiate a payjoin, it
+    //  is not a server that can receive a lot of requests without the user
+    //  being aware of it. Therefore we say a consumer wallet app is an
+    //  interactive receiver and an automatic payment processor is
+    //  non-interactive.
+    //
+    //  The way to check a proposal for these cases are different:
+    //   - For an interactive receiver, you can just call
+    //      `assumeInteractiveReceiver` as used here in the example code.
+    //   - For a non-interactive receiver, you would extract the original tx
+    //      with `extractTxToScheduleBroadcast` and check if it can be
+    //      broadcasted in `checkBroadcastSuitability`. This way, if the sender
+    //      doesn't complete the payjoin, you can still broadcast the original
+    //      tx and get your funds. This protects against sender maliciousness of
+    //      probing your utxo set amongst other things.
+
+    final inputsOwned = await uncheckedProposal.assumeInteractiveReceiver();
+    /*
+      // Non-interactive receiver example code:
+      final originalTx = await uncheckedProposal.extractTxToScheduleBroadcast();
+      final inputsOwned = await uncheckedProposal.checkBroadcastSuitability(
+          canBroadcast: (e) async {
+        // Here you would check if the original tx is a valid tx that pays you 
+        //  and that can be broadcasted.
+        return true;
+      });
+    */
+
     // Receive Check 2: receiver can't sign for proposal inputs
     final mixedInputScripts =
         await inputsOwned.checkInputsNotOwned(isOwned: isOwned);