Add example for progressive call results

examples/rpc_progressive_call_results/callee.dart

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+import "dart:io";
+import "package:xconn/xconn.dart";
+
+const procedureDownload = "io.xconn.progress.download";
+
+Future<void> main() async {
+  var client = Client();
+  var session = await client.connect("ws://localhost:8080/ws", "realm1");
+
+  // Define function to handle received Invocation for "io.xconn.progress.download"
+  Result downloadHandler(Invocation inv) {
+    var totalSize = 1000; // Total file size in "bytes"
+    var chunkSize = 100; // Each chunk is 100 bytes
+    var progress = 0;
+
+    while (progress < totalSize) {
+      progress += chunkSize;
+      inv.sendProgress([progress, totalSize], null); // Send progress
+      sleep(const Duration(milliseconds: 500)); // Simulate time to download each chunk
+    }
+
+    return Result(args: ["Download complete"]);
+  }
+
+  // Register procedure "io.xconn.progress.download"
+  var registration = await session.register(procedureDownload, downloadHandler);
+  print("Registered procedure $procedureDownload successfully");
+
+  // Define a signal handler to catch the interrupt signal (Ctrl+C)
+  ProcessSignal.sigint.watch().listen((signal) async {
+    await session.unregister(registration);
+    await session.close();
+  });
+}

examples/rpc_progressive_call_results/caller.dart

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+import "dart:io";
+
+import "package:xconn/xconn.dart";
+
+const procedureDownload = "io.xconn.progress.download";
+
+Future<void> main() async {
+  var client = Client();
+  var session = await client.connect("ws://localhost:8080/ws", "realm1");
+
+  // Call procedure "io.xconn.progress.download"
+  var result = await session.call(
+    procedureDownload,
+    progressHandler: (Result result) {
+      var progress = result.args[0]; // Current progress
+      var totalSize = result.args[1]; // Total file size
+      print("Download progress: $progress / $totalSize bytes");
+    },
+  );
+
+  print(result.args[0]);
+
+  await session.close();
+  exit(0);
+}