added pdf viewer helper files

Author: Dharanipalani

lib/samples/pdf_viewer/shared/helper.dart

@@ -0,0 +1,88 @@
+import 'package:flutter/material.dart';
+
+/// Shows toast once after the selected text is copied to the Clipboard.
+Widget showToast(bool canShowToast, Alignment alignment, String toastText) {
+  return Visibility(
+      visible: canShowToast,
+      child: Positioned.fill(
+        bottom: 25.0,
+        child: Align(
+          alignment: alignment,
+          child: Flex(
+            direction: Axis.horizontal,
+            mainAxisAlignment: MainAxisAlignment.center,
+            children: <Widget>[
+              Container(
+                padding: const EdgeInsets.only(
+                    left: 16, top: 6, right: 16, bottom: 6),
+                decoration: BoxDecoration(
+                  color: Colors.grey[600],
+                  borderRadius: const BorderRadius.all(
+                    Radius.circular(16.0),
+                  ),
+                ),
+                child: Text(
+                  toastText,
+                  textAlign: TextAlign.center,
+                  style: const TextStyle(
+                      fontFamily: 'Roboto', fontSize: 16, color: Colors.white),
+                ),
+              ),
+            ],
+          ),
+        ),
+      ));
+}
+
+/// Displays the error message
+void showErrorDialog(BuildContext context, String error, String description) {
+  showDialog<dynamic>(
+    context: context,
+    builder: (BuildContext context) {
+      return AlertDialog(
+        insetPadding: const EdgeInsets.all(0),
+        title: Row(
+          mainAxisAlignment: MainAxisAlignment.spaceBetween,
+          children: <Widget>[
+            Text(error),
+            Container(
+              height: 36, // height of close search menu button
+              width: 36, // width of close search menu button
+              child: RawMaterialButton(
+                onPressed: () {
+                  Navigator.of(context).pop();
+                },
+                child: const Icon(
+                  Icons.clear,
+                  size: 20,
+                ),
+              ),
+            ),
+          ],
+        ),
+        content: Container(width: 328.0, child: Text(description)),
+        actions: <Widget>[
+          TextButton(
+            onPressed: () {
+              Navigator.of(context, rootNavigator: true).pop();
+            },
+            child: const Text('OK'),
+          )
+        ],
+        actionsPadding: const EdgeInsets.only(bottom: 10),
+      );
+    },
+  );
+}
+
+/// Represents PDF document.
+class Document {
+  /// Constructs Document instance.
+  Document(this.name, this.path);
+
+  /// Name of the PDF document.
+  final String name;
+
+  /// Path of the PDF document.
+  final String path;
+}