[gxml.wiki] Update examples for GXml.HashMap writting out XML

[Daniel Espinosa Ortiz <despinosa src gnome org>]

commit 528e1a9729d02c1bc88b2f038d20c970c8a96ea4
Author: Daniel Espinosa Ortiz <esodan gmail com>
Date:   Mon Mar 8 23:43:19 2021 +0000

    Update examples for GXml.HashMap writting out XML

 examples.md | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 70 insertions(+), 1 deletion(-)
---
diff --git a/examples.md b/examples.md
index 89211fb..6512564 100644
--- a/examples.md
+++ b/examples.md
@@ -252,13 +252,14 @@ class BookStore : GXml.Element
 {
   [Description (nick="::name")]
   public string name { get; set; }
-  public Book.Array books { get; set; }
+  public Book.HashMap books { get; set; }
   construct {
       try { initialize ("BookStore"); }
       catch (GLib.Error e) {
         warning ("Initialization error for collection type: %s : %s"
             .printf (get_type ().name(), e.message));
       }
+      set_instance_property ('books');
   }
 }
 
@@ -276,4 +277,72 @@ public class App {
 }
 ```
 
+## Serialize GObject classes to XML
+
+```vala
+class Book : GXml.Element
+{
+  [Description(nick="::Name")]
+  public string name { get; set; }
+  [Description(nick="::Year")]
+  public string year { get; set; }
+  [Description(nick="::ISBN")]
+  public string isbn { get; set; }
+  construct {
+    try { initialize ("Book"); } catch { assert_not_reached (); }
+  }
+  public class HashMap : GXml.HashMap {
+    construct {
+      try { initialize_with_key (typeof (Book),"Name"); }
+      catch (GLib.Error e) {
+        warning ("Initialization error for collection type: %s : %s"
+            .printf (get_type ().name(), e.message));
+      }
+    }
+  }
+}
+
+class BookStore : GXml.Element
+{
+  [Description (nick="::name")]
+  public string name { get; set; }
+  public Book.HashMap books { get; set; }
+  construct {
+      try { initialize ("BookStore"); }
+      catch (GLib.Error e) {
+        warning ("Initialization error for collection type: %s : %s"
+            .printf (get_type ().name(), e.message));
+      }
+      set_instance_property ('books');
+  }
+}
+
+public class App {
+
+    public int main () {
+        var file = GLib.File.new_for_path ("document.xml");
+        BookStore store = new BookStore ();
+        Book book1 = store.books.create_item ();
+        book1.name = "Jay I'm here";
+        store.books.append (book1);
+        Book book2 = store.books.create_item ();
+        book2.name = "GXml is Cool";
+        store.books.append (book2);
+        Book book3 = store.books.create_item ();
+        book3.name = "How explore XML using GXml";
+        store.books.append (book3);
+        store.write_file (file);
+    }
+}
+```
+
+You will have a new `document.xml` file with the following content:
+
+```xml
+<BookStore>
+   <Book name = "Jay I'm here"/>
+   <Book name = "GXml is Cool"/>
+   <Book name = "How explore XML using GXml"/>
+</BookStore>
+```
  
\ No newline at end of file