chore(toga): complite 4 step of tutorial

wselfedu/src/wselfedu/app.py

@@ -5,6 +5,13 @@
 from toga.style.pack import COLUMN, ROW
 
 
+def greeting(name):
+    if name:
+        return f"Hello, {name}"
+    else:
+        return "Hello, stranger"
+
+
 class WebSelfEducation(toga.App):
     """Web self education application class."""
 
@@ -35,8 +42,10 @@ def startup(self):
         self.main_window.show()
 
     def say_hello(self, widget):
-        print(f"Hello, {self.name_input.value}")
-
+        self.main_window.info_dialog(
+            greeting(self.name_input.value),
+            "Hi there!",
+        )
 
 def main():
     return WebSelfEducation()