Add example

docs/examples/events/prevent.py

@@ -0,0 +1,26 @@
+from textual.app import App, ComposeResult
+from textual.containers import Horizontal
+from textual.widgets import Button, Input
+
+
+class PreventApp(App):
+    """Demonstrates `prevent` context manager."""
+
+    def compose(self) -> ComposeResult:
+        yield Input()
+        yield Button("Clear", id="clear")
+
+    def on_button_pressed(self) -> None:
+        """Clear the text input."""
+        input = self.query_one(Input)
+        with input.prevent(Input.Changed):  # (1)
+            input.value = ""
+
+    def on_input_changed(self) -> None:
+        """Called as the user types."""
+        self.bell()  # (2)
+
+
+if __name__ == "__main__":
+    app = PreventApp()
+    app.run()