Validation (#2600)

* A few different types of validation

* Rename

* Fix test

* Updating validation framework

* Update lockfile

* Ensure validators can be None

* Reworking the API a little

* Convert Input.Changed to dataclass

* Add utility for getting failures as strings

* Update an example in Validator docstring

* Remove some redundant `pass`es

* Renaming variables

* Validating Input on submit, attaching result to Submitted event

* Testing various validation features

* Update snapshots and deps

* Styling unfocused -invalid Input differently

* Add snapshot test around input validation and associated styles

* Validation docs

* Tidying validation docs in Input widget reference

* Fix mypy issues

* Remove __bool__ from Failure, make validator field required

* Code review changes

* Improving error messages in Validators

docs/examples/widgets/input_validation.py

@@ -0,0 +1,72 @@
+from textual import on
+from textual.app import App, ComposeResult
+from textual.validation import Function, Number, ValidationResult, Validator
+from textual.widgets import Input, Label, Pretty
+
+
+class InputApp(App):
+    # (6)!
+    CSS = """
+    Input.-valid {
+        border: tall $success 60%;
+    }
+    Input.-valid:focus {
+        border: tall $success;
+    }
+    Input {
+        margin: 1 1;
+    }
+    Label {
+        margin: 1 2;
+    }
+    Pretty {
+        margin: 1 2;
+    }
+    """
+
+    def compose(self) -> ComposeResult:
+        yield Label("Enter an even number between 1 and 100 that is also a palindrome.")
+        yield Input(
+            placeholder="Enter a number...",
+            validators=[
+                Number(minimum=1, maximum=100),  # (1)!
+                Function(is_even, "Value is not even."),  # (2)!
+                Palindrome(),  # (3)!
+            ],
+        )
+        yield Pretty([])
+
+    @on(Input.Changed)
+    def show_invalid_reasons(self, event: Input.Changed) -> None:
+        # Updating the UI to show the reasons why validation failed
+        if not event.validation_result.is_valid:  # (4)!
+            self.query_one(Pretty).update(event.validation_result.failure_descriptions)
+        else:
+            self.query_one(Pretty).update([])
+
+
+def is_even(value: str) -> bool:
+    try:
+        return int(value) % 2 == 0
+    except ValueError:
+        return False
+
+
+# A custom validator
+class Palindrome(Validator):  # (5)!
+    def validate(self, value: str) -> ValidationResult:
+        """Check a string is equal to its reverse."""
+        if self.is_palindrome(value):
+            return self.success()
+        else:
+            return self.failure("That's not a palindrome :/")
+
+    @staticmethod
+    def is_palindrome(value: str) -> bool:
+        return value == value[::-1]
+
+
+app = InputApp()
+
+if __name__ == "__main__":
+    app.run()