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

tests/input/test_input_validation.py

@@ -0,0 +1,79 @@
+from textual import on
+from textual.app import App, ComposeResult
+from textual.validation import Number, ValidationResult
+from textual.widgets import Input
+
+
+class InputApp(App):
+    def __init__(self):
+        super().__init__()
+        self.messages = []
+        self.validator = Number(minimum=1, maximum=5)
+
+    def compose(self) -> ComposeResult:
+        yield Input(
+            validators=self.validator,
+        )
+
+    @on(Input.Changed)
+    @on(Input.Submitted)
+    def on_changed_or_submitted(self, event):
+        self.messages.append(event)
+
+
+async def test_input_changed_message_validation_failure():
+    app = InputApp()
+    async with app.run_test() as pilot:
+        input = app.query_one(Input)
+        input.value = "8"
+        await pilot.pause()
+        assert len(app.messages) == 1
+        assert app.messages[0].validation_result == ValidationResult.failure(
+            failures=[
+                Number.NotInRange(
+                    value="8",
+                    validator=app.validator,
+                    description="Must be between 1 and 5.",
+                )
+            ],
+        )
+
+
+async def test_input_changed_message_validation_success():
+    app = InputApp()
+    async with app.run_test() as pilot:
+        input = app.query_one(Input)
+        input.value = "3"
+        await pilot.pause()
+        assert len(app.messages) == 1
+        assert app.messages[0].validation_result == ValidationResult.success()
+
+
+async def test_input_submitted_message_validation_failure():
+    app = InputApp()
+    async with app.run_test() as pilot:
+        input = app.query_one(Input)
+        input.value = "8"
+        await input.action_submit()
+        await pilot.pause()
+        assert len(app.messages) == 2
+        assert app.messages[1].validation_result == ValidationResult.failure(
+            failures=[
+                Number.NotInRange(
+                    value="8",
+                    validator=app.validator,
+                    description="Must be between 1 and 5.",
+                )
+            ],
+        )
+
+
+async def test_input_submitted_message_validation_success():
+    app = InputApp()
+    async with app.run_test() as pilot:
+        input = app.query_one(Input)
+        input.value = "3"
+        await input.action_submit()
+        await pilot.pause()
+        assert len(app.messages) == 2
+        assert app.messages[1].validation_result == ValidationResult.success()