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/snapshot_tests/test_snapshots.py

@@ -84,6 +84,19 @@ def test_input_and_focus(snap_compare):
     assert snap_compare(WIDGET_EXAMPLES_DIR / "input.py", press=press)
 
 
+def test_input_validation(snap_compare):
+    """Checking that invalid styling is applied. The snapshot app itself
+    also adds styling for -valid which gives a green border."""
+    press = [
+        *"-2",  # -2 is invalid, so -invalid should be applied
+        "tab",
+        "3",  # This is valid, so -valid should be applied
+        "tab",
+        *"-2",  # -2 is invalid, so -invalid should be applied (and :focus, since we stop here)
+    ]
+    assert snap_compare(SNAPSHOT_APPS_DIR / "input_validation.py", press=press)
+
+
 def test_buttons_render(snap_compare):
     # Testing button rendering. We press tab to focus the first button too.
     assert snap_compare(WIDGET_EXAMPLES_DIR / "button.py", press=["tab"])