MessageLogApp for testing

[davetapley]

I've been testing my widgets with in isolation using pilot and a 'mini'-app which just mounts the widget-under-test.

To assert what messages are emitted I've come up with MessageLogApp:

from typing import TypeVar, cast, get_origin

from textual import on
from textual.app import App
from textual.message import Message

M = TypeVar('M', bound=Message)


class MessageLog(list[Message]):
    def of_type(self, klass: type[M]) -> list[M]:
        # https://discuss.python.org/t/narrowing-with-isinstance-and-possibly-subscripted-generic/36923/2
        origin = get_origin(klass)
        cls_ = origin if origin else klass
        return [cast(klass, m) for m in self if isinstance(m, cls_)]


T = TypeVar('T')


class MessageLogApp(App[T]):
    messages: MessageLog

    def __init__(self):
        super().__init__()
        self.messages = MessageLog()

    @on(Message)
    def log_message(self, message: Message) -> None:
        self.messages.append(message)

Then I can use that to make an app with my widget-under-test, e.g. TrieTree:

class TrieTreeApp(MessageLogApp[None]):
    def __init__(self, trie: StringTrie[list[Id]]):
        super().__init__()
        self.trie = trie

    def compose(self) -> ComposeResult:
        yield TrieTree(self.trie)

Then I can:

app = TrieTreeApp(self.meta.trie)
async with app.run_test() as pilot:
    # click some stuff
    first, *rest = app.messages.of_type(Tree.NodeSelected[Id])
    self.assertEqual(first.node.data, 'alert1/799')
    self.assertEqual(rest, [])
    app.messages.clear()