Add Blur message to Input widget

[edward-jazzhands]

I realized working with the Input widget that its options for validate_on are "submitted", "changed", and "blur". However, there's only messages for Submitted and Changed. This makes it trickier to synchronize a function with the blur validator than it is with the other two validators, which I believe in this situation does not make sense.

Now I had figured a Blur event most likely does not exist for a reason (i.e. perhaps if one widget has it then why wouldn't all of them). But in this situation I can't help but feel like its existence is warranted in order to make the validate_on=['blur'] parameter be able to be used in a manner that's identical to "submitted" and "changed" without needing to subclass the Input widget.

The problem:

yield Input(value="foo", validate_on=['changed'], ...)

@on(Input.Changed)
def my_input_changed(self, event: Input.Changed):
    # This works

yield Input(value="foo", validate_on=['submitted'], ...)

@on(Input.Submitted)
def my_input_changed(self, event: Input.Submitted):
    # This works

yield Input(value="foo", validate_on=['blur'], ...)

@on(Input.Blur)
def my_input_blur(self, event: Input.Blur):
    # This does not work

[davep]

Now I had figured a Blur event most likely does not exist for a reason

The reason is most likely that the DescendentBlur message is the thing to use. For example:

from textual import on
from textual.app import App, ComposeResult
from textual.containers import Horizontal, Vertical
from textual.events import DescendantBlur
from textual.widgets import Input, Log

class InputBlurTest(App[None]):

    def compose(self) -> ComposeResult:
        with Horizontal():
            with Vertical():
                for n in range(10):
                    yield Input(placeholder=str(n), id=f"input-{n}")
            yield Log()

    @on(DescendantBlur, "Input")
    def log_input_blur(self, message: DescendantBlur) -> None:
        self.query_one(Log).write_line(f"This input lost focus: {message.control!r}")

if __name__ == "__main__":
    InputBlurTest().run()

[TomJGooding]

@davep I think the problem being addressed here is that while...

You can check whether the validation succeeded or failed inside an Input.Changed or Input.Submitted handler by looking at the validation_result attribute on these events.

...you can't do the same when the Input is validated on blur.

[davep]

I was offering a possible explanation as to why that particular message was missing; not suggesting that this PR shouldn't have been PRd.

[willmcgugan]

Good idea, just a request.

[willmcgugan]

Let's call it Blurred, which is more consistent with existing messages, and avoids confusion with the existing Blur message.

[willmcgugan]

@davep I believe @edward-jazzhands was talking about a hypothetical Input.Blur message, and not the standard Blur event.

[edward-jazzhands]

@davep Ah I was not aware of the DescendantBlur and DescendantFocus messages. Didn't notice those in the reference. Yes indeed it would solve the problem. I see now that this would effectively create another message that's doing the same thing as DescendantBlur, but attached specifically to the Input widget, just to give it a message that can be used in a manner that's more similar to the other two messages, and a bit more intuitive to figure out.

Perhaps if this section of the reference (https://textual.textualize.io/widgets/input/#validating-input) was updated to mention the DescendantFocus message then this would be unnecessary. But nevertheless, I've submitted the changes that @willmcgugan requested.