Per character filter for TextEdits

[ickshonpe]

Objective

Add an option to reject TextEdits based on a per character filter.

Solution

• New component: EditableTextFilter. Can be used to set a per character filter for an EditableText entity.
• TextEdit::insert and TextEdit::Paste edits are ignored unless all their characters pass the filter.

The filter does not apply to characters already within the EditableText's text buffer

Initially I thought it would be better to implement the filter at the widget level, then a key press that results in a rejected edit could be propagated, but the clipboard isn't available to the keyboard observer function so paste edits would still need to be filtered when the input buffer is applied.

I made a branch adding a TextEditRejected entity event as an alternative (that would also notify when an edit failed due to the max_characters limit being exceeded), but left it out of this PR to keep this one focused on just the filtering.

Testing

New example:

cargo run --example editable_text_filter

[viridia]

I'd like to ask, is the plan to have both a "per-character filter" and a "buffer validation" filter? Because the latter could subsume the functionality of the former - if I have a mechanism powerful enough to do phone numbers or email addresses, then restricting to hex characters is just a simple case.

[ickshonpe]

I'd like to ask, is the plan to have both a "per-character filter" and a "buffer validation" filter? Because the latter could subsume the functionality of the former - if I have a mechanism powerful enough to do phone numbers or email addresses, then restricting to hex characters is just a simple case.

Yep I'd favour using buffer validation for both, but buffer validation is a bit more complicated as it can't be implemented just by examination of each TextEdit before application. You need to apply the TextEdit and then validate, which requires some way of rolling back the changes if they fail validation. It'll be easy once we have undo implemented though, and this seems okay until then.

[Zeophlite]

I'd prefer this argument to be an Option (not a strong feeling)

[ickshonpe]

I thought about it, but it just seemed simpler this way, instead of having to unwrap.

[ickshonpe]

Another advantage of buffer validation is that we wouldn't have to do these individual checks before TextEdits are applied, we can just have one validation function we call after application, which could handle max_characters as well.

[viridia]

Another advantage of buffer validation is that we wouldn't have to do these individual checks before TextEdits are applied, we can just have one validation function we call after application, which could handle max_characters as well.

Instead of using undo, I'd have a second buffer for the proposed change, and then std::mem::swap them when done.

However, I realized that perhaps character filtering is not redundant with buffer validation. Take the case we've been discussing with the color picker, where you paste a string like "#ffffff" and you want to strip the "#", but allow the hex digits to be pasted. Buffer validation would reject the whole thing. (Again, this is trivial with controlled widgets.)