ComputedTextStyle

[ickshonpe]

Objective

Propagate text styles down the tree, instead of setting them per node.

Work towards #21175

Solution

• New component ComputedTextStyle, required on Text, Text2d, and TextSpan.
• Unrequire TextFont and TextColor from Text, Text2d, and TextSpan.
• New system update_text_styles. It updates each ComputedTextStyle, from either the current entity or the nearest ancestor with a TextFont or TextColor.
• New resource DefaultTextStyle, used if update_text_styles can't find any ancestor with a TextColor or TextFont.

---

update_text_styles isn't optimised at all yet. Every frame (there's no change detection), from every text entity, it walks up the tree until it finds a TextColor and TextFont to use. It's okay as a reference implementation though, more concerned with synchronisation errors at this stage.

Most existing code should work without changes. Because they are no longer required, TextFont and TextColor aren't automatically inserted on text entities when ellided, so instead of the default font, they will use the font and color they inherit from their ancestors instead. And queries for TextFont and TextColor that assume the components presence may also fail now.

Testing

The text2d, feathers and testbed_ui examples all seem to be working correctly with only minor changes.

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[viridia]

This might also be a good place to stash the global default font size, although that might be a different PR. Maybe a TODO?

[ickshonpe]

I was thinking the global default font size would just be taken from the font_size from the DefaultTextStyle's TextFont field. I left this part deliberately underdeveloped though because I wasn't sure what we wanted and it's probably better to deal with in another PR.

[viridia]

I propose adding a release note that covers all of the various font-related work we'd like to do. I'm happy to help with writing the text if needed.

[viridia]

Do we even need ThemedText any more?

[ickshonpe]

Doesn't look like it, I thought I'd leave it for now though, and focus this PR on the text module changes.

[ickshonpe]

I propose adding a release note that covers all of the various font-related work we'd like to do. I'm happy to help with writing the text if needed.

Yeah, that would be a big help, there are so many changes, I'm not looking forward to it. I've got another couple of PRs that should be published tomorrow, that cover the relative font sizes and text span rework.

[viridia]

I was thinking we should file a cover bug which links to all the various font-related changes we want to make in 0.18.

@alice-i-cecile

[ickshonpe]

I don't feel that happy with the name of the DefaultTextStyle resource. Maybe something like GlobalTextStyle be better? It needs to be clear that this isn't a Default text style that gets applied if you don't explicitly style a text node. Instead it's the initial text style values that are propagated down each hierarchy supporting text that is overridden by the TextFont and TextColor components.

Maybe IntialTextStyle or PropagatedTextStyle would be okay, I'm not sure. Or GlobalPropagatedTextStyle but it's getting a bit long then.

Also it should be possible to set a default text style per text implementation as well. Perhaps by adding a type parameter, have DefaultTextStyle<()> be global, which is overridden for Text or Text2d if the DefaultTextStyle<Text> or DefaultTextStyle<Text2d> resources are present, respectively. I'll leave this for a follow up though.

[viridia]

I don't have a problem with the name DefaultTextStyle, but I don't think I really grasp the distinction you are making.

It needs to be clear that this isn't a Default text style that gets applied if you don't explicitly style a text node.

My understanding is that it's an inherited default: it applies to any text entity that doesn't have an explicit style, nor has any ancestor that has an explicit style set.

Part of the problem is that we don't have a word to describe the domain that encompasses all user interface hierarchies. If all Window entities were children of a common parent entity, then we wouldn't need resources, we could just stick these components on the common parent.

Having a separate default for Text2D entities does make sense.

I also think that introducing ComputedTextStyle weakens the rationale for having TextFont contain both font-face and font size. That is: I think that the intuitive API would be one in which font face and font size are specified separately; however in a world where we use components to also track the lifetime of font atlases, it makes sense to combine them in a single component. However, if ComputedTextStyle is taking over the job of tracking those lifetimes, then that reason kind of disappears.