Custom MeasureFunc

[ickshonpe]

Objective

At the moment Bevy UI has a very basic, unmodifiable MeasureFunc.

Problems:

• Text wrapping can't be implemented correctly as the MeasureFunc needs access to the font and text data to choose the correct size for the node.

• Images are difficult to work with. With more complicated layouts users have to set fixed size constraints using min_size and max_size to get the correct dimensions, which isn't acceptable.

• Users can't implement widgets that need to control their own size.

• There are bugs that are hard to find and replicate without modifying the MeasureFunc
  definite measure func #8111, MeasureFunc Text aspect ratio bug #6748, and MeasureFunc given incorrect AvailableSpace DioxusLabs/taffy#406 are examples.

Solution

Create a new trait MeasureNode and add a Box<dyn MeasureNode> field to CalculatedSize.
Have upsert_leaf use the boxed MeasureNode to create the MeasureFunc for the node.

Changelog

• Added a new trait MeasureNode.
• Added new structs ImageMeasure and BasicMeasure that implement MeasureNode.
• Add a field to CalculatedSize called measure that takes a boxed MeasureNode.
• upsert_leaf uses the measure of CalculatedSize to create a MeasureFunc for the node.

[ickshonpe]

This PR is the wrong approach, the whole flex module has big underlying problems I didn't understand before.

[TimJentzsch]

@ickshonpe What are the underlying problems?
Did we capture them in issues already?

[ickshonpe]

@ickshonpe What are the underlying problems? Did we capture them in issues already?

The major problem with this PR is that it clones and double-boxes the MeasureFunc when I think the layout algorithm should be able to access it directly from the CalculatedSize component.

There are a lot of internal performance bugs that are hard to find because they only have an impact when the UI updates and not every frame. I just published #8402 which fixes another unnecessary update bug.

[nicoburns]

@ickshonpe Would it be helpful if MeasureFuncs were able to borrow from their environment? I've been considering adding such a feature to Taffy, although there are a couple of possible designs:

• MeasureFunc's could take an "environment" parameter. The user would would be pass the "environment" (which could be a reference) to compute_layout, which would just pass it in every time it called a MeasureFunc.
• OR Individual MeasureFunc's could be replaced by a single measure func which is passed in to compute_layout and get's called with the node's id as a paramter.