This adds a new Format String node which simplifies constructing strings from
multiple values. The node takes a format string and a dynamic number of
additional parameters as input. The format string determines how the other
inputs are inserted into the string. Only integer, float and string inputs are
supported for now.
It supports two different format syntaxes:
* Python compatible format syntax which also mostly matches the behavior of the
`fmt` C++ library. Most of this is supported, but there are some small
limitations.
* Syntax of the form `###.##` where each `#` stands for a digit. This is the
syntax that was introduced in #134860.
This node greatly simplifies common string operations which would have required
potentially many nodes before to convert numbers to strings and to concatenate
them. It also makes new conversions possible that were not supported before.
This node can also be used to insert e.g. frame numbers into a file path which
was surprisingly complex before.
This node has special behavior for the name of new inputs. For the purpose of
the node, the name of the inputs must be valid identifiers and it's usually
helpful when they are short. New names are therefore initialized to be single
characters. If possible, the first character of the linked input is used. This
works well when connecting e.g. a Separate Vector/Color node. Otherwise, inputs
are named `a` to `z` by default. If that's not possible, the source socket name
is used instead (converted to be a valid identifier). If that still doesn't
work, the name is made unique using the normal `.001` mechanism except that `_`
instead of `.` is used as separator to make the name a valid identifier.
Python Syntax references:
* Python: https://docs.python.org/3/library/string.html#formatspec
* `fmt`: https://fmt.dev/latest/syntax/
More detailed notes about compatibility with the above syntax specifications:
* Conversion using e.g. `!r` like in Python is not supported (maybe the future).
* Sub-attribute access like `{vector.x}` is not supported (maybe the future).
* Using `%` like in Python is not supported (maybe in future).
* Using `#` for an alternate form is not supported. This might help in the
future to make the syntax compatible with #134860.
* Using `L` like in the `fmt` library is not supported because it depends on the
locale which is not good for determinism.
* Grouping with e.g. thousands separators using e.g. `,` or `_` like in Python
is not supported (maybe in future). Need to think about the locale here too.
* Mixing of unnamed (`{}`) and named (`{x} or {0}`) specifiers is allowed.
However, all unnamed specifiers must come before any named specifier.
The implementation uses the `fmt` library for the actual formatting. However,
the inputs are preprocessed to give us more control over the exact supported
syntax and error messages. The code is already somewhat written so that many
strings could be formatted with the same format but that's not actually used yet
because we don't have string fields yet.
Error messages are propagated using a new mechanism that allows a limited form
of error propagation from multi-functions to the node that evaluates them.
Currently, this only works in fairly limited circumstances, e.g. it does not
work during field evaluation. Since this node is never part of field evaluation
yet, that limitation seems ok, but it's something to work on at some point.
Properly supporting that requires some more changes to propagate enough context
information everywhere. Also showing errors of field evaluation on the field
node itself (instead of on the evaluation node) requires even more work because
our current logging system is not setup to support that yet.
This node comes with a few new requirements for the socket items system: names
must be valid identifiers and they are initialized in a non-trivial way.
Overall, this was fairly straight forward to implement but currently it requires
to adding a bunch of new members to all the accessors that don't really need it.
This is something that we should simplify at some point even if I'm not entirely
sure how yet. The same new requirements used in this node would probably also
exist in a potential future expression node.
Pull Request: https://projects.blender.org/blender/blender/pulls/138860
182797ea61