Properties 2.0 - Required unless annotated with default value

[jstarry]

Properties 2.0

Problem

Properties without #[props(required)] or #[props(default = "fn_name")] are implicitly set to their default value.

Proposal

Properties should be treated as required unless indicated otherwise

New syntax:

#[derive(Clone, Properties)]
struct Props {
  #[prop_or(3)],
  countdown: usize,

  #[prop_or_else(Callback::noop)]
  on_click: Callback<()>,

  #[prop_or(true)]
  display: bool,

  #[prop_or_default]
  highlight: bool,

  // implicitly required
  required: MyRequiredValue,

  #[prop_or_default]
  opt_value: Option<Value>,

  // implicitly required
  opt_required: Option<Value>,
}

Old syntax:

#[derive(Clone, Properties)]
struct Props {
  #[props(default = "countdown_default")],
  countdown: usize,

  #[props(default = "Callback::noop")]
  on_click: Callback<()>,

  #[props(default = "display_default")]
  display: bool,

  // implicitly optional and will default to `false`
  highlight: bool,

  #[props(required)]
  required: MyRequiredValue,

  // implicitly optional and will default to `None`
  opt_value: Option<Value>,

  #[props(required)]
  opt_required: Option<Value>,
}

fn countdown_default() -> usize {
  3
}

fn display_default() -> bool {
  true
}

Explanation

Now that we have a default attribute for properties, I think it makes sense to require it for indicating when properties are optional. So this means we would drop support for #[props(required)]

You may ask, why not follow the precedent that serde set for default values? Link

Serde was designed before arbitrary token streams were allowed for helper attributes in Rust 1.34.0: https://blog.rust-lang.org/2019/04/11/Rust-1.34.0.html#custom-attributes-accept-arbitrary-token-streams (kudos to @jplatte for pointing this out)

I propose changing the derive props macro attributes from props to prop_or_else, prop_or_default. Reference: https://doc.rust-lang.org/reference/procedural-macros.html#derive-macro-helper-attributes I feel that or, or_else and or_default naming is more Rust-y and the or_else helps inform developers that the default expression for a prop will not be run at runtime unless a prop is not present.

[jplatte]

It's supposed to be default = …, not default: …, right?

[jstarry]

@jplatte I believe either is supported by Rust now. I thought default: ... looked nicer but default = ... is fine too

[jplatte]

Yeah, either should be possible. I've just never seen the colon syntax used in attributes. AFAIK all builtin attributes use = (e.g. #[path = "foo.rs"], #[cfg(feature = "feat")] ).

[jstarry]

@jplatte @mdtusz @AlephAlpha @hgzimmerman I just updated this issue to be a new proposal for how we handle props and would like input. This builds upon the work that @AlephAlpha already did in #881

[jplatte]

I like the new proposal :)

We could maaaybe have #[prop_or_else(Callback::noop)] (fn) and #[prop_or(true)] (value) to mirror Options / Results API, but I don't know if that really makes things simpler.

[jstarry]

Good call, that would probably be more in line with expected behavior. It'd be closer to Option's API

[mdtusz]

No comments from me. Looks like a great improvement for usability and grok-ability!

[AlephAlpha]

I'm trying to implement this. Should we support the old syntax for backward compatibility?

[jstarry]

No need, would complicate things too much