[eml] Widget prop change handlers & expressions

The problem

For now you are required to define change event for each property by hand, like it is done for <btngroup>:

#[param(value: String => BtnGroup:value)]
#[signal(value_change: ValueChanged<String> => value_changed)]
fn buttongroup(ctx: &mut WidgetContext) { ... }

You can connect to the value_change event later:

let label = commands.spawn_empty().id();
commands.add(eml! {
  <body>
    <label {label}/>
    <btngroup on:value_change=run!(for label |ctx, label:Label| {
       label.value = format!(
         "value changed from {} to {}",
         ctx.old_value(),
         ctx.new_value()
    })/>
  </body>
})

There are several problems with such approach:

you need to define signal for each widget property separately
there is not universal Changed<T> event
you need to explicitly define entity and/or associated component for each callback
... using run!(for ...) macro syntax
you have to call ctx.old/new_value() for fetching the change, and its kinda ugly

The solution

When the entity associations (#68) and default widget component (#todo) will be done, it is easy to implement change handler expression syntax:

#[prop(value: String)
fn buttongroup() { eml! { ... } }

commands.add(eml! {
  <body>
    <entity {label as Label}/>
    <label:label/>
    <btngroup value:changed={|change, label| label.value=format!(
      "value changed from {} to {}", change.from(), change.to()
    )}/>
</body>

Change registrators could be done for each widget's #[prop(...)] automatically. ChangeHandlerExpression parser should detect built-in (without type-spec), custom WorldQuery and SystemParam arguments:

this: current widget entity associated with widget main component (it is used as target entity if no $ident provided).
change: Change<T> value with from() and new() methods
$ident - registered associated target entity (like label in example above)
$ident: &Component - imutable Component access for target entity (like tr: &Transform)
$ident: &mut Component - mutable Component access for target entity (like tr: &mut Transform)
$ident: SystemParam - mutable access to system param (like cmd: Commands)

There is no ctx argument support as far as we do not need context anymore when fetching SystemParam in handlers is done.

Cahnge detection could be easaly done inside the bind systems (e.g. generate Change<T> event when value cahnged.

jkb0o / belly

[eml] Widget prop change handlers & expressions #69

The problem

The solution