What is bind and transform in the context of Variable and Service?

[arzcbnh]

Widget.bind obviously just sets an event listener, so it's very straightforward to understand. They clearly are not the same methods as defined in Widget, since Variable.bind requires no arguments and Service.bind only needs the property name. However, both seem to return something that... resembles a pointer? I know it's not one, though. I don't know how a property expecting a string is able to take this "pointer-like object".

I read the entire documentation, so it might have gone over my head, but the pages for Variable and Service don't really explain what exactly these methods do, only how to place them in code with examples - which is fine and comprehensible, I'm just being curious. I was able to find their definitions on the source code, and it looks like bind is defined with some kind of chain of generics. I've barely learned JavaScript, much less TypeScript, so it's impossible for me to make out what it's doing.

[Aylur]

both of them return an instance of Binding which is basically a struct that holds

• the emitter object that needs to be watched
• its property
• a transformFn which is not meant to be called, only set, is a function that the value will go through and be set on a widget property
• a callable transform method which returns a new instance of Binding with an updated transfromFn

it is possible to chain transforms since they will be composed, I think they are called composite functions in math

this Binding struct is only used in Widget constructors. the above mentioned three properties emitter, prop, transformFn will be "consumed" and be translated into Widget.bind calls which are just again translated into Widget.hook calls

all of this mess to have typesafety through using generics, so we can do

Widget({
  prop: object.bind('prop').transform(prop => prop)
})

instead of the basic imperative gobject way

function transform(prop) {
  return `transformed: ${prop}`
}

const widget = Widget({
  prop: transform(object.prop)
})

const id = object.connect('notify::prop', () => {
  widget.prop = transform(object.prop)
})

widget.connect('destroy', () => {
  object.disconnect(id)
})