package via

import (

"encoding/json"

"github.com/go-via/via/h"

)

// LocalSignal is a client-only reactive value — a Datastar `_`-prefixed signal

// that lives entirely in the browser, is never sent to the server, and needs

// no composition struct field or Mount binding. Use it for ephemeral UI state

// (menu open/closed, hover, active tab) that should react instantly without a

// server round-trip:

//

// open := via.Local("open")

// h.Div(open.Init(false),

// h.Button(open.Toggle(), h.Text("menu")),

// h.Nav(open.Show(), ...),

// )

//

// For state the server must see or persist, use a [Signal] or State* handle

// instead — a LocalSignal is intentionally invisible to actions.

type LocalSignal struct {

name string // wire name without the leading "_"

dollar string // "$_" + name, the expression reference

}

// Local returns a client-only signal handle named "_"+name. name must be a

// valid JS identifier fragment (letters, digits, underscores).

func Local(name string) LocalSignal {

return LocalSignal{name: name, dollar: "$_" + name}

}

// Init declares the signal with an initial value via Datastar's object form

// (data-signals="{_name:<json>}"). Place it once on a container element that

// wraps the uses of this signal.

func (l LocalSignal) Init(v any) h.H {

// json.Marshal only fails for un-encodable kinds (chan/func) — a caller

// error, surfaced loud at first render rather than emitting a broken signal.

b, err := json.Marshal(v)

if err != nil {

panic("via.Local(" + l.name + ").Init: value cannot be JSON-encoded: " + err.Error())

}

return h.Data("signals", "{_"+l.name+":"+string(b)+"}")

}

// Ref returns the "$_name" expression for use in raw Datastar expressions.

func (l LocalSignal) Ref() string { return l.dollar }

// Bind returns a two-way binding attribute for form inputs (data-bind="_name").

func (l LocalSignal) Bind() h.H { return h.Data("bind", "_"+l.name) }

// Text binds the signal's value as the host element's text content.

func (l LocalSignal) Text() h.H { return h.Data("text", l.dollar) }

// Show toggles the host element's display by the signal's truthiness.

func (l LocalSignal) Show() h.H { return h.Data("show", l.dollar) }

// ShowUnless is the negation of [LocalSignal.Show].

//

// EXPERIMENTAL: a young convenience helper; may change before 1.0.

func (l LocalSignal) ShowUnless() h.H { return h.Data("show", "!"+l.dollar) }

// Class toggles the named CSS class by the signal's truthiness. See

// [Signal.Class] for the lower-case attribute-name caveat.

func (l LocalSignal) Class(name string) h.H { return h.Data("class:"+name, l.dollar) }

// Toggle returns an on:click attribute that flips a boolean local signal —

// the canonical client-only toggle with no server round-trip.

func (l LocalSignal) Toggle() h.H { return h.Data("on:click", l.dollar+"=!"+l.dollar) }