add WASM blazor-like thing

go/wasmruntime/persist_wasm.go

@@ -0,0 +1,68 @@
+//go:build js && wasm
+
+package wasmruntime
+
+import (
+	"syscall/js"
+
+	"kjol/vdom"
+)
+
+// State preservation across an in-place hot swap (see the dev server's
+// livereload script). A fresh wasm instance has fresh Go memory, so signal
+// values would reset on every rebuild. To keep them, the outgoing instance
+// snapshots its signals (by creation order, via a vdom.Collector) into
+// sessionStorage on dispose, and the incoming instance restores them on boot.
+//
+// It's best-effort: if an edit adds/removes/reorders signals, the by-order match
+// shifts and unmatched signals fall back to their initial value — the same
+// hook-order caveat as server components.
+
+const stateKey = "__gowasm_hmr_state"
+
+var stateCollector *vdom.Collector
+
+// PreserveState registers the collector whose signals are snapshotted to
+// sessionStorage when this instance is disposed for a hot swap.
+func PreserveState(c *vdom.Collector) { stateCollector = c }
+
+// RestoreState returns the signal snapshot saved by a prior instance before a
+// hot swap, or nil if there is none. It is one-shot: the entry is cleared so a
+// manual page refresh starts from initial state.
+func RestoreState() [][]byte {
+	ss := js.Global().Get("sessionStorage")
+	if !ss.Truthy() {
+		return nil
+	}
+	raw := ss.Call("getItem", stateKey)
+	if !raw.Truthy() {
+		return nil
+	}
+	ss.Call("removeItem", stateKey)
+	arr := js.Global().Get("JSON").Call("parse", raw)
+	if arr.Type() != js.TypeObject {
+		return nil
+	}
+	out := make([][]byte, arr.Length())
+	for i := range out {
+		out[i] = []byte(arr.Index(i).String())
+	}
+	return out
+}
+
+// saveState writes the preserved collector's current signal values to
+// sessionStorage. Called from the dispose hook before the DOM is torn down.
+func saveState() {
+	if stateCollector == nil {
+		return
+	}
+	ss := js.Global().Get("sessionStorage")
+	if !ss.Truthy() {
+		return
+	}
+	arr := js.Global().Get("Array").New()
+	for _, b := range stateCollector.Snapshot() {
+		arr.Call("push", string(b))
+	}
+	ss.Call("setItem", stateKey, js.Global().Get("JSON").Call("stringify", arr))
+}