add WASM blazor-like thing

go/wasmruntime/fetch.go

@@ -0,0 +1,77 @@
+//go:build js && wasm
+
+package wasmruntime
+
+import (
+	"errors"
+	"fmt"
+	"sync"
+	"syscall/js"
+)
+
+// Fetch performs an HTTP GET via the browser Fetch API and returns the response
+// body as a string. It bridges a JS Promise into Go: it registers then/catch
+// callbacks and blocks the calling goroutine on a channel until the request
+// settles.
+//
+// Because it blocks, call it from a goroutine — never from a reactive callback
+// or event handler — and write the result into signals when it returns, e.g.:
+//
+//	go func() {
+//	    body, err := Fetch(url)
+//	    ...
+//	    data.Set(...) // updates the UI reactively
+//	}()
+func Fetch(url string) (string, error) {
+	var (
+		once                  sync.Once
+		done                  = make(chan struct{})
+		body                  string
+		ferr                  error
+		onResp, onText, onErr js.Func
+	)
+	finish := func() { once.Do(func() { close(done) }) }
+
+	onErr = js.FuncOf(func(this js.Value, args []js.Value) any {
+		msg := "fetch error"
+		if len(args) > 0 && args[0].Truthy() {
+			e := args[0]
+			if m := e.Get("message"); m.Truthy() {
+				msg = m.String()
+			} else {
+				msg = e.Call("toString").String()
+			}
+		}
+		ferr = errors.New(msg)
+		finish()
+		return nil
+	})
+
+	onText = js.FuncOf(func(this js.Value, args []js.Value) any {
+		if len(args) > 0 {
+			body = args[0].String()
+		}
+		finish()
+		return nil
+	})
+
+	onResp = js.FuncOf(func(this js.Value, args []js.Value) any {
+		resp := args[0]
+		if !resp.Get("ok").Bool() {
+			ferr = fmt.Errorf("HTTP %d", resp.Get("status").Int())
+			finish()
+			return nil
+		}
+		// resp.text() -> Promise<string>
+		resp.Call("text").Call("then", onText).Call("catch", onErr)
+		return nil
+	})
+
+	js.Global().Call("fetch", url).Call("then", onResp).Call("catch", onErr)
+
+	<-done
+	onResp.Release()
+	onText.Release()
+	onErr.Release()
+	return body, ferr
+}