Enable async bind for Go

examples/bind.go

@@ -1,18 +1,32 @@
 package main
 
-import "github.com/webview/webview"
+import (
+	"time"
+
+	"github.com/webview/webview"
+)
 
 const html = `<button id="increment">Tap me</button>
 <div>You tapped <span id="count">0</span> time(s).</div>
+<button id="compute">Compute</button>
+<div>Result of computation: <span id="compute-result">0</span></div>
 <script>
-  const [incrementElement, countElement] =
-    document.querySelectorAll("#increment, #count");
+  const [incrementElement, countElement, computeElement, computeResultElement] =
+    document.querySelectorAll("#increment, #count, #compute, #compute-result");
   document.addEventListener("DOMContentLoaded", () => {
     incrementElement.addEventListener("click", () => {
       window.increment().then(result => {
         countElement.textContent = result.count;
       });
     });
+    computeElement.addEventListener("click", () => {
+      computeElement.disabled = true;
+      window.compute(6, 7).then(result => {
+		console.log(result);
+        computeResultElement.textContent = result;
+        computeElement.disabled = false;
+      });
+    });
   });
 </script>`
 
@@ -33,6 +47,17 @@ func main() {
 		return IncrementResult{Count: count}
 	})
 
+	// A binding that performs a long-running computation and returns the result
+	w.Bind("compute", func(a, b int) chan webview.BindCallbackResult {
+		ch := make(chan webview.BindCallbackResult)
+		go func() {
+			defer close(ch)
+			time.Sleep(1 * time.Second)
+			ch <- webview.BindCallbackResult{Value: a * b, Error: nil}
+		}()
+		return ch
+	})
+
 	w.SetHtml(html)
 	w.Run()
 }

webview.go

@@ -50,6 +50,16 @@ const (
 	HintMax = C.WEBVIEW_HINT_MAX
 )
 
+type BindCallbackResult struct {
+	// Error is an error returned by the callback. If error is not nil - the
+	// result of the callback is ignored.
+	Error error
+
+	// Value is a value returned by the callback. If error is not nil - the
+	// result of the callback is ignored.
+	Value interface{}
+}
+
 type WebView interface {
 
 	// Run runs the main loop until it's terminated. After this function exits -
@@ -108,7 +118,8 @@ type WebView interface {
 	// JavaScript function.
 	//
 	// f must be a function
-	// f must return either value and error or just error
+	// f can return either value and error or just error
+	// f can return a channel of BindCallbackResult as the value
 	Bind(name string, f interface{}) error
 }
 
@@ -222,21 +233,33 @@ func _webviewBindingGoCallback(w C.webview_t, id *C.char, req *C.char, index uin
 	m.Lock()
 	f := bindings[uintptr(index)]
 	m.Unlock()
-	jsString := func(v interface{}) string { b, _ := json.Marshal(v); return string(b) }
-	status, result := 0, ""
-	if res, err := f(C.GoString(id), C.GoString(req)); err != nil {
-		status = -1
-		result = jsString(err.Error())
-	} else if b, err := json.Marshal(res); err != nil {
-		status = -1
-		result = jsString(err.Error())
-	} else {
-		status = 0
-		result = string(b)
-	}
-	s := C.CString(result)
-	defer C.free(unsafe.Pointer(s))
-	C.webview_return(w, id, C.int(status), s)
+
+	// retain a reference to the id and params strings
+	reqId := C.GoString(id)
+	reqParams := C.GoString(req)
+
+	// run the callback in a separate goroutine
+	go func() {
+		jsString := func(v interface{}) string { b, _ := json.Marshal(v); return string(b) }
+
+		status, result := 0, ""
+		if res, err := f(reqId, reqParams); err != nil {
+			status = -1
+			result = jsString(err.Error())
+		} else if b, err := json.Marshal(res); err != nil {
+			status = -1
+			result = jsString(err.Error())
+		} else {
+			status = 0
+			result = string(b)
+		}
+
+		cId := C.CString(reqId)
+		defer C.free(unsafe.Pointer(cId))
+		s := C.CString(result)
+		defer C.free(unsafe.Pointer(s))
+		C.webview_return(w, cId, C.int(status), s)
+	}()
 }
 
 func (w *webview) Bind(name string, f interface{}) error {
@@ -287,6 +310,25 @@ func (w *webview) Bind(name string, f interface{}) error {
 					return nil, res[0].Interface().(error)
 				}
 				return nil, nil
+			} else if res[0].Type().Kind() == reflect.Chan {
+				if res[0].Type().Elem() != reflect.TypeOf(BindCallbackResult{}) {
+					return nil, errors.New("channel must be of type CallbackResult")
+				}
+
+				// Wait for the channel to receive a value
+				val, ok := res[0].Recv()
+
+				if !ok {
+					return nil, errors.New("channel closed")
+				}
+
+				callbackResult := val.Interface().(BindCallbackResult)
+
+				if callbackResult.Error != nil {
+					return nil, callbackResult.Error
+				}
+
+				return callbackResult.Value, nil
 			}
 			return res[0].Interface(), nil
 		case 2:

webview_test.go

@@ -11,12 +11,20 @@ func Example() {
 	w := New(true)
 	defer w.Destroy()
 	w.SetTitle("Hello")
-	w.Bind("noop", func() string {
+	w.Bind("noop", func(id string) {
 		log.Println("hello")
-		return "hello"
+		w.Return(id, StatusSuccess, "hello")
 	})
-	w.Bind("add", func(a, b int) int {
-		return a + b
+	w.Bind("add", func(id string, a, b int) {
+		w.Return(id, StatusSuccess, a+b)
+	})
+	w.Bind("subtract", func(a, b int) chan BindCallbackResult {
+		ch := make(chan BindCallbackResult)
+		go func() {
+			defer close(ch)
+			ch <- BindCallbackResult{Value: a - b, Error: nil}
+		}()
+		return ch
 	})
 	w.Bind("quit", func() {
 		w.Terminate()
@@ -31,7 +39,10 @@ func Example() {
 						console.log('noop res', res);
 						add(1, 2).then(function(res) {
 							console.log('add res', res);
-							quit();
+							subtract(5, 3).then(function(res) {
+								console.log('subtract res', res);
+								quit();
+							});
 						});
 					});
 				};