Implement DataChannel OnError/OnClosing for WASM

`OnClosing` is only implemented in WASM at this time.

`OnError` isn't tested, but should work as follows the existing
code.

Relates to #479 
Relates to #519

Author: WofWca

datachannel_js.go

@@ -7,6 +7,7 @@
 package webrtc
 
 import (
+	"errors"
 	"fmt"
 	"syscall/js"
 
@@ -26,8 +27,10 @@ type DataChannel struct {
 	// syscall/js API. Initially nil.
 	onOpenHandler       *js.Func
 	onCloseHandler      *js.Func
+	onClosingHandler    *js.Func
 	onMessageHandler    *js.Func
 	onBufferedAmountLow *js.Func
+	onErrorHandler      *js.Func
 
 	// A reference to the associated api object used by this datachannel
 	api *API
@@ -63,6 +66,39 @@ func (d *DataChannel) OnClose(f func()) {
 	d.underlying.Set("onclose", onCloseHandler)
 }
 
+// FYI `OnClosing` is not implemented in the non-JS version of Pion.
+
+func (d *DataChannel) OnClosing(f func()) {
+	if d.onClosingHandler != nil {
+		oldHandler := d.onClosingHandler
+		defer oldHandler.Release()
+	}
+	onClosingHandler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		go f()
+		return js.Undefined()
+	})
+	d.onClosingHandler = &onClosingHandler
+	d.underlying.Set("onclosing", onClosingHandler)
+}
+
+func (d *DataChannel) OnError(f func(err error)) {
+	if d.onErrorHandler != nil {
+		oldHandler := d.onErrorHandler
+		defer oldHandler.Release()
+	}
+	onErrorHandler := js.FuncOf(func(this js.Value, args []js.Value) interface{} {
+		event := args[0]
+		errorObj := event.Get("error")
+		// FYI RTCError has some extra properties, e.g. `errorDetail`:
+		// https://developer.mozilla.org/en-US/docs/Web/API/RTCDataChannel/error_event
+		errorMessage := errorObj.Get("message").String()
+		go f(errors.New(errorMessage))
+		return js.Undefined()
+	})
+	d.onErrorHandler = &onErrorHandler
+	d.underlying.Set("onerror", onErrorHandler)
+}
+
 // OnMessage sets an event handler which is invoked on a binary message arrival
 // from a remote peer. Note that browsers may place limitations on message size.
 func (d *DataChannel) OnMessage(f func(msg DataChannelMessage)) {
@@ -145,12 +181,18 @@ func (d *DataChannel) Close() (err error) {
 	if d.onCloseHandler != nil {
 		d.onCloseHandler.Release()
 	}
+	if d.onClosingHandler != nil {
+		d.onClosingHandler.Release()
+	}
 	if d.onMessageHandler != nil {
 		d.onMessageHandler.Release()
 	}
 	if d.onBufferedAmountLow != nil {
 		d.onBufferedAmountLow.Release()
 	}
+	if d.onErrorHandler != nil {
+		d.onErrorHandler.Release()
+	}
 
 	return nil
 }

examples/data-channels/jsfiddle/main.go

@@ -42,6 +42,12 @@ func main() {
 	sendChannel.OnClose(func() {
 		fmt.Println("sendChannel has closed")
 	})
+	sendChannel.OnClosing(func() {
+		fmt.Println("sendChannel is closing")
+	})
+	sendChannel.OnError(func(err error) {
+		fmt.Println("sendChannel error", err)
+	})
 	sendChannel.OnOpen(func() {
 		fmt.Println("sendChannel has opened")