messenger.go

package flutter

import (
	"fmt"
	"runtime"
	"sync"

	"github.com/go-flutter-desktop/go-flutter/embedder"
	"github.com/go-flutter-desktop/go-flutter/internal/tasker"
	"github.com/go-flutter-desktop/go-flutter/plugin"
)

type messenger struct {
	engine *embedder.FlutterEngine

	channels     map[string]plugin.ChannelHandlerFunc
	channelsLock sync.RWMutex

	// engineTasker holds tasks which must be executed in the engine thread
	engineTasker *tasker.Tasker
}

var _ plugin.BinaryMessenger = &messenger{}

func newMessenger(engine *embedder.FlutterEngine) *messenger {
	return &messenger{
		engine:       engine,
		channels:     make(map[string]plugin.ChannelHandlerFunc),
		engineTasker: tasker.New(),
	}
}

// SendWithReply pushes a binary message on a channel to the Flutter side and
// wait for a reply.
// NOTE: If no value are returned by the flutter handler, the function will
// wait forever. In case you don't want to wait for reply, use Send.
func (m *messenger) SendWithReply(channel string, binaryMessage []byte) (binaryReply []byte, err error) {
	reply := make(chan []byte)
	defer close(reply)
	callbackHandle := &embedder.DataCallback{
		Handle: func(binaryMessage []byte) {
			reply <- binaryMessage
		},
	}
	defer runtime.KeepAlive(callbackHandle)
	responseHandle, err := m.engine.CreatePlatformMessageResponseHandle(callbackHandle)
	if err != nil {
		return nil, err
	}
	defer m.engine.ReleasePlatformMessageResponseHandle(responseHandle)

	msg := &embedder.PlatformMessage{
		Channel:        channel,
		Message:        binaryMessage,
		ResponseHandle: responseHandle,
	}

	if m.engine.TaskRunnerRunOnCurrentThread() {
		err = m.engine.SendPlatformMessage(msg)
	} else {
		replyErr := make(chan error)
		defer close(replyErr)

		postEmptyEvent()
		go m.engineTasker.Do(func() {
			replyErr <- m.engine.SendPlatformMessage(msg)
		})
		err = <-replyErr
	}

	// wait for a reply and return
	return <-reply, nil
}

// Send pushes a binary message on a channel to the Flutter side without
// expecting replies.
func (m *messenger) Send(channel string, binaryMessage []byte) (err error) {
	msg := &embedder.PlatformMessage{
		Channel: channel,
		Message: binaryMessage,
	}

	if m.engine.TaskRunnerRunOnCurrentThread() {
		err = m.engine.SendPlatformMessage(msg)
	} else {
		replyErr := make(chan error)
		defer close(replyErr)

		postEmptyEvent()
		go m.engineTasker.Do(func() {
			replyErr <- m.engine.SendPlatformMessage(msg)
		})
		err = <-replyErr
	}
	if err != nil {
		return err
	}

	return nil
}

// SetChannelHandler satisfies plugin.BinaryMessenger
func (m *messenger) SetChannelHandler(channel string, channelHandler plugin.ChannelHandlerFunc) {
	m.channelsLock.Lock()
	if channelHandler == nil {
		delete(m.channels, channel)
	} else {
		m.channels[channel] = channelHandler
	}
	m.channelsLock.Unlock()
}

func (m *messenger) handlePlatformMessage(message *embedder.PlatformMessage) {
	m.channelsLock.RLock()
	channelHander := m.channels[message.Channel]
	m.channelsLock.RUnlock()

	if channelHander == nil {
		// print a log, but continue on to send a nil reply when required
		fmt.Println("go-flutter: no handler found for channel " + message.Channel)
		return
	}

	var err error
	err = channelHander(message.Message, responseSender{
		engine:       m.engine,
		message:      message,
		engineTasker: m.engineTasker,
	})
	if err != nil {
		fmt.Printf("go-flutter: handling message on channel "+message.Channel+" failed: %v\n", err)
	}
}

type responseSender struct {
	engine       *embedder.FlutterEngine
	message      *embedder.PlatformMessage
	engineTasker *tasker.Tasker
}

func (r responseSender) Send(binaryReply []byte) {
	if !r.message.ExpectsResponse() {
		return // quick path when no response should be sent
	}

	// TODO: detect multiple responses on the same message and spam the log
	// about it.

	postEmptyEvent()
	go r.engineTasker.Do(func() {
		err := r.engine.SendPlatformMessageResponse(r.message.ResponseHandle, binaryReply)
		if err != nil {
			fmt.Printf("go-flutter: failed sending response for message on channel '%s': %v", r.message.Channel, err)
		}
	})
}