Registering a Go app as a protocol handler under Mac OS X

handler.h

#import <Foundation/Foundation.h>

extern void HandleURL(char*);

@interface GoPasser : NSObject
+ (void)handleGetURLEvent:(NSAppleEventDescriptor *)event;
@end

void StartURLHandler(void);

handler.m

#include "handler.h"

@implementation GoPasser
+ (void)handleGetURLEvent:(NSAppleEventDescriptor *)event
{
	HandleURL([[[event paramDescriptorForKeyword:keyDirectObject] stringValue] UTF8String]);
}
@end

void StartURLHandler(void) {
	NSAppleEventManager *appleEventManager = [NSAppleEventManager sharedAppleEventManager];
    [appleEventManager setEventHandler:[GoPasser class]
                           andSelector:@selector(handleGetURLEvent:)
                         forEventClass:kInternetEventClass andEventID:kAEGetURL];
}

Info.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>CFBundleExecutable</key>
    <string>myapp</string>
    <key>CFBundleIdentifier</key>
    <string>com.pocketgophers.myapp</string>
    <key>CFBundleURLTypes</key>
    <array>
        <dict>
            <key>CFBundleURLName</key>
            <string>com.pocketgophers.myapp</string>
            <key>CFBundleURLSchemes</key>
            <array>
                <string>myapp</string>
            </array>
        </dict>
    </array>
    <key>CFBundleInfoDictionaryVersion</key>
    <string>6.0</string>
    <key>CFBundleName</key>
    <string>MyApp</string>
    <key>CFBundlePackageType</key>
    <string>APPL</string>
    <key>CFBundleShortVersionString</key>
    <string>1.0.0</string>
    <key>CFBundleVersion</key>
    <string>20</string>
</dict>
</plist>

main.go

package main

/*
#cgo CFLAGS: -x objective-c
#cgo LDFLAGS: -framework Foundation
#include "handler.h"
*/
import "C"

import (
	"log"

	"github.com/andlabs/ui"
)

// Sources used to figure this out:
// Info.plist: https://gist.github.com/chrissnell/db95a3c5ad6ceca4c673e96cca0f7548
// custom url handler example: http://fredandrandall.com/blog/2011/07/30/how-to-launch-your-macios-app-with-a-custom-url/
// obj-c example: https://gist.github.com/leepro/016d7d6b61021dfc67daf61771c92b3c
// note: import .h, not .m

var labelText chan string

func main() {
	log.SetFlags(log.Lshortfile)

	labelText = make(chan string, 1) // the event handler blocks!, so buffer the channel at least once to get the first message
	C.StartURLHandler()

	err := ui.Main(func() {
		greeting := ui.NewLabel("greeting\nline 2")
		window := ui.NewWindow("Hello", 200, 100, true)
		window.SetChild(greeting)
		window.OnClosing(func(*ui.Window) bool {
			ui.Quit()
			return true
		})
		window.Show()

		go func() {
			for text := range labelText {
				ui.QueueMain(func() {
					greeting.SetText(text)
				})
			}
		}()
	})
	if err != nil {
		log.Fatal(err)
	}
}

//export HandleURL
func HandleURL(u *C.char) {
	labelText <- C.GoString(u)
}

Makefile

myapp.app: *.go *.h *.m Makefile Info.plist
	mkdir -p myapp.app/Contents/MacOS
	go build -i -o myapp.app/Contents/MacOS/myapp
	cp Info.plist myapp.app/Contents/Info.plist

.PHONY: open
open: myapp.app
	open myapp.app

.PHONY: scheme
scheme: myapp.app
	open myapp://hello/world

.PHONY: clean
clean:
	rm -rf myapp.app
	

@ManuelEberhardinger @monmohan (and more likely, anyone who stumbles across this later)

I used mainthread to workaround the issue by separating out the handler code into its own package, and calling its init function in a go routine.

import (
       "github.com/golang-design/mainthread"
       "example.com/module/ui"
       "example.com/module/handler"
)

func main() { mainthread.Init(fn) }

// fn is the actual main function
func fn() {

	// macos requires UI to be run on the main thread so schedule that here (in my case, a tray icon)
	mainthread.Go(ui.Init)

	// moved protocol handler out of main() and into its own package with an init function
	go handler.Init()

        // execute the rest of your main here
        restOfMain()
}

fails on mac for me.

on 11.7.4 ( Big Sur )

make
mkdir -p myapp.app/Contents/MacOS
go build -o myapp.app/Contents/MacOS/myapp .
# myapp
handler.m:7:12: warning: passing 'NS_RETURNS_INNER_POINTER const char *' to parameter of type 'char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
./handler.h:3:28: note: passing argument to parameter here
handler.m:15:40: error: use of undeclared identifier 'kInternetEventClass'; did you mean 'kCoreEventClass'?
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreServices.framework/Frameworks/AE.framework/Headers/AppleEvents.h:65:3: note: 'kCoreEventClass' declared here
handler.m:15:71: error: use of undeclared identifier 'kAEGetURL'; did you mean 'kAEISGetURL'?
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/System/Library/Frameworks/CoreServices.framework/Frameworks/AE.framework/Headers/AERegistry.h:829:3: note: 'kAEISGetURL' declared here