io/transfer,app: [macOS] add support for file opening events

Correctly handle macOS file opening events so that an app containing a
binary using gio can be used to open certain files and even be set as
default app for certain file extensions.
This is done via implementing application:openFile in GioAppDelegate.

File events issued this way will be made available to a gio through the
io/transfer package. This package now also contains documentation for
how to register a macOS app as able to support certain file extensions.

app/os_macos.go

@@ -8,6 +8,10 @@ package app
 import (
 	"errors"
 	"image"
+	"io"
+	"mime"
+	"os"
+	"path"
 	"runtime"
 	"time"
 	"unicode"
@@ -18,6 +22,7 @@ import (
 	"gioui.org/io/key"
 	"gioui.org/io/pointer"
 	"gioui.org/io/system"
+	"gioui.org/io/transfer"
 	"gioui.org/unit"
 
 	_ "gioui.org/internal/cocoainit"
@@ -261,6 +266,10 @@ var viewMap = make(map[C.CFTypeRef]*window)
 // launched is closed when applicationDidFinishLaunching is called.
 var launched = make(chan struct{})
 
+// openFiles captures all the openFile events happening
+// applicationDidFinishLaunching is called.
+var openFiles []string
+
 // nextTopLeft is the offset to use for the next window's call to
 // cascadeTopLeftFromPoint.
 var nextTopLeft C.NSPoint
@@ -844,6 +853,20 @@ func gio_onFinishLaunching() {
 	close(launched)
 }
 
+//export gio_openFile
+func gio_openFile(cfile C.CFTypeRef) {
+	file := nsstringToString(cfile)
+	if len(viewMap) == 0 {
+		// Must do this because openFile is called before
+		// applicationDidFinishLaunching therefore no view is available here.
+		openFiles = append(openFiles, file)
+		return
+	}
+	for _, w := range viewMap {
+		sendOpenFileEvent(w, file)
+	}
+}
+
 func newWindow(win *callbacks, options []Option) error {
 	<-launched
 	errch := make(chan error)
@@ -869,10 +892,29 @@ func newWindow(win *callbacks, options []Option) error {
 		C.makeKeyAndOrderFront(window)
 		layer := C.layerForView(w.view)
 		w.w.Event(ViewEvent{View: uintptr(w.view), Layer: uintptr(layer)})
+		// Now send DataTransfer events for any file which has been used to open
+		// this application from the OS.
+		for _, file := range openFiles {
+			sendOpenFileEvent(w, file)
+		}
 	})
 	return <-errch
 }
 
+func sendOpenFileEvent(w *window, file string) {
+	mimeType := mime.TypeByExtension(path.Ext(file))
+	if mimeType == "" {
+		mimeType = "application/octet-stream"
+	}
+	w.w.Event(transfer.DataEvent{
+		Type: mimeType,
+		URI:  file,
+		Open: func() (io.ReadCloser, error) {
+			return os.Open(file)
+		},
+	})
+}
+
 func newOSWindow() (*window, error) {
 	view := C.gio_createView()
 	if view == 0 {

app/os_macos.m

@@ -390,6 +390,10 @@ - (void)applicationDidHide:(NSNotification *)aNotification {
 - (void)applicationWillUnhide:(NSNotification *)notification {
 	gio_onAppShow();
 }
+- (BOOL)application:(NSApplication *)sender openFile:(NSString *)filename {
+  	gio_openFile((__bridge CFTypeRef)filename);
+  return YES;
+}
 @end
 
 void gio_main() {

app/window.go

@@ -932,6 +932,10 @@ func (w *Window) processEvent(d driver, e event.Event) bool {
 		if handled {
 			w.setNextFrame(time.Time{})
 			w.updateAnimation(d)
+		} else {
+			// Currently this can only happen with open file OS directives on
+			// macOS.
+			w.out <- e2
 		}
 		return handled
 	}

io/router/pointer.go

@@ -872,9 +872,9 @@ func (q *pointerQueue) deliverTransferDataEvent(p *pointerInfo, events *handlerE
 	transferIdx := len(q.transfers)
 	events.Add(p.dataTarget, transfer.DataEvent{
 		Type: src.offeredMime,
-		Open: func() io.ReadCloser {
+		Open: func() (io.ReadCloser, error) {
 			q.transfers[transferIdx] = nil
-			return src.data
+			return src.data, nil
 		},
 	})
 	q.transfers = append(q.transfers, src.data)

io/router/pointer_test.go

@@ -982,8 +982,12 @@ func TestTransfer(t *testing.T) {
 		if got, want := dataEvent.Type, "file"; got != want {
 			t.Fatalf("got %s; want %s", got, want)
 		}
-		if got, want := dataEvent.Open(), ofr; got != want {
-			t.Fatalf("got %v; want %v", got, want)
+		gotReader, gotErr := dataEvent.Open()
+		if gotErr != nil {
+			t.Fatalf("got %s, expected nil", gotErr)
+		}
+		if gotReader != ofr {
+			t.Fatalf("got %v; want %v", gotReader, ofr)
 		}
 
 		// Drag and drop complete.

io/transfer/transfer.go

@@ -14,6 +14,9 @@
 // with the source. When a drag gesture completes, a CancelEvent is sent
 // to the source and all potential targets.
 //
+// DataEvent is also sent when the application is asked by the operating system
+// to open one or more files.
+//
 // Note that the RequestEvent is sent to the source upon drop.
 package transfer
 
@@ -100,10 +103,15 @@ func (CancelEvent) ImplementsEvent() {}
 // DataEvent is sent to the target receiving the transfer.
 type DataEvent struct {
 	// Type is the MIME type of Data.
+	// Type will be "application/octet-stream" for unknown data types.
 	Type string
-	// Open returns the transfer data. It is only valid to call Open in the frame
-	// the DataEvent is received. The caller must close the return value after use.
-	Open func() io.ReadCloser
+	// URI is the identifier of the resource being transferred. It can be set if
+	// the DataEvent concerns a file or an online resource.
+	URI string
+	// Open returns the transfer data. It is only valid to call Open in the
+	// frame the DataEvent is received.
+	// The caller must close the return value after use.
+	Open func() (io.ReadCloser, error)
 }
 
 func (DataEvent) ImplementsEvent() {}

widget/example_test.go

@@ -113,7 +113,11 @@ func ExampleDraggable_Layout() {
 		for _, ev := range gtx.Events(&drop) {
 			switch e := ev.(type) {
 			case transfer.DataEvent:
-				data := e.Open()
+				data, err := e.Open()
+				if err != nil {
+					fmt.Println("DataEvent Open error:", err)
+					break
+				}
 				fmt.Println(data.(offer).Data)
 			}
 		}