app/internal/window: [macOS,iOS] reduce display link starting and stopping

Recent changes to the macOS threading exposed a problem where a
window's display link may fail to start after being started and stopped
in rapid succession.

Introduce a displayLink type that waits a while after the last stop
request before stopping its display link. That seems to be the way
other projects are using display links.

As a bonus, the new implementation avoids the potentially expensive
overhead of frequent starting and stopping the underlying OS thread.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/window/os_darwin.go

@@ -8,13 +8,44 @@ package window
 __attribute__ ((visibility ("hidden"))) void gio_wakeupMainThread(void);
 __attribute__ ((visibility ("hidden"))) NSUInteger gio_nsstringLength(CFTypeRef str);
 __attribute__ ((visibility ("hidden"))) void gio_nsstringGetCharacters(CFTypeRef str, unichar *chars, NSUInteger loc, NSUInteger length);
+__attribute__ ((visibility ("hidden"))) CFTypeRef gio_createDisplayLink(void);
+__attribute__ ((visibility ("hidden"))) void gio_releaseDisplayLink(CFTypeRef dl);
+__attribute__ ((visibility ("hidden"))) int gio_startDisplayLink(CFTypeRef dl);
+__attribute__ ((visibility ("hidden"))) int gio_stopDisplayLink(CFTypeRef dl);
+__attribute__ ((visibility ("hidden"))) void gio_setDisplayLinkDisplay(CFTypeRef dl, uint64_t did);
 */
 import "C"
 import (
+	"errors"
+	"sync"
+	"sync/atomic"
+	"time"
 	"unicode/utf16"
 	"unsafe"
 )
 
+// displayLink is the state for a display link (CVDisplayLinkRef on macOS,
+// CADisplayLink on iOS). It runs a state-machine goroutine that keeps the
+// display link running for a while after being stopped to avoid the thread
+// start/stop overhead and because the CVDisplayLink sometimes fails to
+// start, stop and start again within a short duration.
+type displayLink struct {
+	callback func()
+	// states is for starting or stopping the display link.
+	states chan bool
+	// done is closed when the display link is destroyed.
+	done chan struct{}
+	// dids receives the display id when the callback owner is moved
+	// to a different screen.
+	dids chan uint64
+	// running tracks the desired state of the link. running is accessed
+	// with atomic.
+	running uint32
+}
+
+// displayLinks maps CFTypeRefs to *displayLinks.
+var displayLinks sync.Map
+
 var mainFuncs = make(chan func(), 1)
 
 // runOnMain runs the function on the main thread.
@@ -47,3 +78,93 @@ func nsstringToString(str C.CFTypeRef) string {
 	utf8 := utf16.Decode(chars)
 	return string(utf8)
 }
+
+func NewDisplayLink(callback func()) (*displayLink, error) {
+	d := &displayLink{
+		callback: callback,
+		done:     make(chan struct{}),
+		states:   make(chan bool),
+		dids:     make(chan uint64),
+	}
+	dl := C.gio_createDisplayLink()
+	if dl == 0 {
+		return nil, errors.New("app: failed to create display link")
+	}
+	go d.run(dl)
+	return d, nil
+}
+
+func (d *displayLink) run(dl C.CFTypeRef) {
+	defer C.gio_releaseDisplayLink(dl)
+	displayLinks.Store(dl, d)
+	defer displayLinks.Delete(dl)
+	var stopTimer *time.Timer
+	var tchan <-chan time.Time
+	started := false
+	for {
+		select {
+		case <-tchan:
+			tchan = nil
+			started = false
+			C.gio_stopDisplayLink(dl)
+		case start := <-d.states:
+			switch {
+			case !start && tchan == nil:
+				// stopTimeout is the delay before stopping the display link to
+				// avoid the overhead of frequently starting and stopping the
+				// link thread.
+				const stopTimeout = 500 * time.Millisecond
+				if stopTimer == nil {
+					stopTimer = time.NewTimer(stopTimeout)
+				} else {
+					// stopTimer is always drained when tchan == nil.
+					stopTimer.Reset(stopTimeout)
+				}
+				tchan = stopTimer.C
+				atomic.StoreUint32(&d.running, 0)
+			case start:
+				if tchan != nil && !stopTimer.Stop() {
+					<-tchan
+				}
+				tchan = nil
+				atomic.StoreUint32(&d.running, 1)
+				if !started {
+					started = true
+					if res := C.gio_startDisplayLink(dl); res != 0 {
+						panic("failed to start display link")
+					}
+				}
+			}
+		case did := <-d.dids:
+			C.gio_setDisplayLinkDisplay(dl, C.uint64_t(did))
+		case <-d.done:
+			return
+		}
+	}
+}
+
+func (d *displayLink) Start() {
+	d.states <- true
+}
+
+func (d *displayLink) Stop() {
+	d.states <- false
+}
+
+func (d *displayLink) Close() {
+	close(d.done)
+}
+
+func (d *displayLink) SetDisplayID(did uint64) {
+	d.dids <- did
+}
+
+//export gio_onFrameCallback
+func gio_onFrameCallback(dl C.CFTypeRef) {
+	if d, exists := displayLinks.Load(dl); exists {
+		d := d.(*displayLink)
+		if atomic.LoadUint32(&d.running) != 0 {
+			d.callback()
+		}
+	}
+}