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/gl_macos.m

@@ -19,39 +19,14 @@ static void handleMouse(NSView *view, NSEvent *event, int typ, CGFloat dx, CGFlo
 	gio_onMouse((__bridge CFTypeRef)view, typ, [NSEvent pressedMouseButtons], p.x, p.y, dx, dy, [event timestamp], [event modifierFlags]);
 }
 
-static CVReturn displayLinkCallback(CVDisplayLinkRef displayLink, const CVTimeStamp *inNow, const CVTimeStamp *inOutputTime, CVOptionFlags flagsIn, CVOptionFlags *flagsOut, void *displayLinkContext) {
-	CFTypeRef view = (CFTypeRef *)displayLinkContext;
-	gio_onFrameCallback(view);
-	return kCVReturnSuccess;
-}
-
 @interface GioView : NSOpenGLView 
 @end
 
 @implementation GioView {
-CVDisplayLinkRef displayLink;
 }
 - (instancetype)initWithFrame:(NSRect)frameRect
 				  pixelFormat:(NSOpenGLPixelFormat *)format {
-	self = [super initWithFrame:frameRect pixelFormat:format];
-	if (self) {
-		CVDisplayLinkCreateWithActiveCGDisplays(&displayLink);
-		CVDisplayLinkSetOutputCallback(displayLink, displayLinkCallback, (__bridge void*)self);
-	}
-	return self;
-}
-- (void)dealloc {
-	CVDisplayLinkRelease(displayLink);
-}
-- (void)setAnimating:(BOOL)anim {
-	if (anim) {
-		CVDisplayLinkStart(displayLink);
-	} else {
-		CVDisplayLinkStop(displayLink);
-	}
-}
-- (void)updateDisplay:(CGDirectDisplayID)dispID {
-	CVDisplayLinkSetCurrentCGDisplay(displayLink, dispID);
+	return [super initWithFrame:frameRect pixelFormat:format];
 }
 - (void)prepareOpenGL {
 	[super prepareOpenGL];
@@ -139,16 +114,6 @@ CFTypeRef gio_createGLView(void) {
 	}
 }
 
-void gio_updateDisplayLink(CFTypeRef viewRef, CGDirectDisplayID dispID) {
-	GioView *view = (__bridge GioView *)viewRef;
-	[view updateDisplay:dispID];
-}
-
-void gio_setAnimating(CFTypeRef viewRef, BOOL anim) {
-	GioView *view = (__bridge GioView *)viewRef;
-	[view setAnimating:anim];
-}
-
 CFTypeRef gio_contextForView(CFTypeRef viewRef) {
 	NSOpenGLView *view = (__bridge NSOpenGLView *)viewRef;
 	return (__bridge CFTypeRef)view.openGLContext;

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()
+		}
+	}
+}

app/internal/window/os_ios.go

@@ -22,7 +22,6 @@ __attribute__ ((visibility ("hidden"))) void gio_hideTextInput(CFTypeRef viewRef
 __attribute__ ((visibility ("hidden"))) void gio_addLayerToView(CFTypeRef viewRef, CFTypeRef layerRef);
 __attribute__ ((visibility ("hidden"))) void gio_updateView(CFTypeRef viewRef, CFTypeRef layerRef);
 __attribute__ ((visibility ("hidden"))) void gio_removeLayer(CFTypeRef layerRef);
-__attribute__ ((visibility ("hidden"))) void gio_setAnimating(CFTypeRef viewRef, int anim);
 __attribute__ ((visibility ("hidden"))) struct drawParams gio_viewDrawParams(CFTypeRef viewRef);
 __attribute__ ((visibility ("hidden"))) CFTypeRef gio_readClipboard(void);
 __attribute__ ((visibility ("hidden"))) void gio_writeClipboard(unichar *chars, NSUInteger length);
@@ -46,8 +45,9 @@ import (
 )
 
 type window struct {
-	view C.CFTypeRef
-	w    Callbacks
+	view        C.CFTypeRef
+	w           Callbacks
+	displayLink *displayLink
 
 	layer   C.CFTypeRef
 	visible atomic.Value
@@ -71,6 +71,13 @@ func onCreate(view C.CFTypeRef) {
 	w := &window{
 		view: view,
 	}
+	dl, err := NewDisplayLink(func() {
+		w.draw(false)
+	})
+	if err != nil {
+		panic(err)
+	}
+	w.displayLink = dl
 	wopts := <-mainWindow.out
 	w.w = wopts.window
 	w.w.SetDriver(w)
@@ -81,12 +88,6 @@ func onCreate(view C.CFTypeRef) {
 	w.w.Event(system.StageEvent{Stage: system.StagePaused})
 }
 
-//export gio_onFrameCallback
-func gio_onFrameCallback(view C.CFTypeRef) {
-	w := views[view]
-	w.draw(false)
-}
-
 //export gio_onDraw
 func gio_onDraw(view C.CFTypeRef) {
 	w := views[view]
@@ -139,6 +140,7 @@ func onDestroy(view C.CFTypeRef) {
 	w := views[view]
 	delete(views, view)
 	w.w.Event(system.DestroyEvent{})
+	w.displayLink.Close()
 	C.gio_removeLayer(w.layer)
 	C.CFRelease(w.layer)
 	w.layer = 0
@@ -240,15 +242,11 @@ func (w *window) SetAnimating(anim bool) {
 	if v == 0 {
 		return
 	}
-	var animi C.int
 	if anim {
-		animi = 1
+		w.displayLink.Start()
+	} else {
+		w.displayLink.Stop()
 	}
-	C.CFRetain(v)
-	runOnMain(func() {
-		defer C.CFRelease(v)
-		C.gio_setAnimating(v, animi)
-	})
 }
 
 func (w *window) onKeyCommand(name string) {

app/internal/window/os_ios.m

@@ -9,7 +9,6 @@
 #include "framework_ios.h"
 
 @interface GioView: UIView <UIKeyInput>
-- (void)setAnimating:(BOOL)anim;
 @end
 
 @interface GioViewController : UIViewController
@@ -134,20 +133,9 @@ static void handleTouches(int last, UIView *view, NSSet<UITouch *> *touches, UIE
 }
 
 @implementation GioView
-CADisplayLink *displayLink;
 NSArray<UIKeyCommand *> *_keyCommands;
-
-- (void)onFrameCallback:(CADisplayLink *)link {
-	gio_onFrameCallback((__bridge CFTypeRef)self);
-}
-
-- (instancetype)initWithFrame:(CGRect)frame {
-	self = [super initWithFrame:frame];
-	if (self) {
-		__weak id weakSelf = self;
-		displayLink = [CADisplayLink displayLinkWithTarget:weakSelf selector:@selector(onFrameCallback:)];
-	}
-	return self;
++ (void)onFrameCallback:(CADisplayLink *)link {
+	gio_onFrameCallback((__bridge CFTypeRef)link);
 }
 
 - (void)willMoveToWindow:(UIWindow *)newWindow {
@@ -182,16 +170,6 @@ NSArray<UIKeyCommand *> *_keyCommands;
 }
 
 - (void)dealloc {
-	[displayLink invalidate];
-}
-
-- (void)setAnimating:(BOOL)anim {
-	NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
-	if (anim) {
-		[displayLink addToRunLoop:runLoop forMode:[runLoop currentMode]];
-	} else {
-		[displayLink removeFromRunLoop:runLoop forMode:[runLoop currentMode]];
-	}
 }
 
 - (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
@@ -281,11 +259,6 @@ CFTypeRef gio_readClipboard(void) {
 	}
 }
 
-void gio_setAnimating(CFTypeRef viewRef, int anim) {
-	GioView *view = (__bridge GioView *)viewRef;
-	[view setAnimating:(anim ? YES : NO)];
-}
-
 void gio_showTextInput(CFTypeRef viewRef) {
 	UIView *view = (__bridge UIView *)viewRef;
 	[view becomeFirstResponder];
@@ -335,3 +308,33 @@ struct drawParams gio_viewDrawParams(CFTypeRef viewRef) {
 	params.left = insets.left*scale;
 	return params;
 }
+
+CFTypeRef gio_createDisplayLink(void) {
+	CADisplayLink *dl = [CADisplayLink displayLinkWithTarget:[GioView class] selector:@selector(onFrameCallback:)];
+	dl.paused = YES;
+	NSRunLoop *runLoop = [NSRunLoop mainRunLoop];
+	[dl addToRunLoop:runLoop forMode:[runLoop currentMode]];
+	return (__bridge_retained CFTypeRef)dl;
+}
+
+int gio_startDisplayLink(CFTypeRef dlref) {
+	CADisplayLink *dl = (__bridge CADisplayLink *)dlref;
+	dl.paused = NO;
+	return 0;
+}
+
+int gio_stopDisplayLink(CFTypeRef dlref) {
+	CADisplayLink *dl = (__bridge CADisplayLink *)dlref;
+	dl.paused = YES;
+	return 0;
+}
+
+void gio_releaseDisplayLink(CFTypeRef dlref) {
+	CADisplayLink *dl = (__bridge CADisplayLink *)dlref;
+	[dl invalidate];
+	CFRelease(dlref);
+}
+
+void gio_setDisplayLinkDisplay(CFTypeRef dl, uint64_t did) {
+	// Nothing to do on iOS.
+}

app/internal/window/os_macos.go

@@ -34,8 +34,6 @@ import (
 __attribute__ ((visibility ("hidden"))) void gio_main(CFTypeRef viewRef, const char *title, CGFloat width, CGFloat height);
 __attribute__ ((visibility ("hidden"))) CGFloat gio_viewWidth(CFTypeRef viewRef);
 __attribute__ ((visibility ("hidden"))) CGFloat gio_viewHeight(CFTypeRef viewRef);
-__attribute__ ((visibility ("hidden"))) void gio_setAnimating(CFTypeRef viewRef, BOOL anim);
-__attribute__ ((visibility ("hidden"))) void gio_updateDisplayLink(CFTypeRef viewRef, CGDirectDisplayID dispID);
 __attribute__ ((visibility ("hidden"))) CGFloat gio_getViewBackingScale(CFTypeRef viewRef);
 __attribute__ ((visibility ("hidden"))) CFTypeRef gio_readClipboard(void);
 __attribute__ ((visibility ("hidden"))) void gio_writeClipboard(unichar *chars, NSUInteger length);
@@ -48,9 +46,10 @@ func init() {
 }
 
 type window struct {
-	view  C.CFTypeRef
-	w     Callbacks
-	stage system.Stage
+	view        C.CFTypeRef
+	w           Callbacks
+	stage       system.Stage
+	displayLink *displayLink
 
 	// mu protect the following fields
 	mu            sync.Mutex
@@ -116,16 +115,11 @@ func (w *window) WriteClipboard(s string) {
 func (w *window) ShowTextInput(show bool) {}
 
 func (w *window) SetAnimating(anim bool) {
-	var animb C.BOOL
 	if anim {
-		animb = 1
+		w.displayLink.Start()
+	} else {
+		w.displayLink.Stop()
 	}
-	v := w.view
-	C.CFRetain(v)
-	runOnMain(func() {
-		defer C.CFRelease(v)
-		C.gio_setAnimating(v, animb)
-	})
 }
 
 func (w *window) setStage(stage system.Stage) {
@@ -136,14 +130,6 @@ func (w *window) setStage(stage system.Stage) {
 	w.w.Event(system.StageEvent{Stage: stage})
 }
 
-//export gio_onFrameCallback
-func gio_onFrameCallback(view C.CFTypeRef) {
-	w, exists := lookupView(view)
-	if exists {
-		w.draw(false)
-	}
-}
-
 //export gio_onKeys
 func gio_onKeys(view C.CFTypeRef, cstr *C.char, ti C.double, mods C.NSUInteger) {
 	str := C.GoString(cstr)
@@ -222,6 +208,12 @@ func gio_onFocus(view C.CFTypeRef, focus C.BOOL) {
 	w.w.Event(key.FocusEvent{Focus: focus == C.YES})
 }
 
+//export gio_onChangeScreen
+func gio_onChangeScreen(view C.CFTypeRef, did uint64) {
+	w := mustView(view)
+	w.displayLink.SetDisplayID(did)
+}
+
 func (w *window) draw(sync bool) {
 	w.mu.Lock()
 	wf, hf, scale := w.width, w.height, w.scale
@@ -256,6 +248,7 @@ func configFor(scale float32) config {
 //export gio_onTerminate
 func gio_onTerminate(view C.CFTypeRef) {
 	w := mustView(view)
+	w.displayLink.Close()
 	deleteView(view)
 	w.w.Event(system.DestroyEvent{})
 }
@@ -279,6 +272,13 @@ func gio_onCreate(view C.CFTypeRef) {
 		view:  view,
 		scale: scale,
 	}
+	dl, err := NewDisplayLink(func() {
+		w.draw(false)
+	})
+	if err != nil {
+		panic(err)
+	}
+	w.displayLink = dl
 	wopts := <-mainWindow.out
 	w.w = wopts.window
 	w.w.SetDriver(w)

app/internal/window/os_macos.m

@@ -31,7 +31,7 @@
 - (void)windowDidChangeScreen:(NSNotification *)notification {
 	CGDirectDisplayID dispID = [[[self.window screen] deviceDescription][@"NSScreenNumber"] unsignedIntValue];
 	CFTypeRef view = (__bridge CFTypeRef)self.window.contentView;
-	gio_updateDisplayLink(view, dispID);
+	gio_onChangeScreen(view, dispID);
 }
 - (void)windowDidBecomeKey:(NSNotification *)notification {
 	gio_onFocus((__bridge CFTypeRef)self.window.contentView, YES);
@@ -81,6 +81,34 @@ CGFloat gio_getViewBackingScale(CFTypeRef viewRef) {
 	return [view.window backingScaleFactor];
 }
 
+static CVReturn displayLinkCallback(CVDisplayLinkRef dl, const CVTimeStamp *inNow, const CVTimeStamp *inOutputTime, CVOptionFlags flagsIn, CVOptionFlags *flagsOut, void *displayLinkContext) {
+	gio_onFrameCallback(dl);
+	return kCVReturnSuccess;
+}
+
+CFTypeRef gio_createDisplayLink(void) {
+	CVDisplayLinkRef dl;
+	CVDisplayLinkCreateWithActiveCGDisplays(&dl);
+	CVDisplayLinkSetOutputCallback(dl, displayLinkCallback, nil);
+	return dl;
+}
+
+int gio_startDisplayLink(CFTypeRef dl) {
+	return CVDisplayLinkStart((CVDisplayLinkRef)dl);
+}
+
+int gio_stopDisplayLink(CFTypeRef dl) {
+	return CVDisplayLinkStop((CVDisplayLinkRef)dl);
+}
+
+void gio_releaseDisplayLink(CFTypeRef dl) {
+	CVDisplayLinkRelease((CVDisplayLinkRef)dl);
+}
+
+void gio_setDisplayLinkDisplay(CFTypeRef dl, uint64_t did) {
+	CVDisplayLinkSetCurrentCGDisplay((CVDisplayLinkRef)dl, (CGDirectDisplayID)did);
+}
+
 void gio_main(CFTypeRef viewRef, const char *title, CGFloat width, CGFloat height) {
 	@autoreleasepool {
 		NSView *view = (NSView *)CFBridgingRelease(viewRef);