Re-fix #2449

app/preferences.go

@@ -14,21 +14,37 @@ import (
 type preferences struct {
 	*internal.InMemoryPreferences
 
-	prefLock     sync.RWMutex
-	ignoreChange bool
+	prefLock sync.RWMutex
+	// Normally, any update of preferences is immediately written to disk,
+	// but the initial operation of loading the preferences file performs many separate
+	// changes. To avoid "load->changes!->write" pattern (rewriting preferences file
+	// after each loading), saving file to disk is disabled
+	// during the loading progress. Access guarded by prefLock.
+	loadingInProgress bool
+	// If an application changes its preferences 1000 times per second, we don't want to
+	// rewrite the preferences file after every update. Instead, a time-out mechanism is
+	// implemented in resetSuspend(), limiting the number of file operations per second.
+	suspendChange       bool
+	numSuspendedChanges int
 
 	app *fyneApp
 }
 
 // Declare conformity with Preferences interface
 var _ fyne.Preferences = (*preferences)(nil)
 
-func (p *preferences) resetIgnore() {
+func (p *preferences) resetSuspend() {
 	go func() {
 		time.Sleep(time.Millisecond * 100) // writes are not always atomic. 10ms worked, 100 is safer.
 		p.prefLock.Lock()
-		p.ignoreChange = false
+		p.suspendChange = false
+		changes := p.numSuspendedChanges
+		p.numSuspendedChanges = 0
 		p.prefLock.Unlock()
+
+		if changes > 0 {
+			p.InMemoryPreferences.FireChange()
+		}
 	}()
 }
 
@@ -38,9 +54,9 @@ func (p *preferences) save() error {
 
 func (p *preferences) saveToFile(path string) error {
 	p.prefLock.Lock()
-	p.ignoreChange = true
+	p.suspendChange = true
 	p.prefLock.Unlock()
-	defer p.resetIgnore()
+	defer p.resetSuspend()
 	err := os.MkdirAll(filepath.Dir(path), 0700)
 	if err != nil { // this is not an exists error according to docs
 		return err
@@ -95,9 +111,18 @@ func (p *preferences) loadFromFile(path string) (err error) {
 	}()
 	decode := json.NewDecoder(file)
 
+	p.prefLock.Lock()
+	p.loadingInProgress = true
+	p.prefLock.Unlock()
+
 	p.InMemoryPreferences.WriteValues(func(values map[string]interface{}) {
 		err = decode.Decode(&values)
 	})
+
+	p.prefLock.Lock()
+	p.loadingInProgress = false
+	p.prefLock.Unlock()
+
 	return err
 }
 
@@ -112,10 +137,14 @@ func newPreferences(app *fyneApp) *preferences {
 	}
 
 	p.AddChangeListener(func() {
-		p.prefLock.RLock()
-		shouldIgnoreChange := p.ignoreChange
-		p.prefLock.RUnlock()
-		if shouldIgnoreChange { // callback after loading, no need to save
+		p.prefLock.Lock()
+		shouldIgnoreChange := p.suspendChange || p.loadingInProgress
+		if p.suspendChange {
+			p.numSuspendedChanges++
+		}
+		p.prefLock.Unlock()
+
+		if shouldIgnoreChange { // callback after loading file, or too many updates in a row
 			return
 		}
 

app/preferences_other.go

@@ -17,7 +17,7 @@ func (a *fyneApp) storageRoot() string {
 func (p *preferences) watch() {
 	watchFile(p.storagePath(), func() {
 		p.prefLock.RLock()
-		shouldIgnoreChange := p.ignoreChange
+		shouldIgnoreChange := p.suspendChange
 		p.prefLock.RUnlock()
 		if shouldIgnoreChange {
 			return

internal/preferences.go

@@ -41,7 +41,7 @@ func (p *InMemoryPreferences) WriteValues(fn func(map[string]interface{})) {
 	fn(p.values)
 	p.lock.Unlock()
 
-	p.fireChange()
+	p.FireChange()
 }
 
 func (p *InMemoryPreferences) set(key string, value interface{}) {
@@ -50,7 +50,7 @@ func (p *InMemoryPreferences) set(key string, value interface{}) {
 	p.values[key] = value
 	p.lock.Unlock()
 
-	p.fireChange()
+	p.FireChange()
 }
 
 func (p *InMemoryPreferences) get(key string) (interface{}, bool) {
@@ -68,7 +68,8 @@ func (p *InMemoryPreferences) remove(key string) {
 	delete(p.values, key)
 }
 
-func (p *InMemoryPreferences) fireChange() {
+// FireChange causes InMemoryPreferences to activate all its .changeListeners
+func (p *InMemoryPreferences) FireChange() {
 	p.lock.RLock()
 	defer p.lock.RUnlock()