Add driver priority information

Make system default device selected by default.

mediadevices.go

@@ -181,8 +181,9 @@ func selectBestDriver(filter driver.FilterFn, constraints MediaTrackConstraints)
 
 	driverProperties := queryDriverProperties(filter)
 	for d, props := range driverProperties {
+		priority := float64(d.Info().Priority)
 		for _, p := range props {
-			fitnessDist := constraints.Media.FitnessDistance(p)
+			fitnessDist := constraints.Media.FitnessDistance(p) - priority
 			if fitnessDist < minFitnessDist {
 				minFitnessDist = fitnessDist
 				bestDriver = d

pkg/driver/driver.go

@@ -14,9 +14,22 @@ type AudioRecorder interface {
 	AudioRecord(p prop.Media) (r audio.Reader, err error)
 }
 
+// Priority represents device selection priority level
+type Priority float32
+
+const (
+	// PriorityHigh is a value for system default devices
+	PriorityHigh Priority = 0.1
+	// PriorityNormal is a value for normal devices
+	PriorityNormal Priority = 0.0
+	// PriorityLow is a value for unrecommended devices
+	PriorityLow Priority = -0.1
+)
+
 type Info struct {
 	Label      string
 	DeviceType DeviceType
+	Priority   Priority
 }
 
 type Adapter interface {

pkg/driver/microphone/microphone_linux.go

@@ -27,10 +27,19 @@ func init() {
 	if err != nil {
 		panic(err)
 	}
+	defaultSource, err := pa.DefaultSource()
+	if err != nil {
+		panic(err)
+	}
 	for _, source := range sources {
+		priority := driver.PriorityNormal
+		if defaultSource.ID() == source.ID() {
+			priority = driver.PriorityHigh
+		}
 		driver.GetManager().Register(&microphone{id: source.ID()}, driver.Info{
 			Label:      source.ID(),
 			DeviceType: driver.Microphone,
+			Priority:   priority,
 		})
 	}
 }

pkg/driver/wrapper.go

@@ -9,7 +9,12 @@ import (
 
 func wrapAdapter(a Adapter, info Info) Driver {
 	id := uuid.NewV4().String()
-	d := &adapterWrapper{Adapter: a, id: id, info: info, state: StateClosed}
+	d := &adapterWrapper{
+		Adapter: a,
+		id:      id,
+		info:    info,
+		state:   StateClosed,
+	}
 
 	switch v := a.(type) {
 	case VideoRecorder: