exp/sensor: don't enable a sensor on a sender

The underlying implementation was not enabling the sensors on a
particular sender even though the Enable signature accepts different
instances of Sender to enable.

Consider the following program:

  type A struct{}
  func (a A) Send(ev interface{}) {}

  type B struct{}
  func (b B) Send(ev interface{}) {}

  sensor.Enable(A{}, sensor.Gyroscope, time.Millisecond)
  sensor.Enable(B{}, sensor.Accelerometer, time.Millisecond)

is going to compile but only A will be notified when there are new
gyroscope and accelerometer events.

In order to improve the misleading APIs, this CL introduces a
Notify function that users can register a Sender implementation to
listen the changes. If set nil, the sensor package will keep
reading the events but will won't notify.

  sensor.Notify(A{})
  sensor.Enable(sensor.Gyroscope, time.Millisecond)
  sensor.Enable(sensor.Accelerometer, time.Millisecond)

Change-Id: I25e43349e4ae682930baa2d32430f46f24b588b7
Reviewed-on: https://go-review.googlesource.com/15650
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

exp/sensor/android.go

@@ -101,8 +101,10 @@ func init() {
 	}()
 }
 
-func enable(s Sender, t Type, delay time.Duration) error {
-	startCollecting(s)
+// enable enables the sensor t on sender. A non-nil sender is
+// required before calling enable.
+func enable(t Type, delay time.Duration) error {
+	startCollecting()
 
 	var err error
 	done := make(chan struct{})
@@ -114,7 +116,7 @@ func enable(s Sender, t Type, delay time.Duration) error {
 	return err
 }
 
-func startCollecting(s Sender) {
+func startCollecting() {
 	collectingMu.Lock()
 	defer collectingMu.Unlock()
 
@@ -135,7 +137,7 @@ func startCollecting(s Sender) {
 			}
 			<-done
 			for i := 0; i < n; i++ {
-				s.Send(ev[i])
+				sender.Send(ev[i])
 			}
 		}
 	}()

exp/sensor/darwin_armx.go

@@ -60,7 +60,9 @@ func init() {
 
 const minDelay = 10 * time.Millisecond
 
-func enable(s Sender, t Type, delay time.Duration) error {
+// enable enables the sensor t on sender. A non-nil sender is
+// required before calling enable.
+func enable(t Type, delay time.Duration) error {
 	channels.Lock()
 	defer channels.Unlock()
 
@@ -82,7 +84,7 @@ func enable(s Sender, t Type, delay time.Duration) error {
 	case Magnetometer:
 		C.GoIOS_startMagneto(interval)
 	}
-	go pollSensor(s, t, delay, channels.done[t])
+	go pollSensor(t, delay, channels.done[t])
 	return nil
 }
 
@@ -107,7 +109,7 @@ func disable(t Type) error {
 	return nil
 }
 
-func pollSensor(s Sender, t Type, d time.Duration, done chan struct{}) {
+func pollSensor(t Type, d time.Duration, done chan struct{}) {
 	var lastTimestamp int64
 
 	var timestamp C.int64_t
@@ -133,7 +135,7 @@ func pollSensor(s Sender, t Type, d time.Duration, done chan struct{}) {
 			if ts > lastTimestamp {
 				// TODO(jbd): Do we need to convert the values to another unit?
 				// How does iOS units compare to the Android units.
-				s.Send(Event{
+				sender.Send(Event{
 					Sensor:    t,
 					Timestamp: ts,
 					Data:      []float64{float64(ev[0]), float64(ev[1]), float64(ev[2])},

exp/sensor/notmobile.go

@@ -11,7 +11,7 @@ import (
 	"time"
 )
 
-func enable(s Sender, t Type, delay time.Duration) error {
+func enable(t Type, delay time.Duration) error {
 	return errors.New("sensor: no sensors available")
 }
 

exp/sensor/sensor.go

@@ -7,6 +7,7 @@ package sensor // import "golang.org/x/mobile/exp/sensor"
 
 import (
 	"errors"
+	"sync"
 	"time"
 )
 
@@ -66,20 +67,47 @@ type Event struct {
 
 // TODO(jbd): Move Sender interface definition to a top-level package.
 
+var (
+	// senderMu protects sender.
+	senderMu sync.Mutex
+
+	// sender is notified with the sensor data each time a new event is available.
+	sender Sender
+)
+
 // Sender sends an event.
 type Sender interface {
 	Send(event interface{})
 }
 
+// Notify registers a Sender and sensor events will be sent to s.
+// A typical example of Sender implementations is app.App.
+// Once you call Notify, you are not allowed to call it again.
+// You cannot call Notify with a nil Sender.
+func Notify(s Sender) {
+	senderMu.Lock()
+	defer senderMu.Unlock()
+
+	if s == nil {
+		panic("sensor: cannot set a nil sender")
+	}
+	if sender != nil {
+		panic("sensor: another sender is being notified, cannot set s as the sender")
+	}
+	sender = s
+}
+
 // Enable enables the specified sensor type with the given delay rate.
-// Sensor events will be sent to s, a typical example of Sender
-// implementations is app.App.
-// Enable is not safe for concurrent use.
-func Enable(s Sender, t Type, delay time.Duration) error {
+// Users must set a non-nil Sender via Notify before enabling a sensor,
+// otherwise an error will be returned.
+func Enable(t Type, delay time.Duration) error {
 	if t < 0 || int(t) >= len(sensorNames) {
 		return errors.New("sensor: unknown sensor type")
 	}
-	return enable(s, t, delay)
+	if err := validSender(); err != nil {
+		return err
+	}
+	return enable(t, delay)
 }
 
 // Disable disables to feed the manager with the specified sensor.
@@ -90,3 +118,13 @@ func Disable(t Type) error {
 	}
 	return disable(t)
 }
+
+func validSender() error {
+	senderMu.Lock()
+	defer senderMu.Unlock()
+
+	if sender == nil {
+		return errors.New("sensor: no senders to be notified; cannot enable the sensor")
+	}
+	return nil
+}