fix(darwin): WriteWithoutResponse checks CanSendWriteWithoutResponse before request

gattc.go

@@ -0,0 +1,5 @@
+package bluetooth
+
+import "errors"
+
+var ErrCannotSendWriteWithoutResponse = errors.New("cannot send write without response")

gattc_darwin.go

@@ -199,9 +199,13 @@ func (c DeviceCharacteristic) Write(p []byte) (n int, err error) {
 
 // WriteWithoutResponse replaces the characteristic value with a new value. The
 // call will return before all data has been written. A limited number of such
-// writes can be in flight at any given time. This call is also known as a
-// "write command" (as opposed to a write request).
-func (c DeviceCharacteristic) WriteWithoutResponse(p []byte) (n int, err error) {
+// writes can be in flight at any given time.
+// If the client is not ready to send write without response requests at this time, ErrCannotSendWriteWithoutResponse is returned.
+func (c DeviceCharacteristic) WriteWithoutResponse(p []byte) (int, error) {
+	if !c.service.device.prph.CanSendWriteWithoutResponse() {
+		return 0, ErrCannotSendWriteWithoutResponse
+	}
+
 	c.service.device.prph.WriteCharacteristic(p, c.characteristic, false)
 
 	return len(p), nil
@@ -227,6 +231,14 @@ func (c DeviceCharacteristic) GetMTU() (uint16, error) {
 	return uint16(c.service.device.prph.MaximumWriteValueLength(false)), nil
 }
 
+// CanSendWriteWithoutResponse returns whether a WriteWithoutResponse can be sent
+// at this time. If this returns false, you must wait for some time before
+// sending another WriteWithoutResponse. This is typically because the internal
+// buffer is full.
+func (c DeviceCharacteristic) CanSendWriteWithoutResponse() bool {
+	return c.service.device.prph.CanSendWriteWithoutResponse()
+}
+
 // Read reads the current characteristic value.
 func (c *deviceCharacteristic) Read(data []byte) (n int, err error) {
 	c.readChan = make(chan error)