feat(gattc): add CanSendWriteWithoutResponse for darwin

gattc_darwin.go

@@ -199,8 +199,9 @@ 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).
+// writes can be in flight at any given time.
+// You can use CanSendWriteWithoutResponse to check if you can send more writes.
+// 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) {
 	c.service.device.prph.WriteCharacteristic(p, c.characteristic, false)
 
@@ -227,6 +228,15 @@ 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. You can use this to implement your own flow control when
+// sending many WriteWithoutResponse calls in a row.
+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)