Android Bluetooth Low Energy Getting response to specific request

Question

There's something I'm not quite getting with using Gatt to communicate with a BLE device. According to this: https://developer.android.com/reference/android/bluetooth/BluetoothDevice.html#connectGatt(android.content.Context, boolean, android.bluetooth.BluetoothGattCallback)

BluetoothGatt gatt = device.connectGatt(context,true,new BluetoothGattCallback(){....})

I can connect to a BLE device and give it a callbacks object to be notified on stuff like onCharacteristicRead and onCharacteristicWrite

What I'm not getting is how do which write corresponds to which read callback?

This method signatures are:

public void onCharacteristicRead (BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status)
public void onCharacteristicWrite (BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status)

so if I do:

BluetoothGattCharacteristic char = gatt.getService(UART_SERVICE_UUID).getCharacteristic(UART_TX_CHARACTERISTIC_UUID);
char1.setValue("command1");
gatt.writeCharacteristic(char);
char1.setValue("command2");
gatt.writeCharacteristic(char);

in the onCharacteristicRead callback, how do I know if the characteristic.getStringValue() is for command1 or command2?

Thanks!

Answer 1

There are a couple of important things to know about when using BluetoothGatt.writeCharacteristic() and related BluetoothGatt methods.

1. Please notice that writeCharacteristic() (and many other methods of BluetoothGatt) return a boolean. When the result is false, it means that the operation was not successfully initiated. When does that occur? See the next item.

2. Using aBluetoothGatt object, it is not possible to initiate two operations at the same time. The call to initiate the second will fail and return false. After calling writeCharacteristic(), the code must wait for the callback response (onCharacteristicWrite) before issuing another write. In the sample code for this question the second call to writeCharacteristic() will almost certainly return false for this reason.

In this way, if each BluetoothGatt operation waits for a callback on the previously issued command, you can successfully pair command initiations and callbacks -- really you are forced to.