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Writing and Reading from Characteristics

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Once you are connected to a Gatt Server, you’re going to be interacting with it by writing and reading from the server’s characteristics. To do this, first you have to discover what services are available on this server and which characteristics are avaiable in each service: 

@Override
public void onConnectionStateChange(BluetoothGatt gatt, int status,
       int newState) {
   if (newState == BluetoothProfile.STATE_CONNECTED) {
       Log.i(TAG, "Connected to GATT server.");
       gatt.discoverServices();

   }
. . .

@Override
   public void onServicesDiscovered(BluetoothGatt gatt, int status) {
       if (status == BluetoothGatt.GATT_SUCCESS) {
            List<BluetoothGattService> services = gatt.getServices();
               for (BluetoothGattService service : services) {
                   List<BluetoothGattCharacteristic> characteristics = service.getCharacteristics();
                   for (BluetoothGattCharacteristic characteristic : characteristics) { 
                       ///Once you have a characteristic object, you can perform read/write
                       //operations with it         
                   }
                }
             }
           }

A basic write operation goes like this: 

characteristic.setValue(newValue);
characteristic.setWriteType(BluetoothGattCharacteristic.WRITE_TYPE_DEFAULT);
gatt.writeCharacteristic(characteristic);

When the write process has finished, the onCharacteristicWrite method of your BluetoothGattCallback will be called: 

@Override
public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
    super.onCharacteristicWrite(gatt, characteristic, status);
    Log.d(TAG, "Characteristic " + characteristic.getUuid() + " written);
}

A basic write operation goes like this: 

gatt.readCharacteristic(characteristic);

When the write process has finished, the onCharacteristicRead method of your BluetoothGattCallback will be called: 

@Override
public void onCharacteristicRead(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
  super.onCharacteristicRead(gatt, characteristic, status);
  byte[] value = characteristic.getValue();
  }
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Bluetooth Low Energy (LE):
* Bluetooth Low Energy
* Finding BLE Devices
* Connecting to a GATT Server
* Writing and Reading from Characteristics
* Subscribing to Notifications from the Gatt Server
* Advertising a BLE Device
* Using a Gatt Server
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