Not able to read a characteristic value from BLE device

Question

I need to write and read a characteristic value to Android BLE device. I'm able to write but not read. Here is how I'm writing :

public void writeCustomCharacteristic(int value) {
    if (mBluetoothAdapter == null || mBluetoothGatt == null) {
        Log.w(TAG, "BluetoothAdapter not initialized");
        return;
    }
    /*check if the service is available on the device*/
    BluetoothGattService mCustomService = mBluetoothGatt.getService(UUID.fromString("<MY SERVICE UUID>"));
    if(mCustomService == null){
        Log.w(TAG, "Custom BLE Service not found");
        return;
    }
    /*get the read characteristic from the service*/
    BluetoothGattCharacteristic mWriteCharacteristic = mCustomService.getCharacteristic(UUID.fromString("<MY CHARACTERSTIC UUID>"));
   mWriteCharacteristic.setWriteType(BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE);
       mWriteCharacteristic.setValue(value,BluetoothGattCharacteristic.FORMAT_UINT8,0);
    if(!mBluetoothGatt.writeCharacteristic(mWriteCharacteristic)){
        Log.w(TAG, "Failed to write characteristic");
    }else{
        Log.w(TAG, "Success to write characteristic");
    }
}

And this operation is success if I use

 mWriteCharacteristic.setWriteType(BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE);

If I use any other write types rather than WRITE_TYPE_NO_RESPONSE then it is not triggering onCharacteristicWrite() callback method.

Here is how I'm reading the characteristic :

 private boolean readCharacteristics(int groupPosition,int childPosition){
    try{
        if (mGattCharacteristics != null) {
            final BluetoothGattCharacteristic characteristic = mGattCharacteristics.get(2).get(2);
            final int charaProp = characteristic.getProperties();
            if ((charaProp | BluetoothGattCharacteristic.PROPERTY_READ) > 0) {
                // If there is an active notification on a characteristic, clear
                // it first so it doesn't update the data field on the user interface.
                if (mNotifyCharacteristic != null) {
                    mBluetoothLeService.setCharacteristicNotification(mNotifyCharacteristic, false);
                    mNotifyCharacteristic = null;
                }
                mBluetoothLeService.readCharacteristic(characteristic);
            }
            if ((charaProp | BluetoothGattCharacteristic.PROPERTY_NOTIFY) > 0) {
                mNotifyCharacteristic = characteristic;
                mBluetoothLeService.setCharacteristicNotification(characteristic, true);
            }
            waitSometime(100);
            if ((charaProp | BluetoothGattCharacteristic.PROPERTY_INDICATE) > 0) {
                mNotifyCharacteristic = characteristic;
                mBluetoothLeService.setCharacteristicIndication(characteristic, true);
            }
            waitSometime(100);
            if ((charaProp | BluetoothGattCharacteristic.PROPERTY_WRITE) > 0) {

                byte[] value = characteristic.getValue(); //this is being NULL always

                if (mNotifyCharacteristic != null) {
                    mBluetoothLeService.setCharacteristicNotification(mNotifyCharacteristic, true);
                    mNotifyCharacteristic = null;
                }
                mBluetoothLeService.writeCharacteristic(characteristic, value);
                if(value!=null&&value.length>0) {
                    Toast.makeText(DeviceControlActivity.this,"success reading data",Toast.LENGTH_LONG).show();
                }else{
                  Toast.makeText(DeviceControlActivity.this,"error reading data",Toast.LENGTH_LONG).show();
                }
            }
       /* if ((charaProp | BluetoothGattCharacteristic.PROPERTY_INDICATE) > 0) {
            mNotifyCharacteristic = characteristic;
            //characteristic.setWriteType();
            mBluetoothLeService.setCharacteristicIndication(
                    characteristic, true);
            byte[] value=characteristic.getValue();
        }*/
            return true;
        }
    }catch (Exception e){
        e.printStackTrace();
    }
    return false;
}

characteristic.getValue() returns always null. Where I'm going wrong?

Answer 1

Here is my opinion:

You need to figure out what kind of characteristic you are going to get data? Some of them need to set indication or notify first! Like:

a. Get the descriptor of the characteristic

   BluetoothGattDescriptor descriptor = gattCharacteristic.getDescriptors().get(0); 
     //Usually is Client_Characteristic_Configuration

b. According to the characteristic, set the property() of descriptor true.

if ((gattCharacteristic.PROPERTY_NOTIFY)> 0 ){
    if (descriptor != null) {
       descriptor.setValue((BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE));
       }
   }
else if((gattCharacteristic.PROPERTY_INDICATE) >0 ){
    if(descriptor != null){
       descriptor.setValue(BluetoothGattDescriptor.ENABLE_INDICATION_VALUE);
       }
   }

c. Send the value to the remote BLE device.

gatt.writeDescriptor(descriptor);

After that, you may get the data in callback function:

onCharacteristicChanged

At least above work for me! Hope can help someone~