When does the BluetoothGattCallback and onConnectionStateChange been called in Android for BLE?

Question

I am study about Bluetooth Low Energy (BLE, Bluetooth 4.0) for Android.

I have reference the document like the link: Bluetooth Low Energy.

It use connectGatt() method for connecting to the GATT server on the device-A like the following.

mBluetoothGatt = device_A.connectGatt(this, false, mGattCallback);

And it has onConnectionStateChange , onCharacteristicRead ,etc in the mGattCallback.

The question is:

1. When does the BluetoothGattCallback has be called ?

2. Is the onConnectionStateChange in the mBluetoothGatt will return the message when the device-A has any change for all characteristic on the device-A ?? , or it only return the characteristic which the characteristic I have connect??

Answer 1

First of all after connection (you are connectiong as gatt-client to device as gatt-server) you should call Discovery device's services. After that device can send you onCharacteristicChanged in case of some changes happened. But there is a case when BLE device could be as gatt-client and should send writeCharacteristic to your BluetoothGattServer. In that case you should create gatt-server and receive calls.