Essential Books

Connecting to a GATT Server

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Once you have discovered a desired BluetoothDevice object, you can connect to it by using its connectGatt() method which takes as parameters a Context object, a boolean indicating whether to automatically connect to the BLE device and a BluetoothGattCallback reference where connection events and client operations results will be delivered: 

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
        device.connectGatt(context, false, bluetoothGattCallback, BluetoothDevice.TRANSPORT_AUTO);
    } else {
        device.connectGatt(context, false, bluetoothGattCallback);
    }

Override onConnectionStateChange in BluetoothGattCallback to receive connection an disconnection events: 

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

    } else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
        
        Log.i(TAG, "Disconnected from GATT server.");
    }
}
};
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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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