问题描述：在使用Bluetooth Low Energy通信时，无法调用BluetoothGattCallback onConnectionStateChanged()方法。

解决方案：在连接状态改变时手动调用BluetoothGattCallback onConnectionStateChanged()方法。

示例代码：

private final BluetoothGattCallback mGattCallback = new BluetoothGattCallback() {
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
        if (newState == BluetoothProfile.STATE_CONNECTED) {
            mConnectionState = STATE_CONNECTED;
            mHandler.sendEmptyMessage(MSG_STATE_CONNECTED);
            gatt.discoverServices();
        } else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
            mConnectionState = STATE_DISCONNECTED;
            mHandler.sendEmptyMessage(MSG_STATE_DISCONNECTED);
        } else if (newState == BluetoothProfile.STATE_CONNECTING) {
            mConnectionState = STATE_CONNECTING;
            mHandler.sendEmptyMessage(MSG_STATE_CONNECTING);
        } else if (newState == BluetoothProfile.STATE_DISCONNECTING) {
            mConnectionState = STATE_DISCONNECTING;
            mHandler.sendEmptyMessage(MSG_STATE_DISCONNECTING);
        }
    }
};

private void manuallyInvokeOnConnectionStateChange(int newState){
    mGattCallback.onConnectionStateChange(mBluetoothGatt, null, newState);
}

由于BluetoothGattCallback onConnectionStateChanged()方法在连接状态改变时并不总是会被自动调用，我们可以手动调用该方法以确保其被调用。以上代码中的manuallyInvokeOnConnectionStateChange方法即可手动调用BluetoothGattCallback onConnectionStateChanged()方法。