Why does setCharacteristicNotification() not actua

问题:

The BluetoothLeGatt Android BLE example contains the following code:

public void setCharacteristicNotification(BluetoothGattCharacteristic characteristic,
                                          boolean enabled) {
    if (mBluetoothAdapter == null || mBluetoothGatt == null) {
        Log.w(TAG, "BluetoothAdapter not initialized");
        return;
    }
    mBluetoothGatt.setCharacteristicNotification(characteristic, enabled);

    // This is specific to Heart Rate Measurement.
    if (UUID_HEART_RATE_MEASUREMENT.equals(characteristic.getUuid())) {
        BluetoothGattDescriptor descriptor = characteristic.getDescriptor(
                UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
        descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
        mBluetoothGatt.writeDescriptor(descriptor);
    }
}

My question is basically, why is the marked code specific to Heart Rate Measurement? It seems like having a Client Characteristic Config Descriptor (CCCD) characteristic is the standard way to control characteristic notification, so why doesn't setCharacteristicNotification() take care of writing to it? And since it doesn't do that, what does setCharacteristicNotification() actually do?

I'm pretty new to BLE and there aren't any explanations of it on the internet that don't assume that you already understand it all! So don't assume I know what a CCCD or whatever is! It was difficult enough finding out what CCCD even stands for!

Edit: See also this answer which supports my understanding of CCCDs (and makes me continue to wonder why you have to write to them manually in Android when there is a function that looks like it should do that for you): https://devzone.nordicsemi.com/index.php/what-does-cccd-mean

回答1:

I think is a litte bit late for give an answer but today I had the same doubt and I found a clear answer. Using setCharacteristicNotification() you enable notification localy (on android device) and setting CCC descriptor to ENABLE_NOTIFICATION_VALUE you enable notification on ble peripheral. In fact for enabling CCC notification you have to use setValue() and writeDescriptor() that are methods used for writing characteristics (in this case characteristics descriptors) to remote device. I found this on: http://processors.wiki.ti.com/index.php/SensorTag_User_Guide

回答2:

Here is an excerpt from the O'Reilly book "Getting Started With Bluetooth Low Energy":

To enable notifications on Android, you normally have to locally enable the notification for the particular characteristic you are interested in.

Once that’s done, you also have to enable notifications on the peer device by writing to the device’s client characteristic configuration descriptor (CCCD)

I believe this answers your question.

So

mBluetoothGatt.setCharacteristicNotification(characteristic, enabled); 

refers to the first part

and

mBluetoothGatt.writeDescriptor(descriptor); 

refers to the 2nd.

回答3:

For future peoples coming across this, here's the best answer I could find:

By writing to the Client Characteristic Config descriptor, you, the client, are telling the BLE server to switch configurations. (This made no sense to me either initially, but in english:)

This tells the BLE device to switch modes (configurations) to actively gather and report changes to this characteristic instead of changing and only reporting when requested.

It's poorly named, but digging through the docs it appears this is also going to be used for other possible characteristic changes that the client might request: hence the confusing name.

https://developer.bluetooth.org/gatt/descriptors/Pages/DescriptorViewer.aspx?u=org.bluetooth.descriptor.gatt.client_characteristic_configuration.xml

This begs the question, why call BluetoothGatt.setCharacteristicNotification() if we're only going to duplicate our efforts by modifying the descriptor?! Digging through the BluetoothGatt source shows us that setCharacteristicNotification only prepares the local service to receive notifications, not to enable persistent updates.

回答4:

I know it looks silly, but setting CCCD value is the only way you can tell the API whether you are going to turn on notification or indication.

Currently there is no setCharacteristicIndication. To enable indication, you have to call setCharacteristicNotification (confusing) and then write BluetoothGattDescriptor.ENABLE_INDICATION_VALUE to CCCD, similar to what you did to enable Notification.

回答5:

"The Client Characteristic Configuration descriptor defines how the characteristic may be configured by a specific client."