BLE 监听以及MTU浅析

Posted 2022-01-22 zolty

监听

部分BLE程序会采用 descriptors 进行对固定UUID进行监听，例如：
“00002902-0000-1000-8000-00805f9b34fb”，这里采用了查找所有可用的descriptor UUID，对其监听

mInitialized = gatt.setCharacteristicNotification(characteristic, true);
//下面部分，不是必须的
Log.i(TAG,"setCharacteristicNotification : " + mInitialized);
if(mInitialized)
for(BluetoothGattDescriptor descList : characteristic.getDescriptors())
   BluetoothGattDescriptor descriptor = characteristic.getDescriptor(descList.getUuid());
   Log.i(TAG, "Descriptor Service UUID found : " + descList.getUuid().toString());
   if(descriptor != null)
       Log.i(TAG, "Enable Descriptor Service UUID found : " + descList.getUuid());
       descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
       gatt.writeDescriptor(descriptor);
   

MTU

当你发送byte[] 时，有时候会发现超过20字节后的数据消失了。默认MTU位20字节。所以BLE APP需要手动调节最大包长度

@Override
public void onMtuChanged(BluetoothGatt gatt, int mtu, int status) 
    super.onMtuChanged(gatt, mtu, status);
    Log.d(TAG," onMtuChanged");
    if (BluetoothGatt.GATT_SUCCESS == status) 
        Log.d(TAG, "onMtuChanged success MTU = " + mtu);
    else 
        Log.d(TAG, "onMtuChanged fail ");
    


public void onServicesDiscovered(BluetoothGatt gatt, int status) 
   super.onServicesDiscovered(gatt, status);
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) 
       gatt.requestMtu(131);
   
   //need 2s to config
   try 
       sleep(2000);
    catch (InterruptedException e) 
       e.printStackTrace();
   
   .......

这里将MTU设置为131，注意这里需要2秒让设备加载这个配置。