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# 3.3.2.3.2 Detecting Device Attach

The application will need to know when a Audio v1.0 Device is attached. To receive this attach event from the Audio v1.0 Host Client Driver, the application must register an Attach Event Handler by calling the USB_HOST_AUDIO_V1_0_AttachEventHandlerSet function. This function should be called before the USB_HOST_BusEnable function is called, else the application may miss Audio v1.0 attach events. It can be called multiple times to register multiple event handlers, each for different application clients that need to know about Audio v1.0 Device Attach events.

The total number of event handlers that can be registered is defined by USB_HOST_AUDIO_V1_0_ATTACH_LISTENERS_NUMBER configuration option in `system_config.h`. When a device is attached, the Audio v1.0 Host Client Driver will send the attach event to all the registered event handlers. In this event handler, the USB Audio v1.0 Host Client Driver will pass a USB_HOST_AUDIO V1_0_OBJ that can be opened to gain access to the device. The following code shows an example of how to register attach event handlers.

    /* This code shows an example of Audio v1.0 Attach Event Handler and how this
     * attach event handler can be registered with the Audio v1.0 Host Client Driver */
    bool isAudioDeviceAttached = false;
    USB_HOST_AUDIO_V1_0_OBJ audioDeviceObj;

    /* Audio attach event listener function */
    void APP_USBHostAudioAttachEventListener
    (
        USB_HOST_AUDIO_V1_0_OBJ audioObj,
        USB_HOST_AUDIO_V1_0_EVENT event,
        uintptr_t context
    )
    {
        /* This function gets called when the Audio v1.0 device is attached/detached. In this
         * example we let the application know that a device is attached and we
         * store the Audio v1.0 device object. This object will be required to open the
         * device. */
        switch (event)
        {
            case USB_HOST_AUDIO_V1_0_EVENT_ATTACH:
                if (isAudioDeviceAttached == false)
                {
                    isAudioDeviceAttached = true;
                    audioDeviceObj = audioObj;
                }
                else
                {
                    /* This application supports only one Audio Device . Handle Error Here.*/
                }
            break;
            case USB_HOST_AUDIO_V1_0_EVENT_DETACH:
                if (isAudioDeviceAttached == true)
                {
                    /* This means the device was detached. There is no event data
                     * associated with this event.*/
                    isAudioDeviceAttached = false;
                    break;
                }
            break;
        }
    }

    void APP_Tasks(void)
    {
        switch (appData.state)
        {
            case APP_STATE_BUS_ENABLE:

                /* In this state the application enables the USB Host Bus. Note
                 * how the Audio v1.0 Attach event handler is registered before the bus
                 * is enabled. */

                USB_HOST_AUDIO_V1_0_AttachEventHandlerSet(APP_USBHostAudioAttachEventListener, (uintptr_t) 0);
                USB_HOST_BusEnable(0);
                appData.state = APP_STATE_WAIT_FOR_BUS_ENABLE_COMPLETE;
                break;

            case APP_STATE_WAIT_FOR_BUS_ENABLE_COMPLETE:
                /* Here we wait for the bus enable operation to complete. */
                break;
        }
    }