在使用 Apple PTP 时，发现 withCompleteContentCatalog 方法无法触发。解决方法是，需要通过设置 PTPObsterver 对象的 observerValueAvailable 方法来激活这个方法。 具体代码实现如下：

#import "PTPObserver.h"
#import "PTPDeviceManager.h"

@implementation MyPTPObserver

- (void)observerValueAvailable:(PTPValue *)value
{
    if ([value isKindOfClass:[PTPListOfObjects class]])
    {
        PTPListOfObjects *list = (PTPListOfObjects *)value;
        NSLog(@"Received content catalog. Count: %lu", (unsigned long)[list.objects count]);
        
        // your code to handle the content catalog
    }
}

@end

/**** Rust implementation ****/

pub struct MyPTPObserver {
    observer: Arc>,
}
impl ptp_rs::Callbacks for MyPTPObserver {
    fn object_added(&self, object_handle: u32) {
        // your code to handle new objects
    }
    fn object_removed(&self, object_handle: u32) {
        // your code to handle deleted objects
    }
    fn object_info_changed(&self, object_handle: u32) {
        // your code to handle updated objects
    }
    fn event_fired(&self, event_code: u16) {
        // your code to handle events
    }
    fn observer_value_available(&self, value: PTPValue) {
        if let PTPValue::ListOfObjects(objects) = value {
            println!("Received content catalog. Count: {}", objects.len());

            // your code to handle the content catalog
        }
    }
}

fn main() {
    let device_list = PTPDeviceManager::new().get_devices().unwrap();
    let device = &device_list[0];
    let observer = Arc::new(Mutex::new(MyPTPObserver{ observer: observer.clone() }));
    let ptp = device.get_ptp_session(observer.clone(), MyPTPDeviceCallbacks{});

    // enable the content catalog
    ptp.send_operation(
        PTPOperation::GetObjectHandles(0, 0, PTPObjectFormat::Undefined, "".to_owned())