Ios AVPlayer键值的当前项是否可见？

我使用了一个AvQueuePlayer在后台播放音频。为了保持AvQueuePlayer的播放列表不为空，我使用键值观察来监视AvQueuePlayer的当前项，以便准备加载下一个播放项。有时效果很好，但有时失败了

以下是我收到的坠机报告：

Date/Time:       2011-12-16 23:43:58.963 +0800
OS Version:      iPhone OS 4.3.5 (8L1)
Report Version:  104

Exception Type:  EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x4f2ecfe8
Crashed Thread:  0

Thread 0 name:  Dispatch queue: com.apple.main-thread
Thread 0 Crashed:
0   libobjc.A.dylib                 0x32f0aca4 objc_msgSend + 28
1   Foundation                      0x3165386c NSKVONotify + 24
2   Foundation                      0x31651bb0 -[NSKeyValueObservance observeValueForKeyPath:ofObject:change:context:] + 220
3   Foundation                      0x315fa7ea NSKeyValueNotifyObserver + 210
4   Foundation                      0x315fa492 NSKeyValueDidChange + 230
5   Foundation                      0x315e666a - [NSObject(NSKeyValueObserverNotification) didChangeValueForKey:] + 70
6   AVFoundation                    0x33f190f2 -[AVPlayerItem didChangeValueForKey:] +  26
7   AVFoundation                    0x33f1ceba __avplayeritem_fpItemNotificationCallback_block_invoke_1 + 2310
8   libdispatch.dylib               0x338da8e0 _dispatch_call_block_and_release + 4
9   libdispatch.dylib               0x338d61ee    _dispatch_main_queue_callback_4CF$VARIANT$up + 306
10  CoreFoundation                  0x34c1d934 __CFRunLoopRun + 1328
11  CoreFoundation                  0x34badebc CFRunLoopRunSpecific + 224
12  CoreFoundation                  0x34baddc4 CFRunLoopRunInMode + 52
13  GraphicsServices                0x33e5e418 GSEventRunModal + 108
14  GraphicsServices                0x33e5e4c4 GSEventRun + 56
15  UIKit                           0x31272d62 -[UIApplication _run] + 398
16  UIKit                           0x31270800 UIApplicationMain + 664
17  WeatherTunes                    0x0004b31e main (main.m:17)
18  WeatherTunes                    0x0004b2ac start + 32

以下是我如何使用关键值观察：

[_queuePlayer addObserver:self forKeyPath:@"currentItem" options:NSKeyValueObservingOptionNew context:nil];
[_queuePlayer addObserver:self forKeyPath:@"currentItem.status" options:NSKeyValueObservingOptionNew context:nil];

---

我查阅了AvPlayer的课堂参考资料。然后我发现有人说属性“status”可以被观察到，而它没有提到currentItem。但我发现有些人使用属性currentItem作为可观察项。我现在完全糊涂了。请帮帮我。

我也遇到了同样的问题，我想知道AVQueuePlayer中的一首歌何时结束。我最终还是这样做了，看起来更安全

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(songFinishedWithNotification:)
                                             name:AVPlayerItemDidPlayToEndTimeNotification
                                           object:nil];

---

我也有同样的问题，我想知道一个播放器里的一首歌什么时候结束。我最终还是这样做了，看起来更安全

[[NSNotificationCenter defaultCenter] addObserver:self
                                         selector:@selector(songFinishedWithNotification:)
                                             name:AVPlayerItemDidPlayToEndTimeNotification
                                           object:nil];