iOS如何判断设备处于静音模式

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-(BOOL)silenced 
{
     #if TARGET_IPHONE_SIMULATOR
         // return NO in simulator. Code causes crashes for some reason.
         return NO;
     #endif

    CFStringRef state;
    UInt32 propertySize = sizeof(CFStringRef);
    AudioSessionInitialize(NULL, NULL, NULL, NULL);
    AudioSessionGetProperty(kAudioSessionProperty_AudioRoute, &propertySize;, &state;);
    if(CFStringGetLength(state) > 0)
            return NO;
    else
            return YES;
}