系统声音使用MPVolumeView在8.4版本可以获取到系统音量的值，但在9.0的时候就获取不到

直接上代码：
MPVolumeView volumeView = [[MPVolumeView alloc] init];
UISlider volumeViewSlider = nil;
volumeView.frame = CGRectMake(-1000, -100, 100, 100);
volumeView.hidden = NO;
for (UIView view in [volumeView subviews]){
if ([view.class.description isEqualToString:@"MPVolumeSlider"]){
volumeViewSlider = (UISlider)view;
break;
}
}

// retrieve system volume

// float systemVolume2 = volumeViewSlider.value;
NSLog(@"视频当前音量：%f",systemVolum2);
// change system volume, the value is between 0.0f and 1.0f
[volumeViewSlider setValue:systemVolum2 animated:NO];

// send UI control event to make the change effect right now.
[volumeViewSlider sendActionsForControlEvents:UIControlEventTouchUpInside];

有可能是volumeViewSlider 还未创建，
我的解决方案是 判断volumeViewSlider 不存在 或者 音量为0时，就延时2秒后再次获取