以下是为APP添加控制音量的步骤：
1、导入依赖库，如果不导入就会出现clang: error: linker command failed with exit code 1 (use -v to see invocation)这个错误，如下图所示：

屏幕快照 2016-06-08 下午3.02.32.png

2、导入系统框架#import <MediaPlayer/MediaPlayer.h>

3、实现代码

MPVolumeView *volumeView = [[MPVolumeView alloc] init];
    UISlider* volumeViewSlider = nil;
    for (UIView *view in [volumeView subviews]){
        if ([view.class.description isEqualToString:@"MPVolumeSlider"]){
            volumeViewSlider = (UISlider*)view;
            break;
        }
    }
    NSUserDefaults *def = [NSUserDefaults standardUserDefaults];
    
    float systemVolume = volumeViewSlider.value;
    
    [volumeViewSlider setValue:[[def objectForKey:@"systemVolume"] floatValue] animated:NO];
    
    [volumeViewSlider sendActionsForControlEvents:UIControlEventTouchUpInside];
    if (volumeViewSlider.value == 1) {
        volumeViewSlider.value = 0;
    }else{
    
        volumeViewSlider.value += 0.1f;
        systemVolume += 0.1f;
        NSUserDefaults *de = [NSUserDefaults standardUserDefaults];
    
        [de setFloat:systemVolume forKey:@"systemVolume"];
        }
    
    NSLog(@"%f",volumeViewSlider.value);

但是通过以上代码调节音量不能控制相机拍照。