判断是否存在手电筒

@property(nonatomic, readonly) BOOL hasTorch;

检测手电筒是否可用

例如：例如设备过热,需要降温。此属性键值可观测

@property(nonatomic, readonly, getter=isTorchAvailable) BOOL torchAvailable

检测手电筒是否正在使用

@property(nonatomic, readonly, getter=isTorchActive) BOOL torchActive

手电筒模式

@property(nonatomic) AVCaptureTorchMode torchMode;

手电筒模式：打开 关闭 自动

typedef NS_ENUM(NSInteger, AVCaptureTorchMode) {
    AVCaptureTorchModeOff  = 0,
    AVCaptureTorchModeOn   = 1,
    AVCaptureTorchModeAuto = 2,
} NS_AVAILABLE(10_7, 4_0) __TVOS_PROHIBITED;

设置手电筒亮度[0~1]之间

该值只在AVCaptureTorchModeOn 模式下有效

- (BOOL)setTorchModeOnWithLevel:(float)torchLevel error:(NSError * _Nullable * _Nullable)outError NS_AVAILABLE_IOS(6_0);

更改手电筒模式及亮度的时候必须先锁定设备，修改完后再解锁，否则崩溃

[device lockForConfiguration:nil];
//设置闪光灯为自动
[device setFlashMode:AVCaptureTorchModeOn];
 [captureDevice setTorchModeOnWithLevel:.2 error:nil];
[device unlockForConfiguration];

检测是否支持某种手电筒模式

- (BOOL)isTorchModeSupported:(AVCaptureTorchMode)torchMode;