Swift中Call can throw, but it is not marked with 'try' and the error is not handled错误的解决

最近使用Swift编程中，遇到一个问题，就是出现了 Call can throw, but it is not marked with 'try' and the error is not handled 的错误。
需要获取视频的某一帧的图片，使用copyCGImageAtTime(requestedTime: CMTime, actualTime: UnsafeMutablePointer) throws -> CGImage 但是使用该方法时出现了错误：Call can throw, but it is not marked with 'try' and the error is not handled , 刚开始以为是参数的错误，因为在OC里面方法是这样的 - (nullable CGImageRef)copyCGImageAtTime:(CMTime)requestedTime actualTime:(nullable CMTime *)actualTime error:(NSError * __nullable * __nullable)outError

首先先看一下这个方法的作用是什么？

//  获取单帧的图片
- (nullable CGImageRef)copyCGImageAtTime:(CMTime)requestedTime actualTime:(nullable CMTime *)actualTime error:(NSError * __nullable * __nullable)outError

// 根据视频的路径获取单帧图片
- (UIImage *)getThumbImage:(NSURL *)url {
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:url options:nil];
    AVAssetImageGenerator *gen = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    gen.appliesPreferredTrackTransform = YES;
    CMTime time = CMTimeMakeWithSeconds(0.0, 1);
    NSError *error = nil;
    CMTime actualTime;
    CGImageRef image = [gen copyCGImageAtTime:time actualTime:&actualTime error:&error];
    UIImage *thumb = [[UIImage alloc] initWithCGImage:image];
    CGImageRelease(image);
    return thumb;
}

在OC中这是一个很常见的方法，但转到Swift里面，方法发生了一些变化，但原理是不变的，但是却出现了问题，

问题截图：

出问题了

swift里面方法是这样的

/*! 
@method copyCGImageAtTime:actualTime:error: 
@abstract Returns a CFRetained CGImageRef for an asset at or near the specified time. 
@param requestedTime  The time at which the image of the asset is to be created. 
@param actualTime  A pointer to a CMTime to receive the time at which the image was actually generated. If you are not interested in this information, pass NULL. 
@param outError  An error object describing the reason for failure, in the event that this method returns NULL. 
@result A CGImageRef. 
@discussion Returns the CGImage synchronously. Ownership follows the Create Rule. */ 
 public func copyCGImageAtTime(requestedTime: CMTime, actualTime: UnsafeMutablePointer) throws -> CGImage

这里出现的throws是个什么鬼啊？？ 怎样解决呢？？

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其实很简单，原因就是没有处理错误 。我们根据错误提示，调用可以抛出，但它没有标记和错误处理通过加一个try解决。
（PS: 就像Java中的异常错误处理，也是采用 try ...catch）

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下面是最终解决错误的代码：

func getThunbImage(url: NSURL) -> (UIImage) { 
     let asset: AVURLAsset = AVURLAsset(URL: url, options: nil) 
     let gen: AVAssetImageGenerator = AVAssetImageGenerator(asset: asset) 
    gen.appliesPreferredTrackTransform = true 
    let time: CMTime = CMTimeMakeWithSeconds(0, 1) 
    var actualTime: CMTime = CMTimeMake(0, 0) 
    var thumb: UIImage = UIImage() 
      do { 
           let image: CGImageRef = try gen.copyCGImageAtTime(time, actualTime: &actualTime) 
           thumb = UIImage(CGImage: image) 
      } catch { }
         return thumb 
}