最近使用Swift編程中，遇到一個問題，就是出現了 Call can throw, but it is not marked with 'try' and the error is not handled 的錯誤。
需要獲取視頻的某一幀的圖片，使用copyCGImageAtTime(requestedTime: CMTime, actualTime: UnsafeMutablePointer<CMTime>) throws -> CGImage 但是使用該方法時出現了錯誤：Call can throw, but it is not marked with 'try' and the error is not handled , 剛開始以為是參數的錯誤，因為在OC里面方法是這樣的 - (nullable CGImageRef)copyCGImageAtTime:(CMTime)requestedTime actualTime:(nullable CMTime *)actualTime error:(NSError * __nullable * __nullable)outError

首先先看一下這個方法的作用是什么？

//  獲取單幀的圖片
- (nullable CGImageRef)copyCGImageAtTime:(CMTime)requestedTime actualTime:(nullable CMTime *)actualTime error:(NSError * __nullable * __nullable)outError

// 根據視頻的路徑獲取單幀圖片
- (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中這是一個很常見的方法，但轉到Swift里面，方法發生了一些變化，但原理是不變的，但是卻出現了問題，

問題截圖：

出問題了

swift里面方法是這樣的

/*! 
@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<CMTime>) throws -> CGImage

這里出現的throws是個什么鬼??？？ 怎樣解決呢？？

其實很簡單，原因就是沒有處理錯誤 。我們根據錯誤提示，調用可以拋出，但它沒有標記和錯誤處理通過加一個try解決。
（PS: 就像Java中的異常錯誤處理，也是采用 try ...catch）

下面是最終解決錯誤的代碼：

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 
}