應用與「NSRangeException」崩潰時庫是空的

Question

我使用這個代碼，以獲得從cameraRoll最新照片：

- (void) getTheLatestPhoto{ 
     ALAssetsLibrary *cameraRoll = [[ALAssetsLibrary alloc] init]; 
     [cameraRoll enumerateGroupsWithTypes:ALAssetsGroupSavedPhotos usingBlock:^(ALAssetsGroup *images, BOOL *stop) { 
      [images setAssetsFilter:[ALAssetsFilter allPhotos]]; 
      [images enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:[images numberOfAssets] - 1] options:0 usingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) 
      { 
       if (result) { 

        ALAssetRepresentation *rep = [result defaultRepresentation]; 
        self.sharingImage = [UIImage imageWithCGImage:[rep fullScreenImage]]; 
       } 
      }]; 
     } 
           failureBlock: ^(NSError *error) { 
            NSLog(@"An error occured: %@",error); 
           }]; 
    } 

正因爲如此，如果庫cameraRoll是空的，應用程序將崩潰這個錯誤：

Terminating app due to uncaught exception 'NSRangeException', reason: 'indexSet count or lastIndex must not exceed -numberOfAssets' 

我明白我的索引集比numberOfAssets走高，因此：

[images enumerateAssetsAtIndexes:[NSIndexSet indexSetWithIndex:[images numberOfAssets] - 1] 

在這一點上，我的問題是「我在哪裏提出了一個條件，允許我檢查資產是否爲零？」

如果cameraRoll有超過零，我會讓碼繼續，但如果計數爲零，我想處理這個樣子的反對讓它崩潰：

UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"" message:@"Sorry, there are no photo in Camera Roll" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil, nil]; 
[alert show]; 

感謝你的幫助！

Answer 1

添加if([images numberOfAssets]>0)之前enumerateAssetsAtIndexes線，並繼續代碼...

或if([images numberOfAssets]<=0)然後

UIAlertView *alert = [[UIAlertView alloc]initWithTitle:@"" message:@"Sorry, there are no photo in Camera Roll" delegate:nil cancelButtonTitle:@"OK" otherButtonTitles:nil, nil]; 
[alert show];