当库为空时，应用程序因“NSRangeException”而崩溃

Question

我正在使用此代码从 cameraRoll 获取最新照片：

- (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);
                                }];
    }

事实上，如果库 cameraRoll 为空，应用程序将因以下错误而崩溃：

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

我知道我的 indexSet 高于 numberOfAssets 是因为：

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

此时我的问题是“我应该在哪里设置一个条件来检查资产是否为零？”

如果 cameraRoll 大于零，我会让代码继续，但如果计数为零，我想以这种方式处理问题，反对让它崩溃：

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

感谢您的帮助！

Answer 1

在enumerateAssetsAtIndexes 行之前添加if([images numberOfAssets]>0) 并继续代码...

或if([images numberOfAssets]<=0)然后

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