在我的应用程序中，我允许用户拍照以与给定任务相关联。我的方法是使用此类别创建一个具有自定义名称的 ALAssetsGroup 并将照片保存到其中。当我去抓取所有这些照片并将它们显示为缩略图时，问题就出现了。我正在抓住这样的资产：
- (void)setImagesForTask:(Task *)task {
//clear images associated with the task so we don't double up.
[task clearImages];
//static instance of ALAssetsLibrary
lib = [JobStore defaultAssetsLibrary];
dispatch_queue_t queue = dispatch_get_main_queue();
dispatch_async(queue, ^(void){
[lib enumerateGroupsWithTypes:ALAssetsGroupAlbum
usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
NSLog(@"group:%@", group);
//find my custom photo album and if it's there, enumerate through it.
if (group != nil && [[group valueForProperty:ALAssetsGroupPropertyName] isEqualToString:
[NSString stringWithFormat:@"Media for %@", task.title]]) {
if (group.numberOfAssets != 0) {
[group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
if (result != nil) {
CGImageRef imgRef = [[result defaultRepresentation] fullScreenImage];
UIImage *img = [UIImage imageWithCGImage:imgRef];
//methods to resize the image and get PNG representations…
[task setThumbnailDataFromImage:img];
[task setLargeImageDataFromImage:img];
NSLog(@"saved image to: %@", group);
} else if (result == nil){
NSLog(@"enumeration of assets ended");
}
}];
}
//if the group isn't there...
} else if (group == nil) {
//this method removes a UIView with a UIActivityIndicatorView that appears while enumeration occurs.
[self performSelectorOnMainThread:@selector(removeView)
withObject:nil
waitUntilDone:NO];
NSLog(@"enumeration of groups ended");
}}
failureBlock:^(NSError *error) {
NSLog(@"FAILURE");
}];
});
JobStore *js = [JobStore defaultStore];
[js saveChanges];
}
这是我得到的例外：
2012-04-02 10:05:26.585 MyApp[2746:7d3b] *** Terminating app due to uncaught exception 'NSRangeException', reason: '*** -[NSOrderedSet enumerateObjectsAtIndexes:options:usingBlock:]: index 0 beyond bounds for empty ordered set'
异常在 enumerateAssetsUsingBlock: 方法中引发。如果我将照片添加到一项任务，导航到另一项任务并将照片添加到该任务，则特别容易发生这种情况。一件奇怪的事情是，在抛出异常之前，我的日志中有这个：
2012-04-02 10:05:26.580 MyApp[2746:707] group:ALAssetsGroup - Name:Media for (current task), Type:Album, Assets count:1
所以资产组实际上并不是空的，但是当我尝试在其中枚举资产时，我的应用程序似乎认为那里什么都没有。在我看来，可能是因为我嵌套了这些枚举和/或因为块在后台执行但方法立即返回，所以我试图在我的应用程序知道它包含任何内容之前枚举资产组。有没有最好的方法来解决这个问题？我认为关于这些方法的块是如何执行的，我不理解一些基本的东西。任何投入将不胜感激。