我正在构建一个iOS应用程序，允许用户拍照并将其保存到自定义相册，该相册会在应用中使用相册中的照片填充UICollectionView。我在网上找到了很多关于如何做到这一点的例子，但我无法通过专辑中没有出现的最新图片来解决顽固的问题。我结束了使用通知和串行调度队列的工作，但我认为它可以改进。
以下是我如何保存图片然后重新填充UICollectionView：
当我需要枚举相册时，我构建了一个方法来调用（viewDidLoad，一旦我收到了ALAssetsLibraryChangedNotification）。
-(void)setupCollectionView {
_assets = [@[] mutableCopy];
__block NSMutableArray *tmpAssets = [@[] mutableCopy];
// This grabs a static instance of the ALAssetsLibrary
ALAssetsLibrary *assetsLibrary = [ProfileViewController defaultAssetsLibrary];
// Enumerate through all of the ALAssets (photos) in the user’s Asset Groups (Folders)
[assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
if ([[group valueForProperty:ALAssetsGroupPropertyName] isEqualToString:albumName]) {
// Make sure we have the group... (this may be redundant)
if (group == nil){
return;
}
// I read in some SO posts that setting the filter might help, but I don't think it does
NSLog(@"Refresh pictures - %d",[group numberOfAssets]);
[group setAssetsFilter:[ALAssetsFilter allPhotos]];//this will cause group to reload
if (group!=nil) {
[group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
if(result){
[tmpAssets addObject:result];
}else{
*stop = YES;
}
}];
}
self.assets = tmpAssets;
// Reload the UICollectionView
dispatch_async(dispatch_get_main_queue(), ^{
[self.collectionView reloadData];
[self.collectionView.collectionViewLayout invalidateLayout];
});
}
} failureBlock:^(NSError *error) {
NSLog(@"Error loading images %@", error);
}];
}
最初这似乎工作得很好，但有时候，在将图像写入自定义相册后调用此图片时，刚刚拍摄的照片不会出现在集合视图中。照片将保存到图书馆（它显示在照片应用程序中），退出并重新运行应用程序后，照片会出现，但枚举不包括最新照片。
正如关于此事的一些Stack Overflow帖子的建议，我尝试实施ALAssets＆＃39;通过侦听ALAssetsLibraryChangedNotification来通知，但我遇到了一些问题。这是我收到通知后我正在做的事情：
- (void) handleAssetChangedNotifiation:(NSNotification *)notification {
NSDictionary *userInfo = notification.userInfo;
NSSet *updateAssets = userInfo[ALAssetLibraryUpdatedAssetsKey];
if (updateAssets.count>0) {
dispatch_sync(photoLoadQueue, ^{
[self setupCollectionView];
});
}
}
这对我来说似乎很好。我正在检查ALAssetLibraryUpdatedAssetsKey中是否有任何数据，然后将其丢弃在调度队列上，我发现我开始使用该调度队列时，我收到了几个用于保存单张照片的通知。这是我在将照片写入自定义库后收到的通知：
Received notification: NSConcreteNotification 0x15ed4af0 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {
ALAssetLibraryUpdatedAssetGroupsKey = "{(\n assets-library://group/?id=43E80EF8-EA91-4C71-AFD2-EBDCB53A1D53\n)}";
ALAssetLibraryUpdatedAssetsKey = "{(\n assets-library://asset/asset.JPG?id=63E43AF3-3729-43E9-806C-BE463116FDA2&ext=JPG,\n assets-library://asset/asset.JPG?id=FA2ED24E-DF0F-49A3-85B8-9C181E4077A4&ext=JPG,\n assets-library://asset/asset.JPG?id=A0B66E2E-6EA1-462C-AD93-DB3087BA65D8&ext=JPG\n)}";}}
Received notification: NSConcreteNotification 0x15d538c0 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {
ALAssetLibraryUpdatedAssetsKey = "{(\n assets-library://asset/asset.JPG?id=FBDD2086-EC76-473F-A23E-4F4200C0A6DF&ext=JPG\n)}";}}
Received notification: NSConcreteNotification 0x15dc9230 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {}}
Received notification: NSConcreteNotification 0x15df5b40 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {}}
Received notification: NSConcreteNotification 0x15d6a050 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {}}
Received notification: NSConcreteNotification 0x15d379e0 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {
ALAssetLibraryUpdatedAssetGroupsKey = "{(\n assets-library://group/?id=1E76AFA7-89B4-4277-A175-D7C8E62E49D0&filter=1,\n assets-library://group/?id=1E76AFA7-89B4-4277-A175-D7C8E62E49D0\n)}";
ALAssetLibraryUpdatedAssetsKey = "{(\n assets-library://asset/asset.JPG?id=FBDD2086-EC76-473F-A23E-4F4200C0A6DF&ext=JPG\n)}";
Received notification: NSConcreteNotification 0x15df7bf0 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {
ALAssetLibraryUpdatedAssetGroupsKey = "{(\n assets-library://group/?id=8E390051-E107-42BC-AE55-24BA35966642\n)}";}}
Received notification: NSConcreteNotification 0x15d40360 {name = ALAssetsLibraryChangedNotification; object = <ALAssetsLibrary: 0x15db9e80>; userInfo = {
ALAssetLibraryUpdatedAssetsKey = "{(\n assets-library://asset/asset.JPG?id=5A45779A-C3C1-4545-9BD6-88F094FFA5B9&ext=JPG\n)}";}}
为什么我收到这么多通知？我没有看到一种方法来查找一条特定的消息，一旦图像准备好就会告诉我，所以我使用该调度队列来连续地触发枚举。有没有人对此或解决方案有任何经验？我的工作，但我肯定比我应该做更多的工作。
只是为了完成，这是我保存图像的方法（我可能不应该像我一样嵌套这些块，但我这样做是为了尝试修复添加的图像未被发现）：
- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {
UIImage *originalImage, *editedImage, *imageToSave;
editedImage = (UIImage *) [info objectForKey:UIImagePickerControllerEditedImage];
originalImage = (UIImage *) [info objectForKey:UIImagePickerControllerOriginalImage];
if (editedImage) {
imageToSave = editedImage;
}else {
imageToSave = originalImage;
}
// Get group/asset library/album
__block ALAssetsGroup* groupToAddTo;
ALAssetsLibrary *assetsLibrary = [ProfileViewController defaultAssetsLibrary];
[[NSNotificationCenter defaultCenter] removeObserver:self name:ALAssetsLibraryChangedNotification object:nil];
[assetsLibrary enumerateGroupsWithTypes:ALAssetsGroupAll usingBlock:^(ALAssetsGroup *group, BOOL *stop) {
if ([[group valueForProperty:ALAssetsGroupPropertyName] isEqualToString:albumName]) {
NSLog(@"found album %@", albumName);
groupToAddTo = group;
// save to album
CGImageRef img = [imageToSave CGImage];
// There's some orientation stuff here I pulled out for brevity's sake
[assetsLibrary writeImageToSavedPhotosAlbum:img orientation: alOrientation completionBlock:^(NSURL* assetURL, NSError* error) {
if (error.code == 0) {
[assetsLibrary assetForURL:assetURL resultBlock:^(ALAsset *asset) {
// assign the photo to the album
if ([groupToAddTo valueForProperty:ALAssetsGroupPropertyURL] == nil){
NSLog(@"group properties are nil!");
}else {
if([groupToAddTo addAsset:asset]){
// If the asset writes to the library, listen for the notification
[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(handleAssetChangedNotifiation:) name:ALAssetsLibraryChangedNotification object:nil];
}else {
NSLog(@"Image Not Added!");
}
}
}
failureBlock:^(NSError* error) {
NSLog(@"failed to retrieve image asset:\nError: %@ ", [error localizedDescription]);
}];
}else {
NSLog(@"saved image failed.\nerror code %i\n%@", error.code, [error localizedDescription]);
}
}];
}}
failureBlock:^(NSError* error) {
NSLog(@"failed to enumerate albums:\nError: %@", [error localizedDescription]);}];
[picker dismissViewControllerAnimated:YES completion:NULL];
}
答案 0 :(得分：1)
要意识到的是，与ALAssetsLibrary相关联的块是异步。因此，像这样的代码不起作用：
if (group!=nil) {
[group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
if(result){
[tmpAssets addObject:result];
}else{
*stop = YES;
}
}];
}
self.assets = tmpAssets;
＆＃34; last＆＃34;行self.assets = tmpAssets，在块之前运行 - 因此tmpAssets永远无法正确设置。
对于与ALAssetsLibrary调用关联的所有块，情况也是如此。
一旦掌握了这一点，您就可以重新构建代码，以便一切按照正确的顺序进行。事实上，事实证明这是通过枚举块的额外传递的用途。正如我在书中写的那样：
我最初对这个奇怪的块枚举行为感到困惑，但有一天它的原因在于它闪现出来：这些块都被称为异步（在主线程上），意思是你的其他代码已经完成了运行，所以你有一个额外的通过块作为你第一次做某事的机会，你可能会收集到的所有数据。以前的通行证。
所以你看，你希望你的代码结构更像这样：
if (group!=nil) {
[group enumerateAssetsUsingBlock:^(ALAsset *result, NSUInteger index, BOOL *stop) {
if(result){
[tmpAssets addObject:result];
}else{
self.assets = tmpAssets; // <--- !!!!
// and now proceed to whatever the _next_ step is...!
}
}];
}
所以，你看，你完全走错了路。摆脱所有您的通知，并根据这些新知识完全重构您的所有代码。一旦你理解了这个关于它是如何工作的这个简单但很难记录的事实，ALAssetsLibrary是完全一致的。