init(from:to:changedObjects:)

init(from:to:changedObjects:)

init(from:to:changedObjects:) Initializer of PHFetchResultChangeDetails Creates a change details object that summarizes the differences between two fetch results.

convenience init(from fromResult: PHFetchResult<ObjectType>, to toResult: PHFetchResult<ObjectType>, changedObjects: [ObjectType])

Parameters

• fromResult: A fetch result to be treated as the “before” state in the resulting change details object.
• toResult: A fetch result to be treated as the “after” state in the resulting change details object.
• changedObjects: An collection of objects to manually note as changed between the two fetch results.

Return Value

A change details object.

Discussion

Typically, you use the PHChange class to retrieve PHFetchResultChangeDetails objects describing any changes that have occurred since you performed a fetch, but you can also use this method to get a change details object that summarizes the difference between two arbitrary PHFetchResult objects. In this case, Photos cannot automatically determine when the same objects are present in both fetch results but those objects’ content has changed, so the changedIndexes and changedObjects properties are empty.

However, you can use the changedObjects parameter to manually note objects as changed. For example, consider a view controller that maintains a base fetch result representing a list of albums, and a transient collection that filters the list according to the user’s search terms. For this workflow, you might use a method like the following:

Swift:

func filteredResult(for fetchResult: PHFetchResult<PHCollection>) -> PHFetchResult<PHCollection> {    var filteredAlbums = [PHCollection]()
    fetchResult.enumerateObjects { collection, _, _ in
        if let title = collection.localizedTitle, let searchText = self.searchBar.text, title.hasPrefix(searchText) {
            filteredAlbums.append(collection)
        }
    }
    let filteredList = PHCollectionList.transientCollectionList(with: filteredAlbums, title: nil)
    return PHCollection.fetchCollections(in: filteredList, options: nil)
}

Objective-C:

- (PHFetchResult *)filteredResultForFetchResult:(PHFetchResult *)fetchResult {
    NSMutableArray *filteredAlbums = [NSMutableArray arrayWithCapacity:fetchResult.count];
    for (PHCollection *collection in fetchResult) {
        if ([collection.localizedTitle hasPrefix:self.searchBar.text]) {
            [filteredAlbums addObject:collection];
        }
    }
    PHCollectionList *filteredList = [PHCollectionList transientCollectionListWithCollections:filteredAlbums title:nil];
    return [PHCollection fetchCollectionsInCollectionList:filteredList options:nil];
}

Because transient collections are not stored in the photo library, Photos cannot provide change details for fetch results against a transient collection. The photoLibraryDidChange(:)) method provides change details for the base fetch result, which you can use to request change details for the filtered fetch result:

Swift:

func photoLibraryDidChange(_ change: PHChange) {
    // This method may be called on a background queue; use the main queue to update the UI.
    DispatchQueue.main.async {
        guard let fetchResultChangeDetails = change.changeDetails(for: self.baseFetchResult)
            else { return }

        // Get the new base fetch result and filter it to get a new filtered result.
        self.baseFetchResult = fetchResultChangeDetails.fetchResultAfterChanges
        let previousFilteredResult = self.filteredFetchResult!
        self.filteredFetchResult = self.filteredResult(for: self.baseFetchResult)

        // Get the changed objects between the original and new base fetches
        // and use them for marking changes between the filtered fetches.
        let filteredChangeDetails = PHFetchResultChangeDetails(
            from: previousFilteredResult,
            to: self.filteredFetchResult,
            changedObjects: fetchResultChangeDetails.changedObjects)

        // Use the filtered change details to update the UI.
        self.updateView(for: filteredChangeDetails)
    }
}

Objective-C:

- (void)photoLibraryDidChange:(PHChange *)changeInstance {
    // This method may be called on a background queue; dispatch to the main queue to update the UI.
    dispatch_async(dispatch_get_main_queue(), ^{
 
        PHFetchResultChangeDetails *fetchResultChangeDetails = [changeInstance changeDetailsForFetchResult:self.baseFetchResult];
        if (fetchResultChangeDetails) {
 
            // Get the new base fetch result and filter it to get a new filtered result.
            self.baseFetchResult = fetchResultChangeDetails.fetchResultAfterChanges;
            PHFetchResult *previousFilteredFetchResult = self.filteredFetchResult;
            self.filteredFetchResult = [self filterFetchResult:self.baseFetchResult];
 
            // Get the changed objects between the original and new base fetches,
            // and use them for marking changes between the filtered fetches.
            NSArray *changedObjects = [fetchResultChangeDetails changedObjects];
            PHFetchResultChangeDetails *filteredChangeDetails = [PHFetchResultChangeDetails changeDetailsFromFetchResult:previousFilteredFetchResult toFetchResult:self.filteredFetchResult changedObjects:changedObjects];
 
            // Use the filtered change details to update the UI.
            [self updateViewForChangeDetails:filteredChangeDetails];
        }
    });
}

Because you provide the changedObjects array from the original change details when calling the init(from:to:changedObjects:)) method, Photos can provide a summary of differences between the filtered fetch results that includes changedIndexes and changedObjects information. You can then use this information for efficiently updating your app’s interface — for an example of how to do this, see PHPhotoLibraryChangeObserver.