collection view perform batch update

collection view perform batch update

// Compare old data model with new data model and perform batch update for collection view

/**
 *  Perform batch update on collection view, use old stories and new stories to make comparison
 *  groupId property of story is used for comparison
 *
 *  PRE: make sure collection view's data model is update with newArray
 *  
 *  After animation completed, visible cells' view are updated
 *
 *  @param oldArray Arrary of old stories, contains Story object
 *  @param newArray Arrary of new stories, contains Story object
 */
- (void)performBatchUpdateFromOldDataModel:(NSArray *)oldArray  toNewDataModel:(NSArray *)newArray
{
    // For the first time, when collection view is empty, call reloadData
    if (oldArray.count == 0)
    {
        [self.collectionView reloadData];
        return;
    }
    
    // Prepare groupIds for old stories and new stories, use id to compare
    NSMutableArray *oldStoryIds = [[NSMutableArray alloc] initWithCapacity:oldArray.count];
    for (Story *story in oldArray)
    {
        [oldStoryIds addObject:story.groupId];
    }
    NSMutableArray *newStoryIds = [[NSMutableArray alloc] initWithCapacity:newArray.count];
    for (Story *story in newArray)
    {
        [newStoryIds addObject:story.groupId];
    }

    // If there's no changes, don't update
    if ([oldStoryIds isEqualToArray:newStoryIds])
    {
        return;
    }
    
    // Arrays stores indic to be processed
    NSMutableArray *deletedIndice = [[NSMutableArray alloc] init]; // [1, 2, 3 ...]
    NSMutableArray *insertedIndice = [[NSMutableArray alloc] init]; // [1, 2, 3 ...]
    NSMutableArray *movedIndice = [[NSMutableArray alloc] init]; // [[1, 3], [4, 2] ...]
    
    // Old vs New, to initialize deleted indice and moved indice
    // Init deleted indice, existing in Old but not in New
    // Init moved indice, existing in both but indice are different
    NSUInteger oldStoryIdsCount = [oldStoryIds count];
    for (NSUInteger oldIdIndex = 0; oldIdIndex < oldStoryIdsCount; oldIdIndex++)
    {
        NSNumber *oldId = [oldStoryIds objectAtIndex:oldIdIndex];
        if (![newStoryIds containsObject:oldId])
        {
            // Deleted
            [deletedIndice addObject:@(oldIdIndex)];
        }
        else
        {
            NSMutableArray *OldIndexToNewIndex = [[NSMutableArray alloc] initWithCapacity:2];
            NSUInteger newIdIndex = [newStoryIds indexOfObject:oldId];
            // Moved
            if (newIdIndex != oldIdIndex)
            {
                OldIndexToNewIndex[0] = @(oldIdIndex);
                OldIndexToNewIndex[1] = @(newIdIndex);
                [movedIndice addObject:OldIndexToNewIndex];
            }
        }
    }
    
    // New vs Old, to initialize inserted indice
    // Init inserted indice, existing in New but not in Old
    NSUInteger newStoryIdsCount = [newStoryIds count];
    for (NSUInteger newIdIndex = 0; newIdIndex < newStoryIdsCount; newIdIndex++)
    {
        NSNumber *newId = [newStoryIds objectAtIndex:newIdIndex];
        if (![oldStoryIds containsObject:newId])
        {
            // Inserted
            [insertedIndice addObject:@(newIdIndex)];
        }
    }
    
    // Perform collection view animation
    __weak CurrentReadsViewController *weakSelf = self;
    [self.collectionView performBatchUpdates:^{
        // Delete
        NSArray *deletedIndexPaths = [deletedIndice map:^id(NSNumber *indexToDelete, NSUInteger index) {
            return [NSIndexPath indexPathForItem:[indexToDelete unsignedIntegerValue] inSection:0];
        }];
        [weakSelf.collectionView deleteItemsAtIndexPaths:deletedIndexPaths];
        
        // Insert
        NSArray *insertedIndexPaths = [insertedIndice map:^id(NSNumber *indexToInsert, NSUInteger index) {
            return [NSIndexPath indexPathForItem:[indexToInsert unsignedIntegerValue] inSection:0];
        }];
        [weakSelf.collectionView insertItemsAtIndexPaths:insertedIndexPaths];

        // Move
        for (NSArray *oldIndexToNewIndexPair in movedIndice)
        {
            NSUInteger oldIndex = [oldIndexToNewIndexPair[0] unsignedIntegerValue];
            NSUInteger newIndex = [oldIndexToNewIndexPair[1] unsignedIntegerValue];
            NSIndexPath *fromIndexPath = [NSIndexPath indexPathForItem:oldIndex inSection:0];
            NSIndexPath *toIndexPath = [NSIndexPath indexPathForItem:newIndex inSection:0];
            [weakSelf.collectionView moveItemAtIndexPath:fromIndexPath toIndexPath:toIndexPath];
        }
    } completion:nil];
}