###Note from marko628: this is Udacity's code from the previous version of the iOS Persistence course. I've updated it to Swift 3. The comments below apply to the original Swift 2 version.

###The Color Collection

The color collection app demonstrates two techniques that will be important for the Virtual Tourist app.

• Using UICollectionView with NSFetchedResultsController
• Maintaining a set of "selected" cells

Take a minute to run the app. It adds cells with randomly generated colors when the + button is tapped. When users touch the color cells they are "selected" to be deleted. Tapping the "Remove Selected Colors" button deletes the cells that have been selected.

##Collections and Fetched Results Controllers.

The NSFetchedResultsControllerDelegate methods are designed to work smoothly with UITableViews. They are slightly more complicated to use with Collection Views. The difference is a method on collections views named performBatchUpdates.

This method accepts a closure that should make a complete set of changes. To make this work smoothly with the fetched results controller delegate, we need to store a batch of changes in our view controller, then execute them all inside this closure.

In this example code we accomplish this by keeping three arrays of index paths:

• insertedIndexPaths
• deletedIndexPaths
• updatedIndexPaths

Whenever changes are made to the Color objects in Core Data three steps will take place:

1. The fetched results controller will invoke controllerWillChangeContent on the view controller. We will respond by creating three empty arrays.
2. The fetched results controller will invoke controller(: didChangeObject:atIndexPath:forChangeType:newIndexPath). That represents a change, and we will insert the index path into the appropriate array.
3. The fetched results controller will invoke controllerDidChangeContent on the view controller. We respond by performing the changes saved in the arrays inside the performBatchUpdates method.

Read through these steps in the ColorsViewController and see if that prompts disucssion for the forums.

##Maintaining a set of selected cells

The view controller allows the user to tap color cells to toggle them between being selected and unselected. Selected cells can be deleted using the "Remove Selected Colors" button. The effect is acheived by keeping an array of index paths named selectedIndexes. Whenever a cell is tapped its index path is added to this array (or removed, if it was already present). This toggles the "selected" state for the cell.

The array is used in configureCell to show some visual indication that the cell is selected. In this app the alpha is set to 0.05 on selected cells. They are "grayed out" so to speak. But they could have checkmarks, or borders, or any number of other visual indications.