A student that completes this project shows that they can:

• understand and explain what a collection view is along with common scenarios for its use
• implement UICollectionViewDataSource methods to populate a collection view from model data
• implement and customize basic layouts using UICollectionViewFlowLayout
• implement custom UICollectionViewCells
• implement UICollectionViewDelegate method to react to selecting a collection view cell
• understand and explain the purpose of UserDefaults and the kind of information it should be used to store
• define keys for specific values stored in UserDefaults
• use UserDefaults to store and retrieve preferences

This application allows a user to add photos from their photo library to a collection of photos in a UICollectionView. It will also let the user select a theme and it will persist that preference between launches of the app using UserDefaults

Please look at the screen recording below to know what the finished project should look like:

Please fork and clone this repository. This repository does not have a starter project, so create one inside of the cloned repository folder. Be regularly committing and pushing your code as appropriate.

1. Create a swift file called "Photo.swift".
2. Create a struct Photo in it with the following properties:
   • An imageData: Data variable.
   • A title: String variable.
3. Adopt the `Equatable Protocol.

1. Create a swift file called "PhotoController.swift".
2. Create a class PhotoController.
3. Add a photos varable of type [Photo], and set its initial value to an empty array.
4. Add a "Create" method that initializes a new instance of Photo and appends it to the photos array.
5. Add an "Update" method that takes in Photo,Data, and String parameters.

You will now make a helper class that will contain functionality related to the user's theme preference and give access to that theme information throughout the application. It will use UserDefaults to save and load this preference.

1. Create a "ThemeHelper.swift" file. Add a ThemeHelper class inside of it. 2. Create a string constant called themePreferenceKey. This will be used whenever you need to save the user's theme preference or get it from UserDefaults.
2. Create a function called setThemePreferenceToDark(). Using the set method of UserDefaults (remember that in order to access set, you have to use the standard property on UserDefaults), set the string "Dark" as the value, and the themePreferenceKey as the key.
3. Create another function that does the same thing, but with a different color. Call the function setThemePreferenceToYourColorHere.
4. Finally, create a computed property called themePreference of type String?. This should simply return the string value stored from either of the two methods you wrote above using UserDefaults string(forKey: ...) method. The method should return an optional string because if the user's theme preference hasn't been saved, the value returned from UserDefaults will be nil.
5. In the initializer for this class (you will have to call it), check if the themePreference value is nil. If it is, then call one of the setThemePreference functions you just created. This will make it so the first time the user opens the app a preference will be set for them until they choose to change it.

Keep in mind that the implementation of this class is not the absolute best. You haven't been introduced to a few concepts that would facilitate and keep this class a bit cleaner, especially if this were to be expanded into a full theme helper class for an actual application. Don't worry though, we'll get there!

The layout of this application uses the master-detail pattern. It also includes a view controller that will allow the user to change the application's theme.

Delete the view controller scene that comes with the Main.storyboard.

1. Add a UICollectionViewController scene. Embed it in a navigation controller and set the navigation controller as the initial view controller.
2. Create a Cocoa Touch subclass of UICollectionViewController called PhotosCollectionViewController. Set the collection view controller scene's class to it.
3. Resize the collection view cell however you want, then add a label and an image view to it.
4. Create a Cocoa Touch subclass of UICollectionViewCell called PhotosCollectionViewCell. Set the cell's class to it, then make outlets from the label and image view.
5. Add a bar button item in the left of the navigation bar and set its title to "Select Theme". Add another bar button item in the right of the navigation bar and set its "System Item" to "Add".

Consider changing the collection view's section insets in the Size Inspector so the cells aren't right at the edge of the screen. This is purely for aesthetic purposes.

1. Add a UIViewController scene and create a Cocoa Touch subclass of UIViewController called PhotoDetailViewController.
2. Create the following variables in this subclass:
   • photoController: PhotoController?
   • photo: Photo?
   • themeHelper: ThemeHelper?
3. Set the view controller scene's class to PhotoDetailViewController.
4. Add a UIImageView, a UIButton, and a UITextField. Change the button's title to "Add Photo"
5. Add a UINavigationItem then a UIBarButtonItem to the right of the navigation bar. Set its "System Item" to "Save".
6. Create outlets from the image view and text field.
7. Create an action from the "Add PHoto" button called addPhoto, and an action from the bar button item called savePhoto.
8. Create a "Show" segue from the "Add" bar button item in the collection view scene to this view controller. Give it an identifier.
9. Create another "Show" segue to this view controller from the collection view cell. Give it an identifier.

1. Add another UIViewController scene and create a Cocoa Touch subclass of UIViewController called ThemeSelectionViewController.
2. Create a themeHelper: ThemeHelper? variable in this subclass.
3. Set the view controller scene's class to ThemeSelectionViewController.
4. Add a label that tells the user to select a theme that they would like to use.
5. Add two buttons. One should day "Dark" for the dark theme, and the other should say the other theme that you chose to make in the ThemeHelper such as "Blue".
6. Add an action from each button. Call them selectDarkTheme and selectYourColorHereTheme.
7. Create a "Present Modally" segue from the "Select Theme" bar button item in the collection view scene to this view controller. Give it an identifier.

1. Create a photo: Photo? variable.
2. Create an updateViews() function. You should be familiar with this function.
3. Add a didSet property observer to the photo variable. Call updateViews inside of it.

1. Create a constant called photoController and set its value to a new instance of PhotoController.
2. Create a constant called themeHelper and set its initial value to a new instance of ThemeHelper.
3. Fill in the numberOfItemsInSection There should be as many cells are there are photos in the photoController's photos array.
4. Fill in the cellForItemAt method. This should pass an instance of Photo to the custom cell. Note: You will need to cast the cell as PhotoCollectionViewCell.

Now, we're going to use the ThemeHelper class to let us know what theme the user has selected and make a function that will change the appearance of the view controller based on the theme.

5. Create a function called setTheme(). This function should:

   • Get the current themePreference from the themeHelper. Make sure that it has a value, or return out of the function.
   • Based on the value, change the collection view's background color depending on whether the theme is dark or the other color you selected.

6. Implement the prepareForSegue. You should have three segues to handle. - The segue from the "Add" bar button should pass the themeController, photoController, and the photo.

   • The segue from the cell should pass the the themeController and the photoController.
   • The segue from the "Select Theme" bar button item should pass the themeController.

This view controller will allow the user to (re)select their desired theme.

1. In the selectDarkTheme action, call the themeHelper's setThemePreferenceToDark method. Then call dismiss(animated: Bool, completion: ...). Set animated to true and completion to nil.
2. Do the same thing for the other action, but calling the other setThemePreference method in the themeHelper.

1. Create a setTheme function. This should do the same thing as the setTheme method in your collection view controller, except that it should change the view controller's view's background color instead.
2. Create an updateViews function that takes the values in the photo (if it isn't nil) and sets them in the appropriate UI element. You will need to use the UIImage(data: Data) initializer to convert the photo's imageData to a UIImage. You can then put that UIImage in the image view.
3. The "Save" bar button item's action should either update the photo if it has a value, or create a new instance of photo using the methods in the photoController. "Pop" the view controller afterwards.
4. The addImage action should present a UIImagePickerController that allows the user to select an image to add to the Photo object.
   • Note: Make sure you request authorization to access the photo library, and add the "Privacy - Photo Library Usage Description" key-value pair in the info.plist.
   • Note: You will need to adopt the UIImagePickerControllerDelegate and implement the didFinishPickingMediaWithInfo method to get the image the user selects, then dismiss the image picker. You will also need to adopt the UINavigationControllerDelegate.

• Add persistence using Codable, PropertyListEncoder, PropertyListDecoder, and FileManager to save a plist of all the user's Memory objects.
• Experiment with the methods that UICollectionViewDelegateFlowLayout has to customize your collection view's layout.