Our app works pretty well to save pictures, but as soon as the app quits, all the photos are lost. We need to add a way to save the photos permanently. Our refactoring of the code allows us to work exclusively within PhotoStore now.

Before we write any code, we have to decide how we will store the photos permanently. There are many ways we could choose to save the photos, but one of the easiest ways is to save it to the filesystem, which is what we conceived in our conception phase. Every app is provided with a documents directory that is automatically backed up by the operating system as a part of normal backups. We can store our photos here as files named after the label the user gives them. To avoid any problems with duplicate labels, where we would have multiple files named after the same thing, we can nest every file inside a subdirectory named after the time the photo is saved. The timestamp will always be unique because we will never save two photos at the...