For the first time, our data model is going to be remarkably simple. There's really not a lot to keep track of here. In a way, our data model is exactly replicated by what is in persistent storage—the images themselves.
Just like in prior tasks, we do have a document collection model that reads all the images in persistent storage and lets our document view interact with them. There has been almost no change in this particular model (save for the name), so we won't cover it here.
What we will cover is not quite a data model, but still important. When a user taps the Edit button, we want them to be able to select multiple pictures for a batch operation (such as delete). To do this, we need to keep track of which images are selected, and which ones aren't.
The model itself is so simple that it doesn't actually have its own code file. It's just an array combined with a single property that indicates if we are in selection mode or not. This is what it...