Day-33-34-35-36-Core-Data
- IosDatabases (Swift UI) Module 5
Core data is one of the libraries critical for most of the apps I want to build. I have spent the last couple of weeks working my way through Core data and now have a pretty good grasp of what it is and does. I have one more lesson to go through.
Lesson 1: Basics
Benefits
- First Party Framework
- will not be left behind
- works with other Apple APIs/frameworks
- Not 3rd party library
Drawbacks
- Not for remote data storage
- There is learning curve
- Object graph persistent framework
Graph Persistent framework
Core data manages the SQL-lite database for you behind the scenes.
- Object Graph in Memory
- Managed Object Context
- data management layer (handles saving loading etc)
- Core data Persistant container
Methods
- NSManagedObject can be serialized and stored in CoreData
- Classes are defined as Entities
- Properties are Attributes
Core data will automatically generate classes for you using a GUI.
- the file that is labeled filename.xcdatamodeld
- contains the gui for setting up managed classes (Entities).
Lesson 2
- Define entities and attributes
- Generate the classes in the model
- Get a reference to the Core Data persistent container
- Get the managed object context
Codegen property in the Entity Class property section (right side window)
-
Manual/None
- Editor->Create NSManagedObject subclass
- generates 2 files
- Filename+CoreDataClass.swift
- Filename+CoreDataProperties.swift
- at this time try not to add anything to this file.
-
Class definition
- No files are generated, no class extension possible. These files will only be generated in the product
-
Category/Extension
- if you aren’t going to add anything to the second file
Filename+CoreDataProperties.swift then let XCode just generate it in the product for you. The first file will be generated for you to edit Filename+CoreDataClass.swift
How to add core data after the fact
- Add new file → CoreData (Data Model)
- save with the same name as your project
- Add Code to Persistance.swift
- Make sure the persistant container has the right name
- take code from a fresh project that you added core data from the start.
- add Entities and Attributes
Configuring Entities:
Lesson 3
- Aug 21, 2021
- Added
- Delete
- Update
- Save
File Prep
After I started a new project with Core data it added a bunch of code that can be commented out.
Persistence.swift
Comment out the following blocks of code as it only applies to previews as part of the example Apple provides
// static var preview: PersistenceController = {
// let result = PersistenceController(inMemory: true)
// let viewContext = result.container.viewContext
// for _ in 0..<10 {
// let newItem = Item(context: viewContext)
// newItem.timestamp = Date()
// }
// do {
// try viewContext.save()
// } catch {
// // Replace this implementation with code to handle the error appropriately.
// // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
// let nsError = error as NSError
// fatalError("Unresolved error \(nsError), \(nsError.userInfo)")
// }
// return result
// }()
// if inMemory {
// container.persistentStoreDescriptions.first!.url = URL(fileURLWithPath: "/dev/null")
// }
Environment variable
The environment variable at the top of ContentView.swift sets the variable for the managedObjectContext. IN our case, this variable is named viewContext. But should be placed at the top of your view hierarchy.
@Environment(\.managedObjectContext) private var viewContext
How to create a Person object
let p = Person(context: viewContext)
Fetch all Person objects and put into var people
- put this near Environment variables
@FetchRequest(sortDescriptors: []) var people: FetchedResults<Person>
Previews in ContentView
I was able to get the preview working by changing the .preview to .shared.
struct ContentView_Previews: PreviewProvider {
static var previews: some View {
ContentView().environment(\.managedObjectContext, PersistenceController.shared.container.viewContext)
}
}
Save
It is important that after any action (Add, Delete, Update) that the save method is called.
do {
try viewContext.save()
} catch {
// error with saving
}
Add
private func addItem() {
let p = Person(context: viewContext)
p.age = 20
p.name = "Joe"
}
Delete
Where person is a Person objects from the array persons
viewContext.delete(person)
Update
You can modify the attributes in any of the objects in the array of class objects and the call save.
person.name = "Ted"
Lesson 4 Sorting and Filtering with Core Data.
- Aug 28, 2021
- NSSortDescriptor: sorting
- NSPredicate: Filtering
The FetchRequest described above can be sorteded and filtered using NSSortDescriptor and NSPredicate
- sortDescriptors: [NSSortDescriptor(key: “age”, ascending: true)] is an array so you can add more than 1 NSSortDescriptor
- predicate: NSPredicate(format: “age > 9 && age < 16”)) can be used to filter based on attributes in the core data.
- var people is where you are setting the fetch request.
If you have set filters you want to apply to data with no user input you can put those filters/sorts directly in the fetch request as follows.
@FetchRequest(sortDescriptors: [NSSortDescriptor(key: "age", ascending: true)], predicate: NSPredicate(format: "age > 9 && age < 16")) var people: FetchedResults<Person>
More often however, we want to be able to give the user the ability to sort and filter. In order to do this we need to create some @State variables and update those State variables using .onchange modifiers associated with Text or Stacks.
@State var people = [Person]()
@State var filterByText = ""
VStack {
// Textfield for filter
TextField("Filter Text", text: $filterByText) { _ in
// Fetch new data
// Gets activated upon mouse insertion into text field and upon enter
//fetchData()
} // List of people you fetched
List {
ForEach(people) { person in
}
}
.onChange(of: filterByText, perform: { value in
fetchData()
})
} // End of VStack
func fetchData() {
//Create fetch request
let request = Person.pfetchRequest()
//Set sort descriptors and predicates
request.sortDescriptors = [NSSortDescriptor(key: "age", ascending: true)]
// Don't need single quotes for %@
request.predicate = NSPredicate(format: "name contains %@", filterByText)
//Execute the fetch on the main thread so it updates right away
DispatchQueue.main.async {
do {
let results = try viewContext.fetch(request)
// Update the state property
self.people = results
} catch {
}
}
}
``
