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Day 58

Saurabh Jamadagni
11th October, 2022

NSPredicate

  • Predicates are like tests for objects in the entity.
  • The object will only be shown if it passes the tests.
  • Basically, it's like a SQL query.
  • Use NSPredicate(format: "query inside")
  • But if the query has quotation marks then it can get tricky.
  • Thus use %@ as placeholder.
NSPredicate(format: "universe == %@", "Star Wars"))
  • We can also make use of operations such as IN, BEGINSWITH and NOT. Also, for more complex predicates, use AND.

Tip: Look at NSCompoundPredicate for complex predicates.

Dynamically filtering FetchRequest

  • Create a new SwiftUI view. Let's say it's FilterList
  • Give it a fetch request property.
@FetchRequest var fetchRequest: FetchedResults<Singer>
  • Add an initializer similar to the below.
init(filter: String) {
    _fetchRequest = FetchRequest<Singer>(sortDescriptors: [], predicate: NSPredicate(format: "lastName BEGINSWITH %@", filter))
}
  • Notice the underscore before fetchrequest.
  • When we update a state value, it means now use this value for the state struct.
  • Giving the underscore means use a new state struct entirely.

Note: To learn how to create a FilterList view that works for any entities, i.e. using generics, come back to this day and look into the 'Want to go further' section.

One-to-Many relationships

  • How to establish relationships between entities?
  • In the CoreData editor, under Attributes there will be Relationships. You can also choose between 'To One' or 'To Many' depending on the type of relation in the inspector.
  • Select both entities > Codegen > Manual/None. Editor > Create NSManagedObject Subclass.
  • NSSet is not similar to a SwiftUI Set. So try to use an as operator else nil coalesce it.

Thanks for reading! Let's connect on Twitter 👋