Core Data with SwiftUI Tutorial by Paul Hudson

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Attention

This is a learning note, which means that there may be faults and misunderstanding in it. You are more than welcome to correct me by commenting.

L1: Why does .self work for ForEach?

Hashable is essential for SwiftUI to differentiate and locate each view component.

For more infomation: HASH是什么？“哈希值校验”又是怎么回事儿？

L2: Creating NSManagedObject subclasses

extension Movie {
  @nonobjc public class func fetchRequest() -> NSFetchRequest<Movie> {
    return NsFetchReauest<Movie>(entityName： "Movie")
  }
  // We can make this not optional by simply deleting question mark
  @NSManaged public var title: String?
  @NSManaged public var director: String
  @NSManaged public var year: Int16
  
  // In case of nil collapsing, we can add
  public var wrappedTitle:String {
    title ?? "Unknown Title"
  }
}

extension Movie ：Identifiable {
}

Core data only load data when is actually needed (lazy load). This is good.

L3: Conditional saving of NSManagedObjectContext

struct ContentView: View {
  @Environment(\. managedobjectContext) var moc
  var body:some View {
    Button("Save"){
      // moc.hasChanges help us be sure about saving data only when changed
      if moc.hasChanges {
        try? moc.save()
    }
    }
  }
}

L4: Ensuring Core Data objects are unique using constraints

To make sure some specific attributes have unique values, add attribute name to Constraints in the right column.

L5: Filtering @FetchRequest using NSPredicate

@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"universe == 'Star Wars' ")) var ships: FetchedResults<Ship>

@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"universe == %@ ", "Star Wars")) var ships: FetchedResults<Ship>

// Find all data whose inital character is before F in alphabet
@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"universe < %@ ", "F")) var ships: FetchedResults<Ship>

// IN
@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"universe IN %@ ", ["Aliens","Firefly"])) var ships: FetchedResults<Ship>

// BEGINSWITH is Case sensitive, while BEGINSWITH[c] is not
@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"name BEGINSWITH %@ ", "E")) var ships: FetchedResults<Ship>
@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"name BEGINSWITH %@ ", "e")) var ships: FetchedResults<Ship>
@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"name BEGINSWITH[c] %@ ", "e")) var ships: FetchedResults<Ship>

// CONTAINS
@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"name CONTAINS[c] %@ ", "e")) var ships: FetchedResults<Ship>

// NOT
@FetchRequest(sortDescriptors: []， predicate: NSPredicate(format:"NOT name CONTAINS[c] %@ ", "e")) var ships: FetchedResults<Ship>

// AND, OR

L6: Dynamically filtering @FetchRequest with SwiftUI

// Parent View 
import CoreData
import SwiftUI
struct ContentView: View {
  @Environment(\.managedObjectContext) var moc
  @State private var lastNameFilter = "A"
  
  var body:some View {
    VStack {
      FilteredList(filter: lastNameFilter)
      
      Button("Change filter to S"){
        lastNameFilter = "S"
      }
    }
  }
}

// Child View
import SwiftUI
struct FilteredList: View {
  @FetchRequest var fetchRequest: FetchedResults<Singer>
  
  var body: some View {
    List(fetchRequest, id:\.self){singer in
      Text("\(singer.wrappedFirstName) \(singer. wrappedLastName)")
  }
   
  init(filter： String){
    // The underscore is very essential
    _fetchRequest = FetchRequest<Singer> (sortDescriptors: [], predicate:NSPredicate(format: "lastName BEGINSWITH %@",filter))
  }
}

To handle any kind of Entity and filters, use following codes:

// Parent View 
import CoreData
import SwiftUI
struct ContentView: View {
  @Environment(\.managedObjectContext) var moc
  @State private var lastNameFilter = "A"
  
  var body:some View {
    VStack {
      FilteredList(filterKey："lastName", filterValue: lastNameFilter){ (singer:Singer) in
        Text("\(singer.wrappedFirstName) \(singer. wrappedLastName)")
        
      }
      
      Button("Change filter to S"){
        lastNameFilter = "S"
      }
    }
  }
}

// Child View
import CoreData
import SwiftUI
struct FilteredList<T: NSManagedObject, Content: View>: View {
  @FetchRequest var fetchRequest: FetchedResults<T>
  let content: (T) -> Content
  
  var body: some View {
    List(fetchRequest, id:\.self){item in
      self.content(item)
    }
  }
  init(filterKey：String, filterValue: String, @ViewBuilder content: @escaping (T) -> Content){
    // The underscore is very essential
    _fetchRequest = FetchRequest<T> (sortDescriptors: [], predicate:NSPredicate(format: "%K BEGINSWITH %@",filterKey, filterValue))
  }
}

L7: One-to-many relationships with @FetchRequest and SwiftUI

// Core Data files 
extension Country{
   // Omit some codes here
  
  // shortName must be unique in order to specify the relationship
   @NSManaged public var shortName:String?
   @NSManaged public var candy: NSSet?
   
   public var candyArray: [Candy]{
     let set = candy as? Set<Candy> ?? []
     
     return set.sorted{
       // Remember to declare `wrappedName` in `Candy`
       $0.wrappedName < $1.wrappedName
     }
   }
 }

// View
// Please follow the video step by step