为什么我的 SwiftUI 应用程序中没有更新 ObservedObject 数组？

Question

我正在使用 SwiftUI，试图了解 ObservableObject 的工作原理。我有一组 Person 对象。当我将新的Person 添加到数组中时，它会重新加载到我的视图中，但是如果我更改现有Person 的值，它不会重新加载到视图中。

//  NamesClass.swift
import Foundation
import SwiftUI
import Combine

class Person: ObservableObject,Identifiable{
    var id: Int
    @Published var name: String
    
    init(id: Int, name: String){
        self.id = id
        self.name = name
    }
}

class People: ObservableObject{
    @Published var people: [Person]
    
    init(){
        self.people = [
            Person(id: 1, name:"Javier"),
            Person(id: 2, name:"Juan"),
            Person(id: 3, name:"Pedro"),
            Person(id: 4, name:"Luis")]
    }
}

struct ContentView: View {
    @ObservedObject var mypeople: People
    
    var body: some View {
        VStack{
            ForEach(mypeople.people){ person in
                Text("\(person.name)")
            }
            Button(action: {
                self.mypeople.people[0].name="Jaime"
                //self.mypeople.people.append(Person(id: 5, name: "John"))
            }) {
                Text("Add/Change name")
            }
        }
    }
}

如果我取消注释该行以添加新的Person (John)，Jaime 的名字会正确显示，但是如果我只是更改名称，则视图中不会显示。

恐怕我做错了什么，或者我不明白ObservedObjects 是如何处理数组的。

Answer 1

您可以使用结构而不是类。由于结构体的值语义，对人名的更改被视为对 Person 结构体本身的更改，并且此更改也是对人员数组的更改，因此@Published 将发送通知并重新计算视图主体。

import Foundation
import SwiftUI
import Combine

struct Person: Identifiable{
    var id: Int
    var name: String

    init(id: Int, name: String){
        self.id = id
        self.name = name
    }

}

class Model: ObservableObject{
    @Published var people: [Person]

    init(){
        self.people = [
            Person(id: 1, name:"Javier"),
            Person(id: 2, name:"Juan"),
            Person(id: 3, name:"Pedro"),
            Person(id: 4, name:"Luis")]
    }

}

struct ContentView: View {
    @StateObject var model = Model()

    var body: some View {
        VStack{
            ForEach(model.people){ person in
                Text("\(person.name)")
            }
            Button(action: {
                self.mypeople.people[0].name="Jaime"
            }) {
                Text("Add/Change name")
            }
        }
    }
}

或者（不推荐），Person 是一个类，所以它是一个引用类型。当它发生变化时，People 数组保持不变，因此主题不会发出任何内容。但是，您可以手动调用它，让它知道：

Button(action: {
    self.mypeople.objectWillChange.send()
    self.mypeople.people[0].name="Jaime"    
}) {
    Text("Add/Change name")
}

Answer 2

我认为这个问题有一个更优雅的解决方案。您可以为列表行创建一个自定义视图，而不是尝试将 objectWillChange 消息传播到模型层次结构中，这样每个项目都是一个 @ObservedObject：

struct PersonRow: View {
    @ObservedObject var person: Person

    var body: some View {
        Text(person.name)
    }
}

struct ContentView: View {
    @ObservedObject var mypeople: People

    var body: some View {
        VStack{
            ForEach(mypeople.people){ person in
                PersonRow(person: person)
            }
            Button(action: {
                self.mypeople.people[0].name="Jaime"
                //self.mypeople.people.append(Person(id: 5, name: "John"))
            }) {
                Text("Add/Change name")
            }
        }
    }
}

通常，为 List/ForEach 中的项目创建自定义视图可以监控集合中的每个项目的更改。

Answer 3

对于那些可能会觉得有帮助的人。这是@kontiki 答案的更通用方法。

这样您就不必为不同的模型类类型重复自己

import Foundation
import Combine
import SwiftUI

class ObservableArray<T>: ObservableObject {

    @Published var array:[T] = []
    var cancellables = [AnyCancellable]()

    init(array: [T]) {
        self.array = array

    }

    func observeChildrenChanges<T: ObservableObject>() -> ObservableArray<T> {
        let array2 = array as! [T]
        array2.forEach({
            let c = $0.objectWillChange.sink(receiveValue: { _ in self.objectWillChange.send() })

            // Important: You have to keep the returned value allocated,
            // otherwise the sink subscription gets cancelled
            self.cancellables.append(c)
        })
        return self as! ObservableArray<T>
    }


}

class Person: ObservableObject,Identifiable{
    var id: Int
    @Published var name: String

    init(id: Int, name: String){
        self.id = id
        self.name = name
    }

} 

struct ContentView : View {
    //For observing changes to the array only. 
    //No need for model class(in this case Person) to conform to ObservabeObject protocol
    @ObservedObject var mypeople: ObservableArray<Person> = ObservableArray(array: [
            Person(id: 1, name:"Javier"),
            Person(id: 2, name:"Juan"),
            Person(id: 3, name:"Pedro"),
            Person(id: 4, name:"Luis")])

    //For observing changes to the array and changes inside its children
    //Note: The model class(in this case Person) must conform to ObservableObject protocol
    @ObservedObject var mypeople: ObservableArray<Person> = try! ObservableArray(array: [
            Person(id: 1, name:"Javier"),
            Person(id: 2, name:"Juan"),
            Person(id: 3, name:"Pedro"),
            Person(id: 4, name:"Luis")]).observeChildrenChanges()

    var body: some View {
        VStack{
            ForEach(mypeople.array){ person in
                Text("\(person.name)")
            }
            Button(action: {
                self.mypeople.array[0].name="Jaime"
                //self.mypeople.people.append(Person(id: 5, name: "John"))
            }) {
                Text("Add/Change name")
            }
        }
    }
}

Answer 4

ObservableArray 非常有用，谢谢！这是一个更通用的版本，它支持所有集合，当您需要对通过一对多关系（被建模为集合）间接的 CoreData 值做出反应时，这很方便。

import Combine
import SwiftUI

private class ObservedObjectCollectionBox<Element>: ObservableObject where Element: ObservableObject {
    private var subscription: AnyCancellable?
    
    init(_ wrappedValue: AnyCollection<Element>) {
        self.reset(wrappedValue)
    }
    
    func reset(_ newValue: AnyCollection<Element>) {
        self.subscription = Publishers.MergeMany(newValue.map{ $0.objectWillChange })
            .eraseToAnyPublisher()
            .sink { _ in
                self.objectWillChange.send()
            }
    }
}

@propertyWrapper
public struct ObservedObjectCollection<Element>: DynamicProperty where Element: ObservableObject {
    public var wrappedValue: AnyCollection<Element> {
        didSet {
            if isKnownUniquelyReferenced(&observed) {
                self.observed.reset(wrappedValue)
            } else {
                self.observed = ObservedObjectCollectionBox(wrappedValue)
            }
        }
    }
    
    @ObservedObject private var observed: ObservedObjectCollectionBox<Element>

    public init(wrappedValue: AnyCollection<Element>) {
        self.wrappedValue = wrappedValue
        self.observed = ObservedObjectCollectionBox(wrappedValue)
    }
    
    public init(wrappedValue: AnyCollection<Element>?) {
        self.init(wrappedValue: wrappedValue ?? AnyCollection([]))
    }
    
    public init<C: Collection>(wrappedValue: C) where C.Element == Element {
        self.init(wrappedValue: AnyCollection(wrappedValue))
    }
    
    public init<C: Collection>(wrappedValue: C?) where C.Element == Element {
        if let wrappedValue = wrappedValue {
            self.init(wrappedValue: wrappedValue)
        } else {
            self.init(wrappedValue: AnyCollection([]))
        }
    }
}

它可以如下使用，例如我们有一个包含 Set 的类 Fridge，尽管没有观察每个项目的任何子视图，但我们的视图需要对后者的变化做出反应。

class Food: ObservableObject, Hashable {
    @Published var name: String
    @Published var calories: Float
    
    init(name: String, calories: Float) {
        self.name = name
        self.calories = calories
    }
    
    static func ==(lhs: Food, rhs: Food) -> Bool {
        return lhs.name == rhs.name && lhs.calories == rhs.calories
    }
    
    func hash(into hasher: inout Hasher) {
        hasher.combine(self.name)
        hasher.combine(self.calories)
    }
}

class Fridge: ObservableObject {
    @Published var food: Set<Food>
    
    init(food: Set<Food>) {
        self.food = food
    }
}

struct FridgeCaloriesView: View {
    @ObservedObjectCollection var food: AnyCollection<Food>

    init(fridge: Fridge) {
        self._food = ObservedObjectCollection(wrappedValue: fridge.food)
    }

    var totalCalories: Float {
        self.food.map { $0.calories }.reduce(0, +)
    }

    var body: some View {
        Text("Total calories in fridge: \(totalCalories)")
    }
}