import UIKit
import Combine

//以下是ObservableObject协议
/*
 public protocol ObservableObject : AnyObject {

     /// The type of publisher that emits before the object has changed.
     associatedtype ObjectWillChangePublisher : Publisher = ObservableObjectPublisher where Self.ObjectWillChangePublisher.Failure == Never

     /// A publisher that emits before the object has changed.
     var objectWillChange: Self.ObjectWillChangePublisher { get }
 }
 */

class ViewModel: ObservableObject{
    @Published var publishedString: String = "old published value"
    @Published var publishedNumber: Int = 0
    var number = 0
    var str = "test"
}

let vm: ViewModel = .init()

//objectWillChanged的默认类型是ObjectWillChangePublisher，而ObjectWillChangePublisher的Output是Void
//另外需要注意，改变value的值也会触发objectWillChange
vm.objectWillChange.sink { (output) in
    print("object will change \(output)")
}

//1
vm.publishedString = "new published value"

//官方文档对于Published的描述是：A type that publishes a property marked with an attribute.
//从测试来看，新的订阅是会收到最新的值的
vm.$publishedString.sink { (value) in
    print("test published: \(value)")
}

//2
vm.publishedString = "hi published value"

//3
//改变多个Published属性时，都会触发objectWillChanged
vm.publishedNumber = 1

//改变非Published属性不会触发objectWillChange
print("----------------------")
vm.number = 1
vm.str = "tttt"

打印：

object will change ()
test published: new published value
object will change ()
test published: hi published value
object will change ()
----------------------