Swift 错误消息 Cannot use mutating member on immutable value: 'self' is immutable

Question

我正在尝试在我的视图模型中递增和整数变量，它是一个 var 而不是一个 let，而且递增它的函数是结构中的一个变异函数，但我在我使用的视图中不断收到该错误消息函数。

import Foundation
import SwiftUI


struct Activity: Identifiable, Codable, Equatable, Hashable {
    
    var id = UUID()
    let title: String
    let description: String
    let date: Date
    var completedCounts: Int
    
    var createdDate: String {
        return String(date.formatted(date: .abbreviated, time: .shortened))
    }
    
    func displayCounts(counts: Int) -> String {
        if counts > 1 {
            return "\(counts) times"
        } else {
            return "\(counts) time"
        }
    }
    
    mutating func increaseCount() {
        self.completedCounts += 1
    }
    
}


class Project: ObservableObject {
    @Published  var activities: [Activity] 

************ 我添加模型的视图如下所示 *************

               Button("Save") {
                    if completed == true {
                        completedCounts += 1
                    }
                    let newActivity = Activity(title: title, description: description, date:   date, completedCounts: completedCounts)
                    if title.count > 2 && description.count > 2 {
                        project.addActivity(activity: newActivity)
                        dismiss()

*************** 我想使用模型的视图 *********************

                    if completed == true {
                            activities.increaseCount()