Xamarin表单MessagingCenter订阅两次

我在product page viewmodel的listview中单击一个产品项目，以显示一个弹出窗口（使用rg.plugin popup）来选择一个产品变体。选择变体后，我将使用messagingcenter从变体弹出页面viewmodel发送所选变体到产品页面，在产品页面viewmodel构造函数中订阅。在那里工作正常。当我导航到上一页，然后返回到此产品页，以便向产品添加一个或多个变体时 与以前选择的产品相同，Messagingcenter subscribe呼叫了两次，产品价值增加了两次。尝试在产品页面中以“出现”和“消失”方式取消订阅。仍呼叫两次吗？如何解决这个问题

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

调用弹出窗口：

               var result = await dataService.Get_product_variant(store_id, product_id);
                if (result.status == "success")
                {
                    ind_vis = false;
                    OnPropertyChanged("ind_vis");

                    App.Current.Properties["product_variant_result"] = result;
                    App.Current.Properties["cartitems"] = purchaselist;
                    App.Current.Properties["selected_product"] = product_List2 ;
                    await PopupNavigation.Instance.PushAsync(new Popup_variant());                   
                }

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

弹出视图模型：发送消息

        public Popup_variant_vm()
        {
            Radio_btn = new Command<Product_variant_list2>(Radio_stk_tapped);
            product_variant_list = new List<Product_variant_list2>();
            purchaselist = new ObservableCollection<Product_list2>();                       
            show_variants();            
        }

        internal void Confirm_variant()
        {
            if(App.Current.Properties.ContainsKey("selected_variant"))
            {
                 MessagingCenter.Send<Popup_variant_vm, object>(this, "selected_variant", App.Current.Properties["selected_variant"]); //Message send from popup to product page                    
            }
            else
            {
                DependencyService.Get<IToast>().LongAlert("Please select any size");
            }
        }

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

public Popup\u variant\u vm（）
{
Radio_btn=新命令（Radio_stk_分接）；
产品变量列表=新列表（）；
purchaselist=新的ObservableCollection（）；
显示变量（）；
}
内部无效确认_变量（）
{
if（App.Current.Properties.ContainsKey（“选定的变量”））
{
MessagingCenter.Send（这是“选定的_变体”，App.Current.Properties[“选定的_变体”]）；//从弹出窗口发送到产品页面的消息
}
其他的
{
DependencyService.Get（）.LongAlert（“请选择任意大小”）；
}
}

产品页面视图模型：此处订阅..从上一页导航到此页面时调用两次

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

public Store_page（）
{
初始化组件（）；
}
出现时受保护的覆盖无效（）
{
base.OnAppearing（）；
var vm=新存储\页面\虚拟机（）；
vm.Navigation=导航；
BindingContext=vm；
MessagingCenter.Unsubscribe（此“选定的变体”）；
MessagingCenter.Subscribe（此“选定变量”，异步（发件人，选定变量）=>
{ 
var vm1=作为存储页面的BindingContext\u vm；
vm1？.Addcart2（选定的_变量）；//调用两次
});            
} 

在产品cs页面中取消订阅

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

protected override void OnDisappearing()
        {
            var vm = BindingContext as store_page_vm;
            vm?.Save_cart();    
            MessagingCenter.Unsubscribe<Popup_variant_vm>(this, "selected_variant");    
        }

protected override void OnDisappearing（）
{
var vm=BindingContext作为存储\页面\虚拟机；
vm？.Save_cart（）；
MessagingCenter.Unsubscribe（此“选定的变体”）；
}

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

---

请尝试此操作，并确保您不会多次呼叫Subscribe。

您的取消订阅操作应如下所示，并且应能正常工作：

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

  MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");

MessagingCenter.Unsubscribe（这是“选定的变量”）；

我的解决方案：

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

把取消订阅的句子放进订阅正文

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

 MessagingCenter.Subscribe<object, string>(this, "IdSearch", (sender, arg) =>
            {
              

                listView.ItemsSource = arg;

                MessagingCenter.Unsubscribe<object, string>(this, "IdSearch");

            }, BindingContext);

MessagingCenter.Subscribe（此“IdSearch”，（发件人，参数）=>
{
listView.ItemsSource=arg；
取消订阅（此“IdSearch”）；
}，绑定上下文）；

我已在订阅服务器的应用程序中创建了静态计数器变量，我已完成以下操作：

    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    } 

公共静态类常量
{
公共静态int msgcentersubscribebeconter{get；set；}=0；
}
MessagingCenter.Subscribe（这是“hello”，（发件人，arg）=>
{
常量.msgcentersubscribeccounter++；
if（arg.Equals（“hello”）&&Constants.msgcentersubscribebeconter==1）
{
//在这里处理你的逻辑
}
});

---

重置OnDisappearing（）方法中的计数器，从中调用Send。

当前页面有多少个实例calling@G.hakim

public Store_page（）{InitializeComponent（）；}

仅

BindingContext作为Store_page_vm

调用多个times@G.hakim忘记在

MessagingCenter.Unsubscribe中添加
对象（这是“选定的变量”）
…现在我已经添加了它作为答案，请看它仍然不起作用，订户会被呼叫两次。
    public Store_page()
    {
       InitializeComponent();
    }
    protected override void OnAppearing()
    {
        base.OnAppearing();

        var vm = new store_page_vm();
        vm.Navigation = Navigation;
        BindingContext = vm;

        MessagingCenter.Unsubscribe<Popup_variant_vm, object>(this, "selected_variant");
        MessagingCenter.Subscribe<Popup_variant_vm, object>(this, "selected_variant",async (sender, selected_variant) =>
        { 
            var vm1 = BindingContext as store_page_vm;
            vm1?.Addcart2(selected_variant);// called twice
        });            
    }