更新彈出視圖模型中的屬性不會更新UI

在標題中，我有一個問題，即更新彈出視圖模型中的屬性不會更新 UI。我使用來自 xamarin 社區工具包的彈出視窗。我正在使用執行此任務的命令：

        async Task ShowPopup()
    {
        MessagingCenter.Send(AnimeGroupObservable, "AnimeGroups");
        Shell.Current.ShowPopup(new MediaListGroupsPopup());
    }

它發送帶有有效負載的訊息并顯示彈出視窗。這是彈出視圖模型：

    using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using System.Text;
using System.Windows.Input;
using OtakuApp.Models;
using Xamarin.Forms;

namespace OtakuApp.ViewModels
{
    class MediaListGroupsPopupViewModel : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;

        void OnPropertyChanged([CallerMemberName] string name = "")
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }

        public ObservableCollection<Group> _AnimeGroups = new ObservableCollection<Group>();
        public ObservableCollection<Group> AnimeGroups
        {
            get => _AnimeGroups;
            set
            {
                if (_AnimeGroups == value)
                    return;

                _AnimeGroups = value;
                OnPropertyChanged();
            }
        }

        public String _label;

        public String label
        {
            get => _label;
            set
            {
                if (value == _label)
                    return;

                _label = value;
                OnPropertyChanged();
            }
        }


        public MediaListGroupsPopupViewModel()
        {
            MessagingCenter.Subscribe<ObservableCollection<Group>>(this, "AnimeGroups", (AnimeGroupObservable) =>
            {
                Console.WriteLine(AnimeGroupObservable[0].Name);
                label = AnimeGroupObservable[1].Name;

                MessagingCenter.Unsubscribe<ObservableCollection<Group>>(this, "AnimeGroups");
            });
        }
    }
}

我計劃有一個小的標簽集合視圖可供選擇。但是現在我正在努力更新一個標簽只是為了測驗目的，所以你可以想象我已經嘗試了集合視圖但它沒有用。在代碼中手動將 _label 設定為某些內容表明系結有效。只是因為某種原因沒有更新。

彈出 xaml 檔案：

    <?xml version="1.0" encoding="utf-8" ?>
<xct:Popup
    x:Class="OtakuApp.Popups.MediaListGroupsPopup"
    xmlns="http://xamarin.com/schemas/2014/forms"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:xct="http://xamarin.com/schemas/2020/toolkit"
    Size="300,300">
    <StackLayout>
        <Label Text="{Binding label}" />
    </StackLayout>

</xct:Popup>

所以現在我有兩個問題：

1. 標簽不更新。它系結到具有 INotifyPropertyChanged 的??屬性
2. Weirdly this subscription happens only the second time (and after that too, just not the first time) I open up a popup. Is this because it's in the constructor? If yes, what's the correct way to deal with it?

Also a small question - I have unsubscribe at the end of subscription. When I didn't have it and I printed out AnimeGroupObservable[0].Name, the first time it was printed one time, the second time I open up the popup two times etc. Is the unsubscribe at the end the correct way to fix this?

uj5u.com熱心網友回復：

由于您將單個引數傳遞給單個頁面，因此使用建構式比（這很好，但對于這種情況來說太MessagingCenter過分了）要簡單得多

創建頁面時，在建構式中傳遞引數

Shell.Current.ShowPopup(new MediaListGroupsPopup(AnimeGroupObservable));

然后修改頁面建構式接受引數

public MediaListGroupsPopup(ObservableCollection<Group> groups)
{
  // you did't show how you create your VM, but I assume it's something like this
  this.BindingContext = new MediaListGroupsPopupViewModel(groups);
}

然后修改你的 VM 建構式

public MediaListGroupsPopupViewModel(ObservableCollection<Group> groups)
{
  label = groups[1].Name;
}

如果你真的只使用一個string值，你可以只傳遞它而不是整個ObservableCollection

標籤：xaml xamarin xamarin.forms mvvm xamarin-community-toolkit

上一篇：如何為listViewUWPC#中的每一項自定義UI或DataTemplate

下一篇：基本控制元件（如Button、TextBlock）是否有xaml檔案實作？