每 x 秒刷新 xaml 页面并保持当前扩展器状态

Question

我有一个 xamarin 项目。有一个带有扩展器列表的滚动视图。

我喜欢每 x 秒刷新一次页面，但保持扩展器的状态（isExpanded 布尔值）。

如何检查扩展器（或标签、按钮等）的状态并在每隔 x 秒刷新一次时保持这些值？

我觉得我必须向我的后台代码函数添加一个参数，类似于点击或点击事件期间的 'object sender'。

在后台代码中，我尝试使用

每 x 秒刷新一次页面

Device.StartTimer(TimeSpan.FromSeconds(x),Updatefunction);

目前它们在页面刷新时都有默认的 isExpanded (false) 状态。

Answer 1

您可以在 viewModel 中添加一个 bool 属性，然后将此属性绑定到 <Expander> 选项卡中的 IsExpanded="{Binding Expand1Opened}"。当用户单击 Expander 时，IsExpanded 将取决于 Expand1Opened 属性的值。无论每 x 秒刷新一次页面，它都会保持当前的扩展器状态。我为 Expander 添加 Command，如果单击 Expander，Expand1Opened 属性的值将在 ViewModel 中更改。

 <RefreshView IsRefreshing="{Binding IsRefreshing}"
                     RefreshColor="Teal"
                     Command="{Binding RefreshCommand}">
            <ScrollView>
                <StackLayout>
                    <Expander IsExpanded="{Binding Expand1Opened}" Command="{Binding Expand1OpenedCommand}">
                    <Expander.Header>
                        <Label Text="List1"
               FontAttributes="Bold"
               FontSize="Medium" />
                    </Expander.Header>
                    <Grid Padding="10">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto" />
                            <ColumnDefinition Width="Auto" />
                        </Grid.ColumnDefinitions>
                        <FlexLayout Direction="Row"
                            Wrap="Wrap"
                            AlignItems="Center"
                            AlignContent="Center"
                            BindableLayout.ItemsSource="{Binding Items}"
                            BindableLayout.ItemTemplate="{StaticResource ColorItemTemplate}" />
                    </Grid>
                </Expander>

                    <Expander IsExpanded="{Binding Expand2Opened}" Command="{Binding Expand2OpenedCommand}">
                    <Expander.Header>
                        <Label Text="List2"
               FontAttributes="Bold"
               FontSize="Medium" />
                    </Expander.Header>
                    <Grid Padding="10">
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto" />
                            <ColumnDefinition Width="Auto" />
                        </Grid.ColumnDefinitions>
                        <FlexLayout Direction="Row"
                            Wrap="Wrap"
                            AlignItems="Center"
                            AlignContent="Center"
                            BindableLayout.ItemsSource="{Binding Items}"
                            BindableLayout.ItemTemplate="{StaticResource ColorItemTemplate}" />
                    </Grid>
                </Expander>
                </StackLayout>

            </ScrollView>
        </RefreshView>

这是视图模型。我有两个 <Expander>，所以我添加了两个属性 Expand1Opened 和 Expand2Opened，并添加了两个名为 Expand1OpenedCommand 和 Expand2OpenedCommand 的 Command，如果<Expander>被点击，Expand1OpenedCommand会被调用，然后Expand1Opened的值会改变，如果refreshview被刷新，Expand1Opened的值不会改变，所以expander的状态将被保留。

 public class MainPageViewModel : INotifyPropertyChanged
    {
        const int RefreshDuration = 2;
        int itemNumber = 1;
        readonly Random random;
        bool isRefreshing;

        public bool IsRefreshing
        {
            get { return isRefreshing; }
            set
            {
                isRefreshing = value;
                OnPropertyChanged();
            }
        }

        bool expand1Opened = false;
        public bool Expand1Opened
        {
            get { return expand1Opened; }
            set
            {
                expand1Opened = value;
                OnPropertyChanged();
            }
        }

        bool expand2Opened=false;
        public bool Expand2Opened
        {
            get { return expand2Opened; }
            set
            {
                expand2Opened = value;
                OnPropertyChanged();
            }
        }
        public ObservableCollection<Item> Items { get; private set; }

        public ICommand RefreshCommand => new Command(async () => await RefreshItemsAsync());

        public ICommand Expand1OpenedCommand { get; set; }
        public ICommand Expand2OpenedCommand { get; set; }
        public MainPageViewModel()
        {
            random = new Random();
            Items = new ObservableCollection<Item>();

            Expand1OpenedCommand = new Command((() =>
            {
                expand1Opened = !expand1Opened;
            }));
            Expand2OpenedCommand = new Command((() =>
            {
                expand2Opened = !expand2Opened;
            }));


            AddItems();
        }

        void AddItems()
        {
            for (int i = 0; i < 1; i++)
            {
                Items.Add(new Item
                {
                    Color = Color.FromRgb(random.Next(0, 255), random.Next(0, 255), random.Next(0, 255)),
                    Name = $"Item {itemNumber++}",
                    Isfavourite = false

                }); 
            }
        }

        async Task RefreshItemsAsync()
        {
            IsRefreshing = true;
            await Task.Delay(TimeSpan.FromSeconds(RefreshDuration));
            AddItems();
            IsRefreshing = false;
        }

        #region INotifyPropertyChanged

        public event PropertyChangedEventHandler PropertyChanged;

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

        #endregion
    }
}

这是运行 GIF。

每 x 秒刷新 xaml 页面并保持当前扩展器状态

Refresh xaml page every x seconds and keep the current expander state

xaml

xamarin.forms

refresh

expander