Xamarin TimePicker propertychanged 事件在加载时触发

Question

我正在为 android 开发一个 Xamarin 应用程序，使用 Xamarin 表单来创建我的布局。我遇到了一个问题，我的时间选择器在我的视图单元加载时触发。

我正在做的是创建一个列表视图，然后将项目模板设置为我的自定义视图单元格模板。在我的视图单元格模板中，我正在创建一个 TimePicker 并将 PropertyChanged 事件绑定到它。我还使用从数据库检索的信息设置 TimePicker.Time 属性。此时似乎发生的是我的 PropertyChanged 事件针对将显示在列表视图中的每个项目被触发。这会导致不需要的多个数据库调用。

有没有办法在属性实际更改之前停止调用 PropertyChanged 事件？

我的代码如下。

public class MyCell : ViewCell
{
private readonly TimePicker _myTimePicker;
public MyCell()
{
   _myTimePicker = new TimePicker()
        {
            HorizontalOptions = LayoutOptions.EndAndExpand
        };

        _myTimePicker.SetBinding(TimePicker.TimeProperty, "StartTime");
        _myTimePicker.PropertyChanged += MyTimePicker_PropertyChanged;

        var viewLayout = new StackLayout()
        {
            HorizontalOptions = LayoutOptions.StartAndExpand,
            Orientation = StackOrientation.Horizontal,
            Padding = new Thickness(20),
            Children = { _myTimePicker }
        };
        View = viewLayout;
}

private void MyTimePicker_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {
        if (e.PropertyName == TimePicker.TimeProperty.PropertyName && _myTimePicker.Time != TimeSpan.MinValue)
        {
            var dataAccess = new DataAccessLayer();
            dataAccess.Update(myTimePicker.Time);
        }
    }
}

我不确定为什么属性changed 事件会被触发多次，以及在我实际选择时间之前如何停止它触发。任何帮助将不胜感激。

下面是我用于显示列表视图的表单的代码。我所有的 xaml 都是在代码中定义的。 'MyCell' 使用来自“_dataAccess.GetTimes()”的 return 值。

public class TimeDetails : ContentPage
{
    private ListView _listView;
    private readonly DataAccessLayer _dataAccess = new DataAccessLayer();

    protected override void OnAppearing()
    {
        _listView = new ListView
        {
            RowHeight = 80,
            SeparatorColor = Color.Blue,
            SeparatorVisibility = SeparatorVisibility.Default
        };

        _listView.ItemsSource = _dataAccess.GetTimes();
        _listView.ItemTemplate = new DataTemplate(typeof(MyCell));
        _listView.ItemSelected += ListView_ItemSelected;

        Content = new StackLayout
        {
            VerticalOptions = LayoutOptions.FillAndExpand,
            Children = { _listView }
        };
    }
}

return类型的_dataAcess.GetTimes()是TaskTime的List。 TaskTime 模型如下所示。

public class TaskTime
{
    public int Id { get; set; }
    public string Task { get; set; }
    public TimeSpan StartTime { get; set; }
}

Answer 1

class MyCell : ViewCell
{
    private readonly TimePicker _myTimePicker;
    public MyCell()
    {
        _myTimePicker = new TimePicker()
        {
            HorizontalOptions = LayoutOptions.EndAndExpand
        };

        _myTimePicker.SetBinding(TimePicker.TimeProperty, "StartTime");
        _myTimePicker.PropertyChanged += MyTimePicker_PropertyChanged;
        _myTimePicker.Focused += _myTimePicker_Focused;

        var viewLayout = new StackLayout()
        {
            HorizontalOptions = LayoutOptions.StartAndExpand,
            Orientation = StackOrientation.Horizontal,
            Padding = new Thickness(20),
            Children = { _myTimePicker }
        };
        View = viewLayout;
    }

    bool myTimePickerSelected;
    private void _myTimePicker_Focused(object sender, FocusEventArgs e)
    {
        myTimePickerSelected = true;
    }

    private void MyTimePicker_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e)
    {

        if (e.PropertyName == TimePicker.TimeProperty.PropertyName && myTimePickerSelected)
        {
            //var dataAccess = new DataAccessLayer();
            //dataAccess.Update(myTimePicker.Time);
        }
    }
}

Answer 2

使用聚焦事件和非聚焦事件的更简单的解决方案是：

public class CustomTimePicker : TimePicker
{
    public event EventHandler TimeChanged;

    private TimeSpan StartValue { get; set; }

    public CustomTimePicker ()
    {
        this.Focused += OnFoused;
        this.Unfocused += OnUnfocused;
    }

    private void OnFoused(object sender, FocusEventArgs e)
    {
        StartValue = this.Time;
    }

    private void OnUnfocused(object sender, FocusEventArgs e)
    {
        if (StartValue != this.Time)
        {
            TimeChanged?.Invoke(this, e);
        }
    }
}

Answer 3

我看到的是 OnAppearing() 在所有初始化完成后完成。也许您可以引入一个布尔变量，然后在 PropertyChanged() 方法中使用它。这是一个技巧，因为您必须从 ContentPage 开始，并在初始化完成时将一个值传递给您的控件。我刚刚看到，CellView 中有 OnAppearing() 方法，这样一切都在您的控制中 class.

Xamarin TimePicker propertychanged 事件在加载时触发

Xamarin TimePicker propertychanged event fired on load

android

xamarin

xamarin.forms

timepicker

propertychanged