c# - XAML 框架从未针对表单行为调用 OnDetachingForm

标签 c# xaml xamarin

我正在尝试找到一种使用表单行为的好方法，以确保用户只能在输入控件中键入所需的输入。我的问题是 xaml 框架永远不会调用 OnDetachingFrom 方法。这会导致内存丢失，因为我订阅了 Entry 控件的 TextChanged 事件来修改其行为，但无法取消订阅。

我试图找到一种“干净”的方法来跟踪哪些控件需要在页面从堆栈中弹出时清除其行为(我必须使用主导航页面来跟踪自己)，但我能想到的一切命名每个控件，将控件添加到代码后面的页面上的集合中，使用“Clear”方法在页面上实现一个接口(interface)，该方法对集合中的每个控件执行 xxx.Behaviors.Clear() ，调用 OnDetachingForm每个控件的方法。

这看起来有点可怕，与“干净”相反。我希望有人知道更好的方法。由于这样的设计疏忽，我从来没有真正喜欢过 XAML 和 MVVM。希望我所有的谷歌搜索都错过了一些东西。

对于我的行为，我几乎是从 Microsoft 教程页面复制的。
https://learn.microsoft.com/en-us/xamarin/xamarin-forms/app-fundamentals/behaviors/creating 

namespace StoreTrak.Behaviors
{
    public class IntegerValidationBehavior : Behavior<Entry>
    {
        protected override void OnAttachedTo(Entry bindable)
        {
            if (bindable != null)
                bindable.TextChanged += OnEntryTextChanged;
            base.OnAttachedTo(bindable);
        }

        /// <summary>
        /// This NEVER gets called by the XAML framework.
        /// </summary>
        /// <param name="bindable"></param>
        protected override void OnDetachingFrom(Entry bindable)
        {
            if (bindable != null)
                bindable.TextChanged -= OnEntryTextChanged;
            base.OnDetachingFrom(bindable);
        }

        private static void OnEntryTextChanged(object sender, TextChangedEventArgs args)
        {
            if (string.IsNullOrEmpty(args.NewTextValue))
            {
                ((Entry)sender).Text = "0";
                return;
            }

            if (!int.TryParse(args.NewTextValue, out int x))
                ((Entry)sender).Text = args.OldTextValue;
        }
    }
}

然后我只是做了一个基本的实现。

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
             xmlns:vm="clr-namespace:StoreTrak.ViewModels"
             xmlns:behaviors="clr-namespace:StoreTrak.Behaviors"
             x:Class="StoreTrak.Pages.TestPage">
    <ContentPage.BindingContext>
        <vm:TestViewModel />
    </ContentPage.BindingContext>
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>

        <Label Text="Field 1" Grid.Row="0" Grid.Column="0" />
        <Entry Text="{Binding Field1}" Grid.Row="0" Grid.Column="1" />

        <Label Text="Field 2" Grid.Row="2" Grid.Column="0" />
        <StackLayout Grid.Row="2" Grid.Column="1" Margin="0" Padding="0">
            <Entry x:Name="Field2" Text="{Binding Field2}">
                <Entry.Behaviors>
                    <behaviors:IntegerValidationBehavior />
                </Entry.Behaviors>
            </Entry>
            <Label Text="Error number 1" TextColor="Red" FontSize="Small" IsVisible="False" />
            <Label Text="Error number 2" TextColor="Red" FontSize="Small" IsVisible="True" />
            <Label Text="Error number 3" TextColor="Red" FontSize="Small" IsVisible="False" />
        </StackLayout>
        <Label Text="Field 3" Grid.Row="3" Grid.Column="0" />
        <Entry Text="{Binding Field3}" Grid.Row="3" Grid.Column="1" />
    </Grid>

那么我如何才能触发该事件呢？我能找到的唯一方法是调用

Field2.Behaviors.Clear();

但是我在哪里调用它呢？我不能将它放在 OnApprearing 中，因为它可以在导航到新页面时调用，然后当再次显示此页面时，行为就会消失。

/// <summary>
/// can be called when a new page is added to the stack
/// </summary>
protected override void OnDisappearing()
{
    base.OnDisappearing();
}

所以当页面从堆栈中删除时我需要清除它。我怎么知道什么时候会发生这种情况？我能找到的唯一方法是在主页上监听主导航页面上的事件。
我还创建了一个接口(interface) IPageDispose 并在我的页面上实现它。

    public partial class App : Application
    {
        public App()
        {
            InitializeComponent();

            MainPage = new NavigationPage(new Pages.MainPage());
            if (MainPage is NavigationPage page)
            {
                page.Popped += Page_Popped;
                page.PoppedToRoot += Page_PoppedToRoot;
            }
        }

        /// <summary>
        /// https://www.johankarlsson.net/2017/08/popped-pages-in-xamarin-forms.html
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void Page_PoppedToRoot(object sender, NavigationEventArgs e)
        {
            if (e is PoppedToRootEventArgs args)
            {
                foreach(Page page in args.PoppedPages)
                    Page_Popped(sender, new NavigationEventArgs(page));
            }
        }

        private void Page_Popped(object sender, NavigationEventArgs e)
        {
            if (e.Page is IPageDispose ipd)
            {
                ipd.Dispose();
            }
        }

        protected override void OnStart()
        {
        }

        protected override void OnSleep()
        {
        }

        protected override void OnResume()
        {
        }

        public async static void HandleError(Exception ex)
        {
            Logger.Entry(ex);
            await Application.Current.MainPage.Navigation.PushModalAsync(new Pages.LogPages.LogPage(ex));
        }
    }
}

namespace StoreTrak.Pages
{
    public interface IPageDispose
    {
        void Dispose();
    }
}


public partial class TestPage : ContentPage, IPageDispose
{
    public TestPage()
    {
        InitializeComponent();
    }

    public void Dispose()
    {
        // How do I know which control to clear?
        // give each one a name and hardcode the Clear method?
        throw new NotImplementedException();
    }

现在，我如何知道要清除哪些控件？我做了这个复杂的过程，仍然需要命名控件并在后面的代码中跟踪它们？这比在后面的代码中使用事件监听器更好吗？

    public class _BasePage : ContentPage
    {
        protected void IntegerValidation_TextChanged(object sender, TextChangedEventArgs e)
        {
            if (string.IsNullOrEmpty(e.NewTextValue))
            {
                ((Entry)sender).Text = "0";
                return;
            }

            if (!int.TryParse(e.NewTextValue, out int x))
                ((Entry)sender).Text = e.OldTextValue;
        }

    }
}

最佳答案

感谢您更新问题。

关于OnDetachingFrom方法，我们可以看一下这个官方文档。

The OnDetachingFrom method is fired when a behavior is removed from a control, and is used to perform any required cleanup such as unsubscribing from an event to prevent a memory leak. However, behaviors are not implicitly removed from controls unless the control's Behaviors collection is modified by a Remove or Clear method.

我们会看到，除非调用Remove和Clear方法，否则OnDetachingFrom通常不会被触发。

But where do I call it? I can't put it in OnApprearing because it can be called when a new page is navigated to, then when this page is shown again the behaviors are gone.

我们可以在页面的OnDisappearing方法上调用clear方法，但是还需要在进入页面时添加行为。

例如:

protected override void OnAppearing()
{
    base.OnAppearing();
    myentry.Behaviors.Add(new NumericValidationBehavior());
}


protected override void OnDisappearing()
{
    base.OnDisappearing();
    myentry.Behaviors.Clear();
}

=================================更新=============== =======================

您可以使用Style 和 Trigger 对于 Entry，则无需通过编码为每个 Entry 添加/删除行为。

创建一个NumericValidationBehavior类:

public class NumericValidationBehavior : Behavior<Entry>
{
    public static readonly BindableProperty AttachBehaviorProperty = 
        BindableProperty.CreateAttached ("AttachBehavior", typeof(bool), typeof(NumericValidationBehavior), false, propertyChanged: OnAttachBehaviorChanged);

    public static bool GetAttachBehavior (BindableObject view)
    {
        return (bool)view.GetValue (AttachBehaviorProperty);
    }

    public static void SetAttachBehavior (BindableObject view, bool value)
    {
        view.SetValue (AttachBehaviorProperty, value);
    }

    static void OnAttachBehaviorChanged (BindableObject view, object oldValue, object newValue)
    {
        var entry = view as Entry;
        if (entry == null) {
            return;
        }

        bool attachBehavior = (bool)newValue;
        if (attachBehavior) {
            entry.Behaviors.Add (new NumericValidationBehavior ());
        } else {
            var toRemove = entry.Behaviors.FirstOrDefault (b => b is NumericValidationBehavior);
            if (toRemove != null) {
                entry.Behaviors.Remove (toRemove);
            }
        }
    }

    protected override void OnAttachedTo (Entry entry)
    {
        entry.TextChanged += OnEntryTextChanged;
        base.OnAttachedTo (entry);
    }

    protected override void OnDetachingFrom (Entry entry)
    {
        entry.TextChanged -= OnEntryTextChanged;
        base.OnDetachingFrom (entry);
    }

    void OnEntryTextChanged (object sender, TextChangedEventArgs args)
    {
        double result;
        bool isValid = double.TryParse (args.NewTextValue, out result);
        ((Entry)sender).TextColor = isValid ? Color.Default : Color.Red;
    }
}

然后在ContentPage.Xaml中:

<?xml version="1.0" encoding="UTF-8"?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" xmlns:local="clr-namespace:WorkingWithBehaviors;assembly=NumericValidationBehaviorStyle" x:Class="WorkingWithBehaviors.NumericValidationPage" Title="XAML" IconImageSource="xaml.png">
    <ContentPage.Resources>
        <ResourceDictionary>
            <Style TargetType="Entry">
                <Style.Triggers>
                    <Trigger TargetType="Entry"
                             Property="IsFocused"
                             Value="True">
                       
                        <Setter Property="local:NumericValidationBehavior.AttachBehavior"
                                Value="true" />
                        <!-- multiple Setters elements are allowed -->
                    </Trigger>
                    <Trigger TargetType="Entry"
                             Property="IsFocused"
                             Value="False">
                        
                        <Setter Property="local:NumericValidationBehavior.AttachBehavior"
                                Value="False" />
                    </Trigger>
                </Style.Triggers>
            </Style>
        </ResourceDictionary>
    </ContentPage.Resources>
    
    <StackLayout Padding="10,50,10,0">
        <Label Text="Red when the number isn't valid" FontSize="Small" />
        <Entry Placeholder="Enter a System.Double" />
    </StackLayout>
    
</ContentPage>

关于c# - XAML 框架从未针对表单行为调用 OnDetachingForm，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/63766598/

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