C#、Xamarin表單：初始化時未引發自定義TextChangedEvent

我正在創建一個 Xamarin.Forms MVVM 應用程式（僅使用 Android），只要它們的文本屬性具有特定值，就需要將某些按鈕標為紅色。（目的：提醒用戶按下按鈕并選擇一個值，這將更改按鈕文本屬性并因此洗掉紅色輪廓）

為此，我創建了以下檔案：

CButton擴展默認值的自定義按鈕Button：

    public class CButton : Button
    {
        // this Hides the Default .Text-Property
        public string Text 
        {
            get => base.Text;
            set
            {
                base.Text = value;
                TextChangedEvent(this, new EventArgs());
            }

        }
        // The Raised Event
        protected virtual void TextChangedEvent(object sender, EventArgs e)
        {
            EventHandler<EventArgs> handler = TextChanged;
            handler(sender, e);
        }
        public event EventHandler<EventArgs> TextChanged;
    }

自定義行為利用了引發的 TextChangedEvent

    public class ButtonValBehavior : Behavior<CButton>
    {
        protected override void OnAttachedTo(CButton bindable)
        {
            bindable.TextChanged  = HandleTextChanged;
            base.OnAttachedTo(bindable);
        }
        void HandleTextChanged(object sender, EventArgs e)
        {
            string forbidden = "hh:mm|dd.mm.yyyy";
            if (forbidden.Contains((sender as CButton).Text.ToLower()))
            {
                //Do when Button Text = "hh:mm" || "dd.mm.yyyy"
                (sender as CButton).BorderColor = Color.Gray;
            }
            else
            {
                //Do whenever Button.Text is any other value
                (sender as CButton).BorderColor = Color.FromHex("#d10f32");
            }
        }
        protected override void OnDetachingFrom(CButton bindable)
        {
            bindable.TextChanged -= HandleTextChanged;
            base.OnDetachingFrom(bindable);
        }
    }

ViewModel 的相關部分如下所示：

    public class VM_DIVI : VM_Base
    {
        public VM_DIVI(O_BasisProtokoll base)
        {
            Base = base;
        }

        private O_BasisProtokoll _base = null;
        public O_BasisProtokoll Base
        {
            get => _base;
            set
            {
                _base = value;
                OnPropertyChanged();
            }
        }

        Command _datePopCommand;
        public Command DatePopCommand
        {
            get
            {
                return _datePopCommand ?? (_datePopCommand = new Command(param => ExecuteDatePopCommand(param)));
            }
        }
        void ExecuteDatePopCommand(object param)
        {
            //launch popup
            var p = new PP_DatePicker(param);
            PopupNavigation.Instance.PushAsync(p);
        }
    }

.xmal 看起來如下（b是xmlns命名空間的）：

    <b:CButton x:Name="BTN_ED_Datum"
               Text="{Binding Base.ED_datum, Mode=TwoWay}" 
               Grid.Column="1"
               Command="{Binding DatePopCommand}" 
               CommandParameter="{x:Reference BTN_ED_Datum}">
        <b:CButton.Behaviors>
            <b:ButtonValBehavior/>
        </b:CButton.Behaviors>
    </b:CButton>

This solution works fine whenever the input is caused by user interaction. However, when a Value is assigned during the initialization of the Page no red outline is created, in fact the TextChangedEvent isn't raised. By using breakpoints I noticed that during initialization the Text Property of CButton is never set, eventhough it actually will be in the view.

Despite fiddling around with my solution I cannot make this work on initialization. I tried to work around this issue by outlining every button by default in their constructor, however this will outline every button red, even when their text value doesn't require them to be.

How can I achieve my initial goal?

Many thanks in advance!

uj5u.com熱心網友回復：

已經有一段時間了，但如果我沒記錯的話，我最終做的是：

• 將Text我的自定義按鈕的新-Property更改為CText和
• 確保我已Mode=TwoWay為任何元素激活，默認情況下未啟用它。（在 msdn 上查找系結模式了解更多資訊）
• 制作CText一個可系結的屬性CButton

我的自定義按鈕現在看起來如下：

using System;
using System.Collections.Generic;
using System.Text;
using Xamarin.Forms;

namespace EORG_Anton.Model
{
    public class CButton : Button
    {
        public static readonly BindableProperty CTextProperty = 
            BindableProperty.Create(nameof(CText), 
            typeof(string), 
            typeof(CButton), 
            default(string), 
            BindingMode.TwoWay,
            propertyChanged: OnTextChanged);
        private static void OnTextChanged(BindableObject bindable, object oldValue, object newValue)
        {
            var control = (CButton)bindable;
            var value = (string)newValue;
            control.CText = value;
        }
        public string CText
        {
            get => base.Text;
            set
            {
                base.Text = value;
                TextChangedEvent(this, new EventArgs());
            }

        }
        protected virtual void TextChangedEvent(object sender, EventArgs e)
        {
            EventHandler<EventArgs> handler = TextChanged;
            handler(sender, e);
        }
        public event EventHandler<EventArgs> TextChanged;
    }
}

標籤：button events xamarin.forms data-binding behavior

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