如何将嵌套输入控件的焦点事件关联到自定义Border包装控件？

我正基于Border创建一个自定义控件，用于包裹Entry、Editor、DatePicker、TimePicker等各类数据输入控件。需求是：当嵌套控件的焦点状态发生变化时，修改自定义控件的边框颜色。

控件结构包含边框、占位标签以及输入控件本身，示例代码如下：

<controls:CustomBorder
     Placeholder="{localization:Translate SMSHint}"
     BorderColor="{StaticResource Primary}"
     Padding="10,10,10,6"
     CornerRadius="RoundRectangle 10"
     ErrorVisible="False"
     OptionalVisible="False"
     PlaceholderColor="{StaticResource Gray600}">
     <controls:CustomBorder.NestedContent>
          <Entry
               IsFocused="This Is The Event I want to bind to the border control"
               HorizontalOptions="Fill"
               Keyboard="Default"
               Text="{Binding SMSCode}">
               <Entry.Behaviors>
                    <toolkit:MaskedBehavior Mask="XXX XXX" />
               </Entry.Behaviors>
         </Entry>
     </controls:CustomBorder.NestedContent>
</controls:CustomBorder>

自定义Border已定义FocusedBorderColor绑定属性：

public static readonly BindableProperty FocusedBorderColorProperty = BindableProperty.Create(
            nameof(FocusedBorderColor),
            typeof(Color),
            typeof(CustomBorder),
            Colors.Orange,
            defaultBindingMode: BindingMode.TwoWay);

public Color FocusedBorderColor
{
    get => (Color)GetValue(FocusedBorderColorProperty);
    set => SetValue(FocusedBorderColorProperty, value);
}

请问是否有相关教程可帮助理解此类事件处理器的绑定方法？

直接拆解这类事件绑定的实现步骤，不用找外部教程就能搞懂：

• 第一步：给自定义Border加事件监听逻辑
  让CustomBorder自动监听嵌套控件的焦点变化。先确保NestedContent是可绑定属性，然后在属性变化时，给新嵌套控件挂上Focused和Unfocused事件，旧控件要取消订阅防止内存泄漏：

// 定义NestedContent的绑定属性
public static readonly BindableProperty NestedContentProperty = BindableProperty.Create(
    nameof(NestedContent),
    typeof(View),
    typeof(CustomBorder),
    propertyChanged: OnNestedContentChanged);

public View NestedContent
{
    get => (View)GetValue(NestedContentProperty);
    set => SetValue(NestedContentProperty, value);
}

// 属性变化时处理事件订阅/取消订阅
private static void OnNestedContentChanged(BindableObject bindable, object oldValue, object newValue)
{
    var customBorder = (CustomBorder)bindable;
    if (oldValue is View oldView)
    {
        oldView.Focused -= customBorder.OnNestedControlFocused;
        oldView.Unfocused -= customBorder.OnNestedControlUnfocused;
    }
    if (newValue is View newView)
    {
        newView.Focused += customBorder.OnNestedControlFocused;
        newView.Unfocused += customBorder.OnNestedControlUnfocused;
    }
}

• 第二步：写焦点切换时的颜色切换逻辑
  在事件处理方法里，直接根据焦点状态替换BorderColor。另外建议加个DefaultBorderColor属性，用来存储未聚焦时的默认颜色，方便在XAML中配置：

// 补充默认边框颜色的绑定属性
public static readonly BindableProperty DefaultBorderColorProperty = BindableProperty.Create(
    nameof(DefaultBorderColor),
    typeof(Color),
    typeof(CustomBorder),
    Colors.Gray);

public Color DefaultBorderColor
{
    get => (Color)GetValue(DefaultBorderColorProperty);
    set => SetValue(DefaultBorderColorProperty, value);
}

// 聚焦时切换为聚焦状态颜色
private void OnNestedControlFocused(object sender, FocusEventArgs e)
{
    BorderColor = FocusedBorderColor;
}

// 失焦时恢复默认颜色
private void OnNestedControlUnfocused(object sender, FocusEventArgs e)
{
    BorderColor = DefaultBorderColor;
}

• 第三步：调整XAML使用方式
  现在不用在Entry里写IsFocused的绑定了，直接在CustomBorder里配置默认和聚焦状态的颜色即可：

<controls:CustomBorder
     Placeholder="{localization:Translate SMSHint}"
     DefaultBorderColor="{StaticResource Primary}"
     FocusedBorderColor="{StaticResource Orange}"
     Padding="10,10,10,6"
     CornerRadius="RoundRectangle 10"
     ErrorVisible="False"
     OptionalVisible="False"
     PlaceholderColor="{StaticResource Gray600}">
     <controls:CustomBorder.NestedContent>
          <Entry
               HorizontalOptions="Fill"
               Keyboard="Default"
               Text="{Binding SMSCode}">
               <Entry.Behaviors>
                    <toolkit:MaskedBehavior Mask="XXX XXX" />
               </Entry.Behaviors>
         </Entry>
     </controls:CustomBorder.NestedContent>
</controls:CustomBorder>

• 核心逻辑说明
  这类控件的本质是父控件主动监听子控件的事件，通过绑定属性实现UI状态联动。重点要注意子控件的事件订阅和取消，避免内存泄漏；用BindableProperty定义属性，才能让控件支持XAML配置和MVVM模式。

内容的提问来源于stack exchange，提问作者Matthew Bailey